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250 ksps, 10-/12-bit adcs in 6-lead sc70 ad7910/ad7920 rev. c information furnished by analog devices is believed to be accurate and reliable. however, no responsibility is assumed by analog devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. specifications subject to change without notice. no license is granted by implication or otherwise under any patent or patent rights of analog devices. trademarks and registered trademarks are the property of their respective owners. one technology way, p.o. box 9106, norwood, ma 02062-9106, u.s.a. tel: 781.329.4700 www.analog.com fax: 781.461.3113 ? 2005 analog devices, inc. all rights reserved. features throughput rate: 250 ksps specified for v dd of 2.35 v to 5.25 v low power 3.6 mw typ at 250 ksps with 3 v supplies 12.5 mw typ at 250 ksps with 5 v supplies wide input bandwidth 71 db snr at 100 khz input frequency flexible power/serial clock speed management no pipeline delays high speed serial interface spi?-/qspi?-/microwire?-/dsp-compatible standby mode: 1 a max 6-lead sc70 package 8-lead msop package applications battery-powered systems personal digital assistants medical instruments mobile communications instrumentation and control systems data acquisition systems high speed modems optical sensors functional block diagram t/h control logic 10-/12-bit successive approximation adc gnd v dd ad7910/ad7920 sdata sclk cs v in 02976-001 figure 1. general description the ad7910/ad7920 1 are, respectively, 10-bit and 12-bit, high speed, low power, successive approximation adcs. the parts operate from a single 2.35 v to 5.25 v power supply and feature throughput rates up to 250 ksps. the parts contain a low noise, wide bandwidth track-and-hold amplifier that can handle input frequencies in excess of 13 mhz. the conversion process and data acquisition are controlled using cs and the serial clock, allowing the devices to interface with microprocessors or dsps. the input signal is sampled on the falling edge of cs and the conversion is initiated at this point. there are no pipeline delays associated with the part. the ad7910/ad7920 use advanced design techniques to achieve very low power dissipation at high throughput rates. the reference for the part is taken internally from v dd . this allows the widest dynamic input range to the adc. thus, the analog input range for the part is 0 to v dd . the conversion rate is determined by the sclk. product highlights 1. 10-/12-bit adcs in sc70 and msop packages. 2. low power consumption. 3. flexible power/serial clock speed management. the conversion rate is determined by the serial clock, allowing the conversion time to be reduced through the serial clock speed increase. this allows the average power consumption to be reduced when power-down mode is used while not converting. the part also features a power-down mode to maximize power efficiency at lower throughput rates. current consumption is 1 a maximum and 50 na typically when in power-down mode. 4. reference derived from the power supply. 5. no pipeline delay. the parts feature a standard successive approximation adc with accurate control of the sampling instant via a cs input and once-off conversion control. 1 protected by u.s. patent no. 6,681,332.
ad7910/ad7920 rev. c | page 2 of 24 table of contents features .............................................................................................. 1 applications ....................................................................................... 1 functional block diagram .............................................................. 1 general description ......................................................................... 1 product highlights ........................................................................... 1 specifications ..................................................................................... 3 ad7910 .......................................................................................... 3 ad7920 .......................................................................................... 4 timing specifications .................................................................. 6 timing examples .............................................................................. 7 timing example 1 ........................................................................ 7 timing example 2 ........................................................................ 7 absolute maximum ratings ............................................................ 8 esd caution .................................................................................. 8 pin configurations and function descriptions ........................... 9 typical performance characteristics ........................................... 10 ter mi nolo g y .................................................................................... 12 circuit information ........................................................................ 13 converter operation ...................................................................... 14 adc transfer function ............................................................. 14 typical connection diagram ................................................... 14 analog input ............................................................................... 15 digital inputs .............................................................................. 15 modes of operation ....................................................................... 16 normal mode .............................................................................. 16 power-down mode .................................................................... 16 power-up time .......................................................................... 16 power vs. throughput rate ........................................................... 18 serial interface ................................................................................ 19 microprocessor interfacing ........................................................... 20 ad7910/ad7920 to tms320c541 interface ......................... 20 ad7910/ad7920 to adsp-218x .............................................. 20 ad7910/ad7920 to dsp563xx interface ............................... 21 application hints ........................................................................... 22 grounding and layout .............................................................. 22 evaluating performance ............................................................ 22 outline dimensions ....................................................................... 23 ordering guide .......................................................................... 24 revision history 9/05rev. b to rev. c updated formatting ..........................................................universal updated outline dimensions....................................................... 23 changes to ordering guide.......................................................... 24 3/04rev. a to rev. b added u.s. patent number ............................................................ 1 changes to note 5 ............................................................................ 2 changes to note 6 of ad7920 specifications............................... 4 changes to note 1 of timing specifications ................................ 4 changes to absolute maximum ratings ...................................... 6 changes to ordering guide............................................................ 6 8/03rev. 0 to rev. a changes to ordering guide.............................................................6 changes to evaluating the ad7910/ad7920 performance section.............................................................................................. 18 updated outline dimensions ...................................................... 19
ad7910/ad7920 rev. c | page 3 of 24 specifications ad7910 v dd = 2.35 v to 5.25 v, f sclk = 5 mhz, f sample = 250 ksps, t a = t min to t max , unless otherwise noted. table 1. parameter 1 a grade 1 , 2 unit test conditions/comments dynamic performance f in = 100 khz sine wave signal-to-noise + distortion (sinad) 3 61 db min total harmonic distortion (thd) 3 ?72 db max peak harmonic or spurious noise (sfdr) 3 ?73 db max intermodulation distortion (imd) 3 second-order terms ?82 db typ fa = 100.73 khz, fb = 90.7 khz third-order terms ?82 db typ fa = 100.73 khz, fb = 90.7 khz aperture delay 10 ns typ aperture jitter 30 ps typ full power bandwidth 13.5 mhz typ @ 3 db 2 mhz typ @ 0.1 db dc accuracy resolution 10 bits integral nonlinearity 0.5 lsb max differential nonlinearity 0.5 lsb max guaranteed no missed codes to 10 bits offset error 3 , 4 1 lsb max gain error 3 , 4 1 lsb max total unadjusted error (tue) 3 , 4 1.2 lsb max analog input input voltage ranges 0 to v dd v dc leakage current 0.5 a max input capacitance 20 pf typ track-and-hold in track, 6 pf typ when in hold logic inputs input high voltage, v inh 2.4 v min input low voltage, v inl 0.8 v max v dd = 5 v 0.4 v max v dd = 3 v input current, i in , sclk pin 0.5 a max typically 10 na, v in = 0 v or v dd input current, i in , cs pin 10 na typ input capacitance, c in 5 5 pf max logic outputs output high voltage, v oh v dd ? 0.2 v min i source = 200 a, v dd = 2.35 v to 5.25 v output low voltage, v ol 0.4 v max i sink = 200 a floating-state leakage current 1 a max floating-state output capacitance 5 5 pf max output coding straight (natural) binary conversion rate conversion time 2.8 s max 14 sclk cycles with sclk at 5 mhz track-and-hold acquisition time 3 250 ns max throughput rate 250 ksps max power requirements v dd 2.35/5.25 v min/max i dd digital i/ps = 0 v or v dd normal mode (static) 2.5 ma typ v dd = 4.75 v to 5.25 v, sclk on or off 1.2 ma typ v dd = 2.35 v to 3.6 v, sclk on or off normal mode (operational) 3 ma max v dd = 4.75 v to 5.25 v, f sample = 250 ksps 1.4 ma max v dd = 2.35 v to 3.6 v, f sample = 250 ksps full power-down mode 1 a max typically 50 na
ad7910/ad7920 rev. c | page 4 of 24 parameter 1 a grade 1 , 2 unit test conditions/comments power dissipation 6 normal mode (operational) 15 mw max v dd = 5 v, f sample = 250 ksps 4.2 mw max v dd = 3 v, f sample = 250 ksps full power-down 5 w max v dd = 5 v 3 w max v dd = 3 v 1 temperature range from ?40c to +85c. 2 operational from v dd = 2.0 v, with input high voltage (v inh ) 1.8 v min. 3 see the terminology section. 4 sc70 values guaranteed by characterization. 5 guaranteed by characterization. 6 see the power vs. throughput rate section. ad7920 v dd = 2.35 v to 5.25 v, f sclk = 5 mhz, f sample = 250 ksps, t a = t min to t max , unless otherwise noted. table 2. parameter 1 a grade , 1 2 b grade 1 , 2 unit test conditions/comments dynamic performance f in = 100 khz sine wave signal-to-noise + distortion (sinad) 3 70 70 db min v dd = 2.35 v to 3.6 v, t a = 25c 69 69 db min v dd = 2.4 v to 3.6 v 71.5 71.5 db typ v dd = 2.35 v to 3.6 v 69 69 db min v dd = 4.75 v to 5.25 v, t a = 25c 68 68 db min v dd = 4.75 v to 5.25 v signal-to-noise ratio (snr) 3 71 71 db min v dd = 2.35 v to 3.6 v, t a = 25c 70 70 db min v dd = 2.4 v to 3.6 v 70 70 db min v dd = 4.75 v to 5.25 v, t a = 25c 69 69 db min v dd = 4.75 v to 5.25 v total harmonic distortion (thd) 3 ?80 ?80 db typ peak harmonic or spurious noise (sfdr) 3 ?82 ?82 db typ intermodulation distortion (imd) 3 second-order terms ?84 ?84 db typ fa = 100.73 khz, fb = 90.72 khz third-order terms ?84 ?84 db typ fa = 100.73 khz, fb = 90.72 khz aperture delay 10 10 ns typ aperture jitter 30 30 ps typ full power bandwidth 13.5 13.5 mhz typ @ 3 db 2 2 mhz typ @ 0.1 db dc accuracy b grade 4 resolution 12 12 bits integral nonlinearity 3 1.5 lsb max 0.75 lsb typ differential nonlinearity ?0.9/+1.5 lsb max guaranteed no missed codes to 12 bits 0.75 lsb typ offset error 3 , 5 1.5 lsb max 1.5 0.2 lsb typ gain error 3 , 5 1.5 lsb max 1.5 0.5 lsb typ total unadjusted error (tue) 3 , 5 2 lsb max analog input input voltage ranges 0 to v dd 0 to v dd v dc leakage current 0.5 0.5 a max input capacitance 20 20 pf typ track-and-hold in track, 6 pf typ when in hold
ad7910/ad7920 rev. c | page 5 of 24 parameter 1 a grade , 1 2 b grade 1 , 2 unit test conditions/comments logic inputs input high voltage, v inh 2.4 2.4 v min 1.8 1.8 v min v dd = 2.35 v input low voltage, v inl 0.8 0.8 v max v dd = 3.6 v to 5.25 v 0.4 0.4 v max v dd = 2.35 v to 3.6 v input current, i in , sclk pin 0.5 0.5 a max typically 10 na, v in = 0 v or v dd input current, i in , cs pin 10 10 na typ input capacitance, c in 6 5 5 pf max logic outputs output high voltage, v oh v dd ? 0.2 v dd ? 0.2 v min i source = 200 a, v dd = 2.35 v to 5.25 v output low voltage, v ol 0.4 0.4 v max i sink = 200 a floating-state leakage current 1 1 a max floating-state output capacitance 6 5 5 pf max output coding straight (natural) binary conversion rate conversion time 3.2 3.2 s max 16 sclk cycles with sclk at 5 mhz track-and-hold acquisition time 3 250 250 ns max throughput rate 250 250 ksps max see the serial interface section power requirements v dd 2.35/5.25 2.35/5.25 v min/max i dd digital i/ps = 0 v or v dd normal mode (static) 2.5 2.5 ma typ v dd = 4.75 v to 5.25 v, sclk on or off 1.2 1.2 ma typ v dd = 2.35 v to 3.6 v, sclk on or off normal mode (operational) 3 3 ma max v dd = 4.75 v to 5.25 v, f sample = 250 ksps 1.4 1.4 ma max v dd = 2.35 v to 3.6 v, f sample = 250 ksps full power-down mode 1 1 a max typically 50 na power dissipation 7 normal mode (operational) 15 15 mw max v dd = 5 v, f sample = 250 ksps 4.2 4.2 mw max v dd = 3 v, f sample = 250 ksps full power-down 5 5 w max v dd = 5 v 3 3 w max v dd = 3 v 1 temperature range from ?40c to +85c. 2 operational from v dd = 2.0 v, with input low voltage (v inl ) 0.35 v max. 3 see the terminology section. 4 b grade, maximum specifications apply as typical figures when v dd = 4.75 v to 5.25 v. 5 sc70 values guaranteed by characterization. 6 guaranteed by characterization. 7 see the power vs. throughput rate section.
ad7910/ad7920 rev. c | page 6 of 24 timing specifications v dd = 2.35 v to 5.25 v, t a = t min to t max , unless otherwise noted. table 3. parameter 1 ad7910/ad7920 limit at t min , t max unit description f sclk 2 10 khz min 3 5 mhz max t convert 14 t sclk ad7910 16 t sclk ad7920 t quiet 50 ns min minimum quiet time required between bus relinquish and start of next conversion t 1 10 ns min minimum cs pulse width t 2 10 ns min cs to sclk setup time t 3 4 22 ns max delay from cs until sdata three-state disabled t 4 40 ns max data access time after sclk falling edge t 5 0.4 t sclk ns min sclk low pulse width t 6 0.4 t sclk ns min sclk high pulse width t 7 5 , 6 sclk to data valid hold time 10 ns min v dd 3.3 v 9.5 ns min 3.3 v < v dd 3.6 v 7 ns min v dd > 3.6 v t 8 6 , 7 36 ns max sclk falling edge to sdata three-state see note 7 ns min sclk falling edge to sdata three-state t power-up 8 1 s max power-up time from full power-down 1 guaranteed by characterization. all input signals are specified with tr = tf = 5 ns (10% to 90% of v dd ) and timed from a voltage level of 1.6 v. 2 mark/space ratio for the sc lk input is 40/60 to 60/40. 3 minimum f sclk at which specifications are guaranteed. 4 measured with the load circuit of figure 2 and defined as the time required for the output to cross 0.8 v or 1.8 v when v dd = 2.35 v and 0.8 v or 2.0 v for v dd > 2.35 v. 5 measured with a 50 pf load capacitor. 6 t 8 is derived from the measured time taken by the data outputs to change 0.5 v when loaded with the circuit of figure 2. the meas ured number is then extrapolated back to remove the effects of charging or discharging the 50 pf capacitor. this means that the time, t 8 , shown in the timing specificatio ns is the true bus relinquish time of the part and is in dependent of the bus loading. 7 t 7 values apply to t 8 minimum values also. 8 see power-up time section. 200 ai ol 200 ai oh 1.6v to output pin c l 50pf 02976-002 figure 2. load circuit for digi tal output timing specifications
ad7910/ad7920 rev. c | page 7 of 24 timing examples figure 3 and figure 4 show some of the timing parameters from table 3. timing example 1 from figure 4 , having f sclk = 5 mhz and a throughput rate of 250 ksps gives a cycle time of t 2 + 12.5(1/f sclk ) + t acq = 4 s. with t 2 = 10 ns min, this leaves t acq to be 1.49 s. this 1.49 s satisfies the requirement of 250 ns for t acq . from figure 4 , t acq comprises 2.5(1/f sclk ) + t 8 + t quiet , where t 8 = 36 ns max. this allows a value of 954 ns for t quiet , satisfying the minimum requirement of 50 ns. timing example 2 the ad7920 can also operate with slower clock frequencies. from figure 4 , having f sclk = 3.4 mhz and a throughput rate of 150 ksps gives a cycle time of t 2 + 12.5(1/f sclk ) + t acq = 6.66 s. with t 2 = 10 ns min, this leaves t acq to be 2.97 s. this 2.97 s satisfies the requirement of 250 ns for t acq . from figure 4 , t acq comprises 2.5(1/f sclk ) + t 8 + t quiet , t 8 = 36 ns max. this allows a value of 2.19 s for t quiet , satisfying the minimum requirement of 50 ns. as in this example and with other slower clock values, the signal may already be acquired before the conversion is complete, but it is still necessary to leave 50 ns minimum t quiet between conversions. in this example, the signal should be fully acquired at approximately point c in figure 4 . cs sclk sdata t 2 t 6 t 3 t 4 t 7 t 5 t 8 t convert t quiet zero zero zero db11 db10 db2 db1 db0 b three-state three- state z 4 leading zeros 12345 13141516 t 1 02976-003 figure 3. ad7920 serial interface timing diagram cs s clk t 2 t convert b 12345 13141516 c t 8 t quiet t acq 12.5(1/f sclk ) 1/throughput 02976-004 figure 4. serial interface timing example
ad7910/ad7920 rev. c | page 8 of 24 absolute maximum ratings t a = 25c, unless otherwise noted. table 4. parameter rating v dd to gnd ?0.3 v to +7 v analog input voltage to gnd ?0.3 v to v dd + 0.3 v digital input voltage to gnd ?0.3 v to +7 v digital output voltage to gnd ?0.3 v to v dd + 0.3 v input current to any pin except supplies 1 10 ma operating temperature range commercial (a, b grade) ?40c to +85c storage temperature range ?65c to +150c junction temperature 150c msop package ja thermal impedance 205.9c/w jc thermal impedance 43.74c/w sc70 package ja thermal impedance 340.2c/w jc thermal impedance 228.9c/w lead temperature, soldering reflow (10 sec to 30 sec) 235 (0/+5)c esd 3.5 kv 1 transient currents of up to 100 ma will not cause scr latch-up. stresses above those listed under absolute maximum ratings may cause permanent damage to the device. this is a stress rating only; functional operation of the device at these or any other conditions above those indicated in the operational section of this specification is not implied. exposure to absolute maximum rating conditions for extended periods may affect device reliability. esd caution esd (electrostatic discharge) sensitive device. electros tatic charges as high as 4000 v readily accumulate on the human body and test equipment and can discharge wi thout detection. although this product features proprietary esd protection circuitry, permanent dama ge may occur on devices subjected to high energy electrostatic discharges. therefore, proper esd precautions are recommended to avoid performance degradation or loss of functionality.
ad7910/ad7920 rev. c | page 9 of 24 pin configurations and function descriptions v dd 1 gnd 2 v in 3 cs 6 sdata 5 sclk 4 ad7910/ ad7920 top view (not to scale) 02976-005 figure 5. 6-lead sc70 pin configuration v dd 1 sdata 2 cs 3 nc 4 v in 8 gnd 7 sclk 6 nc 5 nc = no connect ad7910/ ad7920 top view (not to scale) 02976-006 figure 6. 8-lead msop pin configuration table 5. pin function descriptions pin no. 6-lead sc70 pin no. 8-lead msop mnemonic description 6 3 cs chip select. active low logic input. this input provides the dual function of initiating conversions on the ad7910/ad7920 and framing the serial data transfer. 1 1 v dd power supply input. the v dd range for the ad7910/ad7920 is from 2.35 v to 5.25 v. 2 7 gnd analog ground. ground reference point for all circ uitry on the ad7910/ad7920. all analog input signals should be referred to this gnd voltage. 3 8 v in analog input. single-ended analog in put channel. the input range is 0 to v dd . 5 2 sdata data out. logic output. the conversion result fr om the ad7910/ad7920 is provided on this output as a serial data stream. the bits are clocked out on the fa lling edge of the sclk input. the data stream from the ad7920 consists of four leading zeros followed by the 12 bits of conversion data, which is provided msb first. the data stream from the ad7910 consists of four leading zeros followed by the 10 bits of conversion data followed by two trailing zeros, which is also provided msb first. 4 6 sclk serial clock. logic input. sclk provides the serial cl ock for accessing data from the part. this clock input is also used as the clock source fo r the ad7910/ad7920 conversion process. n/a 4, 5 nc no connect.
ad7910/ad7920 rev. c | page 10 of 24 typical performance characteristics figure 7 and figure 8 show a typical fft plot for the ad7920 and ad7910, respectively, at a 250 ksps sampling rate and a 100 khz input frequency. figure 9 shows the signal-to-(noise + distortion) ratio performance vs. input frequency for various supply voltages while sampling at 250 ksps with an sclk frequency of 5 mhz for the ad7920. figure 10 and figure 11 show typical inl and dnl performance for the ad7920. figure 12 shows a graph of the total harmonic distortion vs. analog input frequency for different source impedances when using a supply voltage of 3.6 v and sampling at a rate of 250 ksps. see the analog input section. figure 13 shows a graph of the total harmonic distortion vs. analog input signal frequency for various supply voltages while sampling at 250 ksps with an sclk frequency of 5 mhz. frequency (khz) ?5 ?55 ?115 0 125 25 snr (db) 50 75 100 ?15 ?35 ?75 ?95 8192 point fft v dd = 2.7v f sample = 250ksps f in = 100khz sinad = 72.05db thd = ?82.87db sfdr = ?87.24db 02976-007 figure 7. ad7920 dynamic performance at 250 ksps frequency (khz) ?45 ?105 snr (db) ?5 15 ?25 ?65 ?85 0 125 25 50 75 100 02976-008 8192 point fft v dd = 2.35v f sample = 250ksps f in = 100khz sinad = 61.67db thd = ?79.59db sfdr = ?82.93db figure 8. ad7910 dynamic performance at 250 ksps frequency (khz) ?72.0 10 1000 sinad (db) 100 ?73.0 ?73.5 v dd = 5.25v v dd = 2.35v v dd = 2.7v v dd = 4.75v v dd = 3.6v ?72.5 ?71.0 ?71.5 02976-009 figure 9. ad7920 sinad vs. input frequency at 250 ksps code 1.0 0.4 ?0.2 0 1024 inl error (lsb) 512 0.8 0.6 0.2 0 ?0.4 ?0.6 ?0.8 ?1.0 1536 2048 2560 3072 3584 4096 v dd = 2.35v temp = 25 c f sample = 250ksps 02976-010 figure 10. ad7920 inl performance
ad7910/ad7920 rev. c | page 11 of 24 code 1.0 0.4 ?0.2 0 1024 dnl error (lsb) 512 0.8 0.6 0.2 0 ?0.4 ?0.6 ?0.8 ?1.0 1536 2048 2560 3072 3584 4096 v dd = 2.35v temp = 25 c f sample = 250ksps 02976-011 figure 11. ad7920 dnl performance input frequency (khz) ?30 ?60 10 1000 thd (db) 100 ?10 ?20 ?40 ?50 ?70 ?80 ?90 v dd = 3.6v r in = 10k r in = 1k r in = 130 r in = 13 r in = 0 02976-012 figure 12. thd vs. analog input frequency for various source impedances input frequency (khz) ?75 ?90 10 1000 thd (db) 100 ?65 ?70 ?80 ?85 v dd = 5.25v v dd = 2.35v v dd = 2.7v v dd = 4.75v v dd = 3.6v 02976-013 figure 13. thd vs. analog input frequency for various supply voltages
ad7910/ad7920 rev. c | page 12 of 24 terminology integral nonlinearity the maximum deviation from a straight line passing through the endpoints of the adc transfer function. for the ad7920 and ad7910, the endpoints of the transfer function are zero scale, a point 1 lsb below the first code transition, and full scale, a point 1 lsb above the last code transition. differential nonlinearity the difference between the measured and the ideal 1 lsb change between any two adjacent codes in the adc. offset error the deviation of the first code transition (00 . . . 000) to (00 . . . 001) from the ideal, that is, gnd + 1 lsb. gain error the deviation of the last code transition (111 . . . 110) to (111 . . . 111) from the ideal, that is, v ref ? 1 lsb after the offset error has been adjusted out. track-and-hold acquisition time the track-and-hold amplifier returns to track mode at the end of conversion. track-and-hold acquisition time is the time required for the output of the track-and-hold amplifier to reach its final value, within 0.5 lsb, after the end of conversion. see the serial interface section for more details. signal-to-(noise + distortion) ratio the measured ratio of signal-to-(noise + distortion) at the output of the a/d converter. the signal is the rms amplitude of the fundamental. noise is the sum of all nonfundamental signals up to half the sampling frequency (f s /2), excluding dc. the ratio is dependent on the number of quantization levels in the digiti- zation process. the more levels, the smaller the quantization noise. the theoretical signal-to-(noise + distortion) ratio for an ideal n-bit converter with a sine wave input is given by: signal-to-(noise + distortion) = (6.02 n + 1.76) db thus, for a 12-bit converter this is 74 db, and for a 10-bit converter this is 62 db. tot a l un a dju s te d e r ror a comprehensive specification that includes gain error, linearity error, and offset error. total harmonic distortion (thd) total harmonic distortion is the ratio of the rms sum of harmonics to the fundamental. it is defined as: 1 2 6 2 5 2 4 2 3 2 2 log20)( v vvvvv dbthd ++++ = where: v 1 is the rms amplitude of the fundamental. v 2 , v 3 , v 4 , v 5 , and v 6 are the rms amplitudes of the second through the sixth harmonics. peak harmonic or spurious noise peak harmonic or spurious noise is the ratio of the rms value of the next largest component in the adc output spectrum (up to f s /2 and excluding dc) to the rms value of the fundamental. normally, the value of this specification is determined by the largest harmonic in the spectrum, but for adcs whose harmonics are buried in the noise floor, it is a noise peak. intermodulation distortion with inputs consisting of sine waves at two frequencies, fa and fb, any active device with nonlinearities creates distortion products at sum and difference frequencies of mfa nfb where m, n = 0, 1, 2, 3, and so on. intermodulation distortion terms are those for which neither m nor n are equal to zero. for example, the second- order terms include (fa + fb) and (fa ? fb), while the third-order terms include (2fa + fb), (2fa ? fb), (fa + 2fb), and (fa ? 2fb). the ad7910/ad7920 are tested using the ccif standard, where two input frequencies are used (see fa and fb in the specifications page). in this case, the second-order terms are usually distanced in frequency from the original sine waves, while the third-order terms are usually at a frequency close to the input frequencies. as a result, the second- and third-order terms are specified separately. the calculation of the intermodulation distortion is as per the thd specification, the ratio of the rms sum of the individual distortion products to the rms amplitude of the sum of the fundamentals, expressed in db.
ad7910/ad7920 rev. c | page 13 of 24 circuit information the ad7910/ad7920 are fast, micropower, 10-bit/12-bit, single-supply a/d converters, respectively. the parts can be operated from a 2.35 v to 5.25 v supply. when operated from either a 5 v supply or a 3 v supply, the ad7910/ad7920 are capable of throughput rates of 250 ksps when provided with a 5 mhz clock. the ad7910/ad7920 provide the user with an on-chip track- and-hold, a/d converter, and a serial interface housed in a tiny 6-lead sc70 package or 8-lead msop package, which offers the user considerable space saving advantages over alternative solutions. the serial clock input accesses data from the part but also provides the clock source for the successive approximation a/d converter. the analog input range is 0 v to v dd . an external reference is not required for the adc and there is no reference on-chip. the reference for the ad7910/ad7920 is derived from the power supply and thus gives the widest dynamic input range. the ad7910/ad7920 also feature a power-down option to allow power saving between conversions. the power-down feature is implemented across the standard serial interface, as described in the modes of operation section.
ad7910/ad7920 rev. c | page 14 of 24 converter operation the ad7910/ad7920 are successive approximation analog-to- digital converters based around a charge redistribution dac. figure 14 and figure 15 show simplified schematics of the adc. figure 14 shows the adc during its acquisition phase. when sw2 is closed and sw1 is in position a, the comparator is held in a balanced condition, and the sampling capacitor acquires the signal on v in . charge redistribution dac control logic comparator sw2 sampling capacitor acquisition phase sw1 a b agnd v dd /2 v in 02976-014 figure 14. adc acquisition phase when the adc starts a conversion (see figure 15 ), sw2 opens and sw1 moves to position b, causing the comparator to become unbalanced. the control logic and charge redistribution dac are used to add and subtract fixed amounts of charge from the sampling capacitor to bring the comparator back into a balanced condition. when the comparator is rebalanced, the conversion is complete. the control logic generates the adc output code. figure 16 shows the adc transfer function. charge redistribution dac control logic comparator sw2 sampling capacitor conversion phase sw1 a b agnd v dd /2 v in 02976-015 figure 15. adc conversion phase adc transfer function the output coding of the ad7910/ad7920 is straight binary. the designed code transitions occur at the successive integer lsb values, that is, 1 lsb, 2 lsbs, and so on. the lsb size is v dd /4096 for the ad7920 and v dd /1024 for the ad7910. the ideal transfer characteristic for the ad7910/ad7920 is shown in figure 16 . 000...000 0v adc code analog input 111...111 000...001 000...010 111...110 111...000 011...111 1lsb +v dd ? 1lsb 1lsb = v dd /1024 (ad7910) 1lsb = v dd /4096 (ad7920) 02976-016 figure 16. transfer characteristic typical connection diagram figure 17 shows a typical connection diagram for the ad7910/ ad7920. v ref is taken internally from v dd and, as such, v dd should be well decoupled. this provides an analog input range of 0 v to v dd . the conversion result is output in a 16-bit word with four leading zeros followed by the msb of the 12-bit or 10-bit result. two trailing zeros follow the 10-bit result from the ad7910. alternatively, because the supply current required by the ad7910/ad7920 is so low, a precision reference can be used as the supply source to the ad7910/ad7920. an ref19x voltage reference (ref195 for 5 v or ref193 for 3 v) can be used to supply the required voltage to the adc (see figure 17 ). this configuration is especially useful if the power supply is quite noisy or if the system supply voltages are at a value other than 5 v or 3 v (for example, 15 v). the ref19x outputs a steady voltage to the ad7910/ad7920. if the low dropout ref193 is used, the current it needs to supply to the ad7910/ad7920 is typically 1.2 ma. when the adc is converting at a rate of 250 ksps, the ref193 needs to supply a maximum of 1.4 ma to the ad7910/ad7920. the load regulation of the ref193 is typically 10 ppm/ma (ref193, v s = 5 v), which results in an error of 14 ppm (42 v) for the 1.4 ma drawn from it. this corresponds to a 0.057 lsb error for the ad7920 with v dd = 3 v from the ref193 and a 0.014 lsb error for the ad7910. for applications where power consumption is of concern, the power-down mode of the adc and the sleep mode of the ref19x reference should be used to improve power performance. see the modes of operation section.
ad7910/ad7920 rev. c | page 15 of 24 ad7910/ ad7920 v dd v in serial interface 0v to v dd input c/ p gnd sclk sdata cs 0.1 f 10 f 1 f tant 0.1 f 680nf 3v 5v supply 1.2ma ref193 02976-017 figure 17. ref193 as power supply table 6 provides typical performance data with various references used as a v dd source for a 100 khz input tone at room temperature, under the same setup conditions. table 6. ad7920 typical perf ormance for various voltage references ic reference tied to v dd ad7920 snr performance (db) ad780 @ 3 v 72.65 ref193 72.35 ad780 @ 2.5 v 72.5 ref192 72.2 ref43 72.6 analog input figure 18 shows an equivalent circuit of the analog input structure of the ad7910/ad7920. the two diodes, d1 and d2, provide esd protection for the analog input. care must be taken to ensure that the analog input signal never exceeds the supply rails by more than 300 mv. this causes these diodes to become forward biased and start conducting current into the substrate. the maximum current these diodes can conduct without causing irreversible damage to the parties is 10 ma. capacitor c1 in figure 18 is typically about 6 pf and can be attributed primarily to pin capacitance. resistor r1 is a lumped component made up of the on resistance of a switch. this resistor is typically about 100 . capacitor c2 is the adc sampling capacitor and has a capacitance of 20 pf typically. for ac applications, removing high frequency components from the analog input signal is recommended by use of a band-pass filter on the relevant analog input pin. in applications where harmonic distortion and signal-to-noise ratio are critical, the analog input should be driven from a low impedance source. large source impedances significantly affect the ac performance of the adc. this can necessitate the use of an input buffer amplifier. the choice of the op amp is a function of the particular application. c1 6pf c2 20pf r1 d1 d2 conversion phase?switch open track phase?switch closed v dd v in 02976-018 figure 18. equivalent analog input circuit table 7 provides some typical performance data with various op amps used as the input buffer for a 100 khz input tone at room temperature, under the same setup conditions. table 7. ad7920 typical perf ormance for various input buffers, v dd = 3 v op amp in te input buffer ad7920 snr performance (db) ad711 72.3 ad797 72.5 ad845 71.4 when no amplifier is used to drive the analog input, the source impedance should be limited to low values. the maximum source impedance depends on the amount of total harmonic distortion (thd) that can be to lerated. the thd increases as the source impedance increases, and performance degrades (see figure 12 ). digital inputs the digital inputs applied to the ad7910/ad7920 are not limited by the maximum ratings that limit the analog input. instead, the digital inputs applied can go to 7 v and are not restricted by the v dd + 0.3 v limit as on the analog input. for example, if the ad7910/ad7920 were operated with a v dd of 3 v, then 5 v logic levels could be used on the digital inputs. however, it is important to note that the data output on sdata still have 3 v logic levels when v dd = 3 v. another advantage of sclk and cs not being restricted by the v dd + 0.3 v limit is that power supply sequencing issues are avoided. if cs or sclk is applied before v dd , there is no risk of latch-up as there would be on the analog inputs if a signal greater than 0.3 v were applied prior to v dd .
ad7910/ad7920 rev. c | page 16 of 24 modes of operation the mode of operation of the ad7910/ad7920 is selected by controlling the logic state of the cs signal during a conversion. there are two possible modes of operation, normal mode and power-down mode. the point at which cs is pulled high after the conversion is initiated determines whether the ad7910/ad7920 enters power-down mode. similarly, if the device is already in power-down mode, cs can control whether it returns to normal operation or remains in power-down mode. these modes of operation are designed to provide flexible power management options. these options can be chosen to optimize the power dissipation/throughput rate ratio for different application requirements. normal mode this mode is intended for fastest throughput rate performance because the user does not have to worry about any power-up times; the ad7910/ad7920 remains fully powered all the time. figure 19 shows the general diagram of the operation of the ad7910/ad7920 in this mode. the conversion is initiated on the falling edge of cs as described in the serial interface section. to ensure that the part remains fully powered up at all times, cs must remain low until at least 10 sclk falling edges have elapsed after the falling edge of cs . if cs is brought high any time after the tenth sclk falling edge but before the end of the t convert , then the part remains powered up but the conversion is terminated and sdata goes back into three-state. for the ad7920, 16 serial clock cycles are required to complete the conversion and access the co mplete conversion result. for the ad7910, a minimum of 14 serial clock cycles is required to complete the conversion and access the complete conversion result. cs can idle high until the next conversion or can idle low until cs returns high sometime prior to the next conversion, effectively idling cs low. once a data transfer is complete (sdata has returned to three- state), another conversion can be initiated after the quiet time, t quiet , has elapsed by bringing cs low again. power-down mode this mode is intended for use in applications where slower throughput rates are required; either the adc is powered down between conversions, or a series of conversions can be performed at a high throughput rate and the adc is powered down for a relatively long duration between these bursts of several conversions. when the ad7910/ad7920 is in power- down mode, all analog circuitry is powered down. to enter power-down mode, the conversion process must be interrupted by bringing cs high anywhere after the second falling edge of sclk, and before the tenth falling edge of sclk, as shown in figure 20 . once cs is brought high in this window of sclks, the part enters power-down mode, the conversion that was initiated by the falling edge of cs is terminated, and sdata goes back into three-state. if cs is brought high before the second sclk falling edge, the part remains in normal mode and does not power down. this avoids accidental power-down due to glitches on the cs line. to exit this mode of operation and power up the ad7910/ ad7920 again, a dummy conversion is performed. on the falling edge of cs , the device begins to power up, and continues to power up as long as cs is held low until after the falling edge of the tenth sclk. the device is fully powered up once 16 sclks have elapsed and valid data results from the next conversion, as shown in figure 21 . if cs is brought high before the tenth sclk falling edge, the ad7910/ad7920 goes back into power-down mode again. this avoids accidental power-up due to glitches on the cs line or an inadvertent burst of eight sclk cycles while cs is low. although the device can begin to power up on the falling edge of cs , it powers down again on the rising edge of cs as long as it occurs before the tenth sclk falling edge. power-up time the power-up time of the ad7910/ad7920 is 1 s, which means that one dummy cycle is always sufficient to allow the device to power up. once the dummy cycle is complete, the adc fully powered up and the input signal acquired properly. the quiet time, t quiet , must still be allowed from the point where the bus goes back into three-state after the dummy conversion, to the next falling edge of cs . when powering up from the power-down mode with a dummy cycle, as in figure 21 , the track-and-hold that was in hold mode while the part was powered down returns to track mode after the first sclk edge the part receives after the falling edge of cs . this is shown as point a in figure 21 . although at any sclk frequency one dummy cycle is sufficient to power up the device and acquire v in , it does not necessarily mean that a full dummy cycle of 16 sclks must always elapse to power up the device and fully acquire v in ; 1 s is sufficient to power the device up and acquire the input signal. therefore, if a 5 mhz sclk frequency is applied to the adc, the cycle time is 3.2 s. in one dummy cycle, 3.2 s, the part powers up and v in is fully acquired. however, after 1 s with a 5 mhz sclk, only five sclk cycles have elapsed. at this stage, the adc is fully powered up and the signal is acquired. in this case, the cs can be brought high after the tenth sclk falling edge and brought low again after a time, t quiet , to initiate the conversion.
ad7910/ad7920 rev. c | page 17 of 24 valid data cs sclk s dat a 11 0 1 2 1 4 1 6 ad7910/ad7920 02976-019 figure 19. normal mode operation three-state sdata sclk cs 11 0 1 2 1 4 1 6 2 02976-020 figure 20. entering power-down mode sclk cs sdat a the part begins to power up the part is fully powered up with v in fully acquired a 10 12 14 16 1 16 valid data invalid data 02976-021 1 figure 21. exiting power-down mode when power supplies are first applied to the ad7910/ad7920, the adc can power up in either power-down mode or in normal mode. because of this, it is best to allow a dummy cycle to elapse to ensure the part is fully powered up before attempting a valid conversion. likewise, if the intention is to keep the part in power- down mode while not in use and the user wishes the part to power up in power-down mode, the dummy cycle can be used to ensure the device is in power-down mode by executing a cycle such as that shown in figure 20 . once supplies are applied to the ad7910/ad7920, the power-up time is the same as that when powering up from power-down mode. it takes approximately 1 s to power up fully if the part powers up in normal mode. it is not necessary to wait 1 s before executing a dummy cycle to ensure the desired mode of operation. instead, the dummy cycle can occur directly after power is supplied to the adc. if the first valid conversion is performed directly after the dummy conversion, care must be taken to ensure that adequate acquisition time is allowed. as mentioned earlier, when powering up from the power-down mode, the part returns to track upon the first sclk edge applied after the falling edge of cs . however, when the adc powers up initially after supplies are applied, the track-and-hold is in track. this means, assuming the user has the facility to monitor the adc supply current, if the adc powers up in the desired mode of operation and thus a dummy cycle is not required to change mode then a dummy cycle is required to place the track-and- hold into track.
ad7910/ad7920 rev. c | page 18 of 24 power vs. throughput rate by using the power-down mode on the ad7910/ad7920 when not converting, the average power consumption of the adc decreases at lower throughput rates. figure 22 shows how, as the throughput rate is reduced, the device remains in its power- down state longer and the average power consumption over time drops accordingly. for example, if the ad7910/ad7920 is operated in a continuous sampling mode with a throughput rate of 100 ksps and an sclk of 5 mhz (v dd = 5 v), and the device is placed in the power-down mode between conversions, the power consumption is calculated as follows. the power dissipation during normal mode is 15 mw (v dd = 5 v). the power dissipation includes the power dissipated while the part is entering power-down mode, the power dissipated during the dummy conversion (when the part is exiting power-down mode and powering up), and the power dissipated during conversion. as mentioned in the power-down mode section, to enter power-down mode, cs has to be brought high anywhere between the second and tenth sclk falling edge. therefore, the power consumption when entering power-down mode varies depending on the number of sclk cycles used. in this example, five sclk cycles are used to enter power-down mode. this gives a time period of 5 (1/f sclk ) = 1 s. the power-up time is 1 s, which implies that only five sclk cycles are required to power up the part. however, cs has to remain low until at least the tenth sclk falling edge when exiting power-down mode. this means that a minimum of nine sclk cycles have to be used to exit power-down mode and power up the part. so, if nine sclk cycles are used, the time to power up the part and exit power-down mode is 9 (1/f sclk ) = 1.8 s. finally, the conversion time is 16 (1/f sclk ) = 3.2 s. therefore, the ad7910/ad7920 can be said to dissipate 15 mw for 3.2 s + 1.8 s + 1 s = 6 s during each conversion cycle. if the throughput rate is 100 ksps, the cycle time is 10 s and the average power dissipated during each cycle is (6/10) (15 mw) = 9 mw. the power dissipation when the part is in power-down has not been taken into account because the shutdown current is so low and it does not have any effect on the overall power dissipation value. if v dd = 3 v, sclk = 5 mhz, and the device is again in power- down mode between conversions, the power dissipation during normal operation is 4.2 mw. assuming the same timing conditions as before, the ad7910/ad7920 can now be said to dissipate 4.2 mw for 6 s during each conversion cycle. with a throughput rate of 100 ksps, the average power dissipated during each cycle is (6/10) (4.2 mw) = 2.52 mw. figure 22 shows the power vs. throughput rate when using the power- down mode between conversions with both 5 v and 3 v supplies. power-down mode is intended for use with throughput rates of approximately 160 ksps and under because at higher sampling rates there is no power saving made by using the power-down mode. throughput rate (ksps) 100 0.1 0 power (mw) 10 1 0.01 20 v dd = 5v, sclk = 5mhz v dd = 3v, sclk = 5mhz 40 60 80 100 120 140 160 180 02976-022 figure 22. power vs. throughput rate
ad7910/ad7920 rev. c | page 19 of 24 serial interface figure 23 and figure 24 show the detailed timing diagram for serial interfacing to the ad7920 and ad7910, respectively. the serial clock provides the conversion clock and also controls the transfer of information from the ad7910/ad7920 during conversion. the cs signal initiates the data transfer and conversion process. the falling edge of cs puts the track-and-hold into hold mode and takes the bus out of three-state; the analog input is sampled at that point. the conversion is also initiated at this point. for the ad7920, the conversion requires 16 sclk cycles to complete. once 13 sclk falling edges have elapsed, track-and- hold goes back into track on the next sclk rising edge, as shown in figure 23 at point b. on the 16th sclk falling edge, the sdata line goes back into three-state. if the rising edge of cs occurs before 16 sclks have elapsed then the conversion is terminated and the sdata line goes back into three-state; otherwise, sdata returns to three-state on the 16th sclk falling edge, as shown in figure 23 . sixteen serial clock cycles are required to perform the conversion process and to access data from the ad7920. for the ad7910, the conversion requires 14 sclk cycles to complete. once 13 sclk falling edges have elapsed, track-and- hold goes back into track on the next sclk rising edge, as shown in figure 24 at point b. if the rising edge of cs occurs before 14 sclks have elapsed, the conversion is terminated and the sdata line goes back into three-state. if 16 sclks are used in the cycle, sdata returns to three-state on the 16th sclk falling edge, as shown in figure 24 . cs going low clocks out the first leading zero to be read in by the microcontroller or dsp. the remaining data is then clocked out by subsequent sclk falling edges beginning with the second leading zero. thus, the first falling clock edge on the serial clock has the first leading zero provided and also clocks out the second leading zero. the final bit in the data transfer is valid on the 16th falling edge, having been clocked out on the previous (15th) falling edge. in applications with a slower sclk, it is possible to read in data on each sclk rising edge. in this case, the first falling edge of sclk clocks out the second leading zero, which could be read in the first rising edge. however, the first leading zero that was clocked out when cs went low is missed unless it was not read in the first falling edge. the 15th falling edge of sclk clocks out the last bit and it could be read in the 15th rising sclk edge. if cs goes low just after the sclk falling edge has elapsed, cs clocks out the first leading zero as before, and it can be read on the sclk rising edge. the next sclk falling edge clocks out the sec- ond leading zero and it could be read on the following rising edge. cs sclk s dat a t 2 t 6 t 3 t 4 t 7 t 5 t 8 t convert t quiet zero zero zero db11 db10 db2 db1 db0 b three-state three- state z 4 leading zeros 15 14 13 5 4 3 2 1 16 t 1 1/throughput 02976-023 figure 23. ad7920 serial interface timing diagram cs sclk s dat a t 2 t 6 t 3 t 4 t 7 t 5 t 8 t convert t quiet zero zero zero db9 db8 db0 zero zero b three-state three- state z 4 leading zeros 2 trailing zeros 15 14 13 5 4 3 2 1 16 t 1 1/throughput 02976-024 figure 24. ad7910 serial interface timing diagram
ad7910/ad7920 rev. c | page 20 of 24 microprocessor interfacing the serial interface on the ad7910/ad7920 allows the parts to be directly connected to a range of different microprocessors. this section explains how to interface the ad7910/ad7920 with some of the more common microcontroller and dsp serial interface protocols. ad7910/ad7920 to tms320c541 interface the serial interface on the tms320c541 uses a continuous serial clock and frame synchronization signals to synchronize the data transfer operations with peripheral devices like the ad7910/ad7920. the cs input allows easy interfacing between the tms320c541 and the ad7910/ad7920 without any glue logic required. the serial port of the tms320c541 is set up to operate in burst mode (fsm = 1 in the serial port control register, spc) with internal serial clock clkx (mcm = 1 in spc register) and internal frame signal (txm = 1 in the spc), so both pins are configured as outputs. for the ad7920, the word length should be set to 16 bits (fo = 0 in the spc register). this dsp allows frames with a word length of 16 bits or 8 bits. therefore, in the case of the ad7910 where just 14 bits could be required, the fo bit would be set up to 16 bits also. this means that to obtain the conversion result, 16 sclks are needed and two trailing zeros are clocked out in the two last clock cycles. to summarize, the values in the spc register are: fo = 0 fsm = 1 mcm = 1 txm = 1 the format bit, fo, can be set to 1 to set the word length to eight bits to implement the power-down mode on the ad7910/ad7920. figure 25 shows the connection diagram. it should be noted that for signal processing applications, it is imperative that the frame synchronization signal from the tms320c541 provides equidistant sampling. ad7910/ad7920* sclk sdata cs clkx clkr fsx fsr tms320c541* *additional pins omitted for clarity dr 02976-025 figure 25. interfacing to the tms320c541 ad7910/ad7920 to adsp-218x the adsp-218x family of dsps is interfaced directly to the ad7910/ad7920 without any glue logic required. the sport control register should be set up as follows: tfsw = rfsw = 1, alternate framing invrfs = invtfs = 1, active low frame signal dtype = 00, right justify data isclk = 1, internal serial clock tfsr = rfsr = 1, frame every word irfs = 0, sets up rfs as an input itfs = 1, sets up tfs as an output slen = 1111, 16 bits for the ad7920 slen = 1101, 14 bits for the ad7910 to implement power-down mode, slen should be set to 0111 to issue an 8-bit sclk burst. the connection diagram is shown in figure 26 . the adsp-218x has the tfs and rfs of the sport tied together, with tfs set as an output and rfs set as an input. the dsp operates in alternate framing mode and the sport control register is set up as described. the frame synchronization signal generated on the tfs is tied to cs and, as with all signal processing applications, equidistant sampling is necessary. however, in this example, the timer interrupt is used to control the sampling rate of the adc and, under certain conditions, equidistant sampling can not be achieved. ad7910/ad7920 * sclk sdata cs sclk dr rfs tfs adsp-218x* 02976-026 *additional pins omitted for clarity figure 26. interfacing to the adsp-218x the timer registers are loaded with a value that provides an interrupt at the required sample interval. when an interrupt is received, a value is transmitted with tfs/dt (adc control word). the tfs is used to control the rfs and thus the reading of data. the frequency of the serial clock is set in the sclkdiv register. when the instruction to transmit with tfs is given, that is, tx0 = ax0, the state of the sclk is checked. the dsp waits until the sclk has gone high, low, and high before transmission starts. if the timer and sclk values are chosen such that the instruction to transmit occurs on or near the rising edge of sclk, the data can be transmitted or it can wait until the next clock edge.
ad7910/ad7920 rev. c | page 21 of 24 for example, the adsp-2111 has a master clock frequency of 16 mhz. if the sclkdiv register is loaded with the value 3, an sclk of 2 mhz is obtained and eight master clock periods elapse for every one sclk period. if the timer registers are loaded with the value 803, 100.5 sclks occur between interrupts and subsequently between transmit instructions. this situation results in nonequidistant sampling as the transmit instruction is occurring on an sclk edge. if the number of sclks between interrupts is a whole integer figure of n, equidistant sampling is implemented by the dsp. ad7910/ad7920 to dsp563xx interface the diagram in figure 27 shows how the ad7910/ad7920 can be connected to the synchronous serial interface (ssi) (synchronous serial interface) of the dsp563xx family of dsps from motorola. the ssi is operated in synchronous and normal mode (syn = 1 and mod = 0 in control register b, crb) with internally generated word frame sync for both tx and rx (bit fsl1 = 0 and bit fsl0 = 0 in the crb). set the word length in the control register a (cra) to 16 by setting bits wl2 = 0, wl1 = 1, and wl0 = 0 for the ad7920. this dsp does not offer the option for a 14-bit word length, so the ad7910 word length is set to 16 bits like the ad7920. for the ad7910, the conversion process uses 16 sclk cycles, with the last two clock periods clocking out two trailing zeros to fill the 16-bit word. to implement the power-down mode on the ad7910/ad7920, the word length can be changed to eight bits by setting bits wl2 = 0, wl1 = 0, and wl0 = 0 in cra. the fsp bit in the crb register can be set to 1, which means the frame goes low and a conversion starts. likewise, by means of bits scd2, sckd, and shfd in the crb register, it is established that pin sc2 (the frame sync signal) and sck in the serial port is configured as outputs and the msb is shifted first. to summarize: mod = 0 syn = 1 wl2, wl1, wl0 depend on the word length fsl1 = 0, fsl0 = 0 fsp = 1, negative frame sync scd2 = 1 sckd = 1 shfd = 0 it should be noted that for signal processing applications, it is imperative that the frame synchronization signal from the dsp563xx provides equidistant sampling. ad7910/ad7920* sdata sclk cs dsp563xx* sck srd sc2 *additional pins omitted for clarity 02976-027 figure 27. interfacing to the dsp563xx
ad7910/ad7920 rev. c | page 22 of 24 application hints grounding and layout the printed circuit board that houses the ad7910/ad7920 should be designed such that the analog and digital sections are separated and confined to certain areas of the board. this facilitates the use of ground planes that can be easily separated. a minimum etch technique is generally best for ground planes as it gives the best shielding. digital and analog ground planes should be joined at only one place. if the ad7910/ad7920 is in a system where multiple devices require an agnd to dgnd connection then the connection should still be made at one point only, a star ground point that should be established as close to the ad7910/ad7920 as possible. avoid running digital lines under the device as these couple noise onto the die. the analog ground plane should be allowed to run under the ad7910/ad7920 to avoid noise coupling. the power supply lines to the ad7910/ad7920 should use as large a trace as possible to provide low impedance paths and reduce the effects of glitches on the power supply line. fast switching signals like clocks should be shielded with digital ground to avoid radiating noise to other sections of the board, and clock signals should never be run near the analog inputs. avoid crossover of digital and analog signals. traces on opposite sides of the board should run at right angles to each other. this reduces the effects of feedthrough through the board. a micro- strip technique is by far the best but is not always possible with a double-sided board. in this technique, the component side of the board is dedicated to ground planes while signals are placed on the solder side. good decoupling is also very important. the supply should be decoupled with, for example, a 680 nf 0805 to gnd. when using the sc70 package in applications where the size of the components is of concern, a 220 nf 0603 capacitor, for example, could be used instead. however, in that case, the decoupling can not be as effective and can result in an approximate sinad degradation of 0.3 db. to achieve the best performance from these decoupling components, the user should endeavor to keep the distance between the decoupling capacitor and the v dd and gnd pins to a minimum with short track lengths connecting the respective pins. figure 28 and figure 29 show the recommended positions of the decoupling capacitor for the msop and sc70 packages, respectively. as can be seen in figure 28 , for the msop package, the decoupling capacitor is placed as close as possible to the ic, with short track lengths to v dd and gnd pins. the decoupling capacitor could also be placed on the underside of the pcb directly underneath the ic, between the v dd and gnd pins attached by vias. this method would not be recommended on pcbs above a standard 1.6 mm thickness. the best performance is seen with the decoupling capacitor on the top of the pcb next to the ic. 02976-028 figure 28. recommended supply decoupling scheme for the ad7910/ad7920 msop package similarly, for the sc70 package, the decoupling capacitor should be located as close as possible to the v dd and gnd pins. because of its pinout, that is, v dd being next to gnd, the decoupling capacitor can be placed extremely close to the ic. the decoupling capacitor could be placed on the underside of the pcb directly under the v dd and gnd pins, but, as before, the best performance is seen with the decoupling capacitor on the same side as the ic. 02976-029 figure 29. recommended supply decoupling scheme for the ad7910/ad7920 sc70 package evaluating performance the evaluation board package includes a fully assembled and tested evaluation board, documentation, and software for controlling the board from the pc via the eval-board controller. to demonstrate/evaluate the ac and dc performance of the ad7910/ad7920, the evaluation board controller can be used in conjunction with the ad7910/ad7920cb evaluation boards as well as many other an alog devices evaluation boards ending in the cb designator. the software allows the user to perform ac (fast fourier transform) and dc (histogram of codes) tests on the ad7910/ad7920. see the evaluation board technical note for more information.
ad7910/ad7920 rev. c | page 23 of 24 outline dimensions compliant to jedec standards mo-203-ab 0.22 0.08 0.30 0.15 1.00 0.90 0.70 seating plane 4 5 6 3 2 1 pin 1 0.65 bsc 1.30 bsc 0.10 max 0.10 coplanarity 0.40 0.10 1.10 0.80 2.20 2.00 1.80 2.40 2.10 1.80 1.35 1.25 1.15 0.46 0.36 0.26 figure 30. 6-lead thin shrink small outline transistor package [sc70] (ks-6) dimensions shown in millimeters compliant to jedec standards mo-187-aa 0.80 0.60 0.40 8 0 4 8 1 5 pin 1 0.65 bsc seating plane 0.38 0.22 1.10 max 3.20 3.00 2.80 coplanarity 0.10 0.23 0.08 3.20 3.00 2.80 5.15 4.90 4.65 0.15 0.00 0.95 0.85 0.75 figure 31. 8-lead mini small outline package [msop] (rm-8) dimensions shown in millimeters
ad7910/ad7920 rev. c | page 24 of 24 ? 2005 analog devices, inc. all rights reserved. trademarks and registered trademarks are the property of their respective owners. c02976-0-9/05(c) ordering guide model temperature range linearity error (lsb) 1 package description package option branding ad7910aks-500rl7 ?40c to +85c 0.5 max 6-lead sc70 ks-6 cva ad7910aks-reel ?40c to +85c 0.5 max 6-lead sc70 ks-6 cva ad7910aks-reel7 ?40c to +85c 0.5 max 6-lead sc70 ks-6 cva ad7910aksz-500rl7 2 ?40c to +85c 0.5 max 6-lead sc70 ks-6 c49 ad7910aksz-reel 2 ?40c to +85c 0.5 max 6-lead sc70 ks-6 c49 ad7910aksz-reel7 2 ?40c to +85c 0.5 max 6-lead sc70 ks-6 c49 ad7910arm ?40c to +85c 0.5 max 8-lead msop rm-8 cva ad7910arm-reel ?40c to +85c 0.5 max 8-lead msop rm-8 cva ad7910arm-reel7 ?40c to +85c 0.5 max 8-lead msop rm-8 cva ad7910armz 2 ?40c to +85c 0.5 max 8-lead msop rm-8 c49 ad7920aks-500rl7 ?40c to +85c 0.75 typ 6-lead sc70 ks-6 cua ad7920aks-reel ?40c to +85c 0.75 typ 6-lead sc70 ks-6 cua ad7920aks-reel7 ?40c to +85c 0.75 typ 6-lead sc70 ks-6 cua ad7920aksz-500rl7 2 ?40c to +85c 0.75 typ 6-lead sc70 ks-6 c47 ad7920aksz-reel7 2 ?40c to +85c 0.75 typ 6-lead sc70 ks-6 c47 ad7920bks-500rl7 ?40c to +85c 1.5 max 6-lead sc70 ks-6 cub ad7920bks-reel ?40c to +85c 1.5 max 6-lead sc70 ks-6 cub ad7920bks-reel7 ?40c to +85c 1.5 max 6-lead sc70 ks-6 cub ad7920bksz-500rl7 2 ?40c to +85c 1.5 max 6-lead sc70 ks-6 c4b ad7920bksz-reel 2 ?40c to +85c 1.5 max 6-lead sc70 ks-6 c4b ad7920bksz-reel7 2 ?40c to +85c 1.5 max 6-lead sc70 ks-6 c4b ad7920brm ?40c to +85c 1.5 max 8-lead msop rm-8 cub ad7920brm-reel ?40c to +85c 1.5 max 8-lead msop rm-8 cub ad7920brm-reel7 ?40c to +85c 1.5 max 8-lead msop rm-8 cub AD7920BRMZ 2 ?40c to +85c 1.5 max 8-lead msop rm-8 c4b AD7920BRMZ-reel 2 ?40c to +85c 1.5 max 8-lead msop rm-8 c4b AD7920BRMZ-reel7 2 ?40c to +85c 1.5 max 8-lead msop rm-8 c4b eval-ad7910cb 3 evaluation board eval-ad7920cb 3 evaluation board eval-control brd2 4 controller board 1 linearity error refers to integral nonlinearity. 2 z = pb-free part. 3 this can be used as a standalone evaluation board or in conjunction with the eval-control brd2 for evaluation/demonstration pu rposes. 4 this board is a complete unit that allows a pc to control and communicate with all analog devices evaluation boards ending in the cb designator. to order a complete evaluation kit, a particular adc evaluation board must be ordered, for examp le., eval-ad7920cb, the eval-control brd2, and a 12 v ac transformer. see relevant evaluation board technical note for more information.

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