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APPLICATION NOTE
MX029
1. Introduction
Dual Digitally Controlled Amplifier
The MX029 is a single-chip LSI circuit comprised of two individual, digitally controlled gain sections. Control of each gain section is accomplished through the serial interface. Each section has 48 distinct gain steps (+/-48dB of range in 2dB steps) plus MUTE. As shown in Figure 1 both gain stages have switchable inputs. This switching allows for selection of three different input signals on one channel and two on the other channel. One of the channels also has output switching. These features make it very easy to use the MX029 as a summing amplifier. The MX029 can also be used as a digital to analog converter(DAC). For this application we take advantage of the 48 distinct gain settings. With a single DC input voltage the variable gain of the amplifier can be used to generate an analog output signal. This application note describes both applications, the MX029 as a summing amplifier and DAC. Whenever the MX029 is used in any application the input signal should be as noise free as possible. Cutting off the serial clock when it is not being used will reduce noise. With high gain settings a very small amount of noise is amplified and becomes a problem. When serial loading the MX029 make sure that the serial clock, serial data and LOAD/LATCH timing is correct. This seems to be a common problem for users, especially the timing of the LOAD/LATCH line. Use a storage scope to check your timing.
SERIAL CLOCK DATA INPUT LOAD/LATCH
SERIAL INTERFACE
VDD
STAGE 1 CONTROL REGISTER
VSS BIAS VSS BIAS
CH1, INPUT 1 CH1, INPUT 2 CH1, INPUT 3
TO STAGE 1 CONTROL REGISTER
CH1, OUTPUT 1
G
CH1, OUTPUT 2
VSS
BIAS
STAGE 1 CONTROL REGISTER
VSS CH2, INPUT 1
BIAS
G
CH2, INPUT 2
CH2, OUTPUT 1
UNIVERSAL AMP IN
UNIVERSAL AMP OUT
BIAS
Figure 1: Device Block Diagram.
(c) 1997 MX*COM, INC. www.mxcom.com Tele: 800 638 5577 910 744 5050 Fax: 910 744 5054 Doc. # 20830071.001 4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA.All trademarks and service marks are held by their respective companies.
MX029 Dual Digitally Controlled Amplifier
2
Application Note
2. The MX029 as a Summing Amplifier
The MX029 can be used as a summing amplifier. Using channel one you can sum up to three different signals. With channel two you can sum two signals. As seen in Figure 2 very few external components are needed for the MX029 to be used as a summing amplifier. The MX029 has an internal resistor that creates a voltage divider with R IN. The configuration and value of this internal resistor is different in gain and attenuation mode, see Figure 2 and Figure 3. In gain mode the internal resistor is approximately 200K and in attenuation mode the internal resistance is approximately 100K. With a larger value of RIN there will be more of a drop across RIN. On the other hand, if the value of RIN is too small the input source may not be able to drive RIN due to source loading. Recommended values for RIN are 10k to 20k. The equation used to calculate the signal level at the input of the MX029 for a two input summing amplifier is:
VIN 029 =
VIN 1 ( RIN 2 RL ) VIN 2 ( RIN 1 RL ) + RIN 1 ( RIN 2 + RL ) + RIN 2 RL RIN 2 ( RIN 1 + RL ) + RIN 1 RL
Where RL is approximately 200K in gain mode and 100K in attenuation mode. For a three input summing amplifier use the following equation :
VIN 029 =
VIN 1 ( R ' // RL ) V ( R ' ' // RL ) V ( R ' ' ' // RL ) + IN 2 + IN 3 RIN 1 + ( R ' // RL ) RIN 2 + ( R ' ' // RL ) RIN 3 + ( R ' ' ' // RL )
Where R' is the parallel combination of RIN2 and RIN3 (RIN2// RIN3) and R'' = RIN1//RIN3 and R''' = RIN1//RIN2 and RL is approximately 200K in gain mode and 100K in attenuation mode.
V IN1 V IN2
R IN1
V BIAS V IN029
+
Internal to the MX029
R IN2
RL V BIAS
200K
Figure 2: Gain Mode Configuration.
V IN1 V IN2
R IN1 V IN029 R IN2 RL
+
Internal to the MX029
100K V BIAS
Figure 3: Attenuation Mode Configuration.
(c) 1997 MX*COM, INC. www.mxcom.com Tele: 800 638 5577 910 744 5050 Fax: 910 744 5054 Doc. # 20830071.001 4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA.All trademarks and service marks are held by their respective companies.
MX029 Dual Digitally Controlled Amplifier
3
Application Note
3. The MX029 as a digital to analog converter
The MX029 can be used as a digital to analog converter. The input should be DC coupled not AC coupled. If the part is AC coupled it will not function as a DAC.
V BIAS V REF RL V BIAS
+
Internal to the MX029
Figure 4: Using the MX029 as a DAC.
5
4.75
4.5
4.25
4
Volts
3.75
3.5
3.25
3
2.75
2.5 -48-46-44-42-40-38-36-34-32-30-28-26-24-22-20-18-16-14-12-10 -8 -6 -4 -2 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48
n (gain parameter)
Figure 5: Volts vs. Gain Setting with VREF at 2.51V. When using the MX029 as a DAC there will be one input voltage(VREF). To generate the output signal this input voltage is stepped up or down using the gain settings of the MX029. Each gain setting corresponds to a step on the output signal, see Figure 5. The minimum duration of each step(sample time) is approximately 8s (giving a maximum sample rate of 125KHz). This is the time required to load and latch the 14 bits of serial data. This 125kHz sample rate can only be realized with negative gain settings. With gain settings 0dB to +30dB the amplifier's bandwidth will be the limiting factor and the maximum sample rate is approximately 20kHz. With gain settings above +30dB the maximum sample rate will be considerably less.
(c) 1997 MX*COM, INC. www.mxcom.com Tele: 800 638 5577 910 744 5050 Fax: 910 744 5054 Doc. # 20830071.001 4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA.All trademarks and service marks are held by their respective companies.
MX029 Dual Digitally Controlled Amplifier
4
Application Note
The analog GND of the MX029 is at VDD/2 (VBIAS). With a 5 volt power supply this voltage is 2.5 volts. With VBIAS at VDD/2 a sign bit is needed to get output levels above and below VBIAS , see Figure 6. When in gain (attenuation) mode the difference of VBIAS and the reference voltage(VREF) will be amplified (attenuated).
n
VOUT = (VREF - VBIAS )10 20 + VBIAS
Where n is the gain setting -48 to +48. For n = 0, VOUT = VREF, for n < 0, VOUT is between VREF and VBIAS, and for n > 1, VOUT is between VREF and a supply rail. For example if VREF is 2.4 volts and VBIAS is 2.5 volts, with a gain setting of +24dB the output signal will (2.4 - 2.5)10 1.2 + 2.5V = 0.915 volts. To calculate the gain of each 2dB step use the following formula:
n
Gain = 10 20
Where n is the gain setting -48 to +48. Figure 7 is an example of an output signal generated by changing the gain settings where the full dynamic range of the MX029 is used. To generate the output signal above VBIAS , VREF needs to be approximately 10 millivolts above VBIAS. And to generate the output signal below VBIAS , VREF needs to be approximately 10 millivolts below VBIAS. When using input levels 10 millivolts above or below VBIAS, the input referred offset voltage of the amplifier must be compensated for by adjusting VREF accordingly. An input referred offset voltage of 10 millivolts is not uncommon for an MX029. For this reason it is not very practical to use values of VREF that are so close to VBIAS. In this application the higher and lower gain settings will typically not be used. It is important to recognize that above VBIAS the range is approximately 2.5 volts to 4.7 volts and below VBIAS it is approximately 0.3 volts to 2.5 volts. With a sign bit the range is approximately 4.4 volts.
nMAX = 20 log
2.2V | VREF - VBIAS |
Where nMAX is the maximum gain setting with a particular VREF.
V REF -V REF Sign Bit
V BIAS
+
Internal to the MX029 V BIAS
Figure 6: Using two VREFs to generate an output that goes above and below VBIAS.
(c) 1997 MX*COM, INC. www.mxcom.com Tele: 800 638 5577 910 744 5050 Fax: 910 744 5054 Doc. # 20830071.001 4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA.All trademarks and service marks are held by their respective companies.
MX029 Dual Digitally Controlled Amplifier
5 4.75 4.5 4.25 4 3.75 3.5 3.25 3 2.75
5
Application Note
Volts
2.5 2.25 2 1.75 1.5 1.25 1 0.75 0.5 0.25 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2
Time (ms)
Figure 7: Example Output Signal. Figure 7 is just one example of an output signal. There are many different output signals that can be generated. Before you start programming the MX029 take some time to think about what input signal level is necessary above or below V BIAS to obtain the output signal that you desire.
(c) 1997 MX*COM, INC. www.mxcom.com Tele: 800 638 5577 910 744 5050 Fax: 910 744 5054 Doc. # 20830071.001 4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA.All trademarks and service marks are held by their respective companies.

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