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industrial organic chemical sensor sgas707 datasheet ? 2017 integrated device technology, inc . 1 october 2 5 , 2017 description the idt sgas70 7 is a solid - state chemiresistor sensor designed to detect volatile organic chemicals ( vocs ) in air. the s ensor uses an integrated heater with highly sensitive polymer - mox composite material designed for detection of vocs . the chemiresistor sensors in the idt sgas family are based on the principle that metal - oxide materials undergo surface inter - actions (physisorption and chemisorption) wit h gas molecules at elevated temperatures, resulting in a measurable change in electrical resistance. as metal - oxide materials are polycrystalline (i.e., composed of multiple grains with distinct grain boundaries), the adsorbed gases have significant electr onic effects on the individual grains. these gas - solid interactions result in a change in electron (or hole) density at the surface (i.e., a space charge forms), which in turn changes the electrical conductivity of the oxide. idt has developed a set of nan ostructured gas - sensing materials with excellent sensitivity and stability. figure 1 . product photo feature s ? high sensitivity to a wide range of vocs ? non - specific: r esponds to many different organic vapors ? typical response time < 1 minute to 90% full scale ? environmental t emperature range: 0 c to 4 0c ? environmental humidity range: 0 % to 90% rh, noncondensing typical applications ? indoor air quality ? ventilation control ? air purification ? gas concentration detection examples of target gases ? formaldehyde ? toluene ? xylenes ? acetone ? isobutylene ? octane ? alcohols available support ? evaluation k it C smod7 07 smart sensing module ? application n otes ? instruction v ideos ? reference design
sgas707 datasheet ? 2017 integrated device technology, inc . 2 october 2 5 , 2017 contents 1. pin assignments ................................ ................................ ................................ ................................ ................................ ........................... 3 2. pin descriptions ................................ ................................ ................................ ................................ ................................ ............................ 3 3. sensor specifications ................................ ................................ ................................ ................................ ................................ ................... 3 4. sensor characteristics ................................ ................................ ................................ ................................ ................................ ................. 4 5. basic measurement circuit ................................ ................................ ................................ ................................ ................................ ........... 5 6. heater driver circuits and control ................................ ................................ ................................ ................................ ................................ 6 6.1 constant voltage drive ................................ ................................ ................................ ................................ ................................ ........ 6 6.2 constant - current drive ................................ ................................ ................................ ................................ ................................ ........ 6 6.3 pulse - width modulation ................................ ................................ ................................ ................................ ................................ ....... 7 6.4 operating the sensor at temperature extremes ................................ ................................ ................................ ................................ . 8 7. sensing characteristics ................................ ................................ ................................ ................................ ................................ ................ 9 7.1 sensitivity ................................ ................................ ................................ ................................ ................................ ............................ 9 7.2 cross - sensitivity ................................ ................................ ................................ ................................ ................................ ................ 12 8. maximum esd ratings ................................ ................................ ................................ ................................ ................................ .............. 13 9. mechanical stress testing ................................ ................................ ................................ ................................ ................................ ......... 13 10. package drawing and dimensions ................................ ................................ ................................ ................................ ............................. 14 11. ordering information ................................ ................................ ................................ ................................ ................................ ................... 15 12. revision history ................................ ................................ ................................ ................................ ................................ .......................... 15 list of figures figure 1. product photo ................................ ................................ ................................ ................................ ................................ ...................... 1 figure 2. to - 39 (to4) pin assignments for sgas707 C top view ................................ ................................ ................................ ................... 3 figure 3. typical sensor response characteristic ................................ ................................ ................................ ................................ ............. 5 figure 4. basic measurement circuit ................................ ................................ ................................ ................................ ................................ .. 5 figure 5. three - terminal voltage regulator ................................ ................................ ................................ ................................ ...................... 6 figure 6. voltage - controlled constant - current circuit ................................ ................................ ................................ ................................ ....... 7 figure 7. recommended applied heater voltage as a function of environmental temperature ................................ ................................ ...... 8 figure 8. typical sensor response to a variety of organic chemicals ................................ ................................ ................................ ............. 9 figure 9. typical sensor sensitivity to a variety of organic chemicals ................................ ................................ ................................ ........... 10 figure 10. effect of different humidity levels on the sensor signal at ambient temperature ................................ ................................ ........... 11 figure 11. response of the sgas707 sensor to other industrial gases ................................ ................................ ................................ .......... 12 figure 12. to - 39 package (to4) outline drawing psc - 4676 ................................ ................................ ................................ ........................... 14 list of tables table 1. to - 39 (to4) pin descript ions ................................ ................................ ................................ ................................ ............................. 3 table 2. electrical characteristics ................................ ................................ ................................ ................................ ................................ ..... 3 table 3. temperature specifications ................................ ................................ ................................ ................................ ................................ . 4 table 4. maximum esd ratings ................................ ................................ ................................ ................................ ................................ ..... 13 table 5. mechanical stres s test conditions ................................ ................................ ................................ ................................ ................... 13
sgas707 datasheet ? 2017 integrated device technology, inc . 3 october 2 5 , 2017 1. pin assignments figure 2 . to - 39 (to4) pin assignments for sgas707 C top view 2. pin descriptions table 1 . to - 39 (to4) pin descriptions note: see figure 2 for the connections described below. pin number name description 1 heater + positive input for the v h heater voltage supply 2 sensor + high - side of the resistive sensor element; positive input for sensing voltage v c 3 heater C negative (ground) input for the v h heater voltage supply 4 sensor C low - side of the resistive sensor element; connects to the middle of the resistor divider circuit and produces sensing voltage output (v out ) 3. sensor specifications note: a ll measurements were made in dry gas at room temperature. specifications are subject to change . table 2 . electrical characteristics symbol parameter conditions minimum typical maximum units p h heater p ower c onsumption v h = 3.5 v 400 mw v h recommended h eater v oltage t sensor = 15 0oc 3.5 vdc r h heater r esistance at room temperature 28 30 32 ? v c recommended s ensing v oltage 2.5 5.0 vdc r 500 r esistance in 500 ppm e thanol 5 500 k ? r 100 /r 9 00 resolution: r esistance in 10 0 ppm e thanol / resistance in 90 0 ppm 3.8 p i n 2 p i n 3 p i n 1 p i n 4 t a b
sgas707 datasheet ? 2017 integrated device technology, inc . 4 october 2 5 , 2017 table 3 . temperature specifications symbol parameter conditions minimum typical maximum units t op sensor operation temperature v h = 3.5v 150 c t amb recommended environmental temperature range 0 40 c t stor maximum storage temperature range - 50 125 c the sensor is not intended for continuous operation above or below the environmental temperature specification, but exposure for short durations will not will not harm the sensor. 4. sensor characteristics idts solid - state chemiresistive sensors are an advanced type of gas - sensitive resistor; i.e. they sense the p resence of a target gas through a change in resistance of the sensing element. most sensors exhibit reduced resistance as gas concentration increases, typicall y over several orders of magnitude across the sensing range. solid - state chemiresistive sensors s how a reduced resistance with increasing gas concentration according to equation 1: r s = a ? c - equation 1 where r s is resistance, c is concentration, and a and ? are constants. although several refined versions of this equation are available for specific sensors or sensing materials, the fundamental resistance versus concentration relationship for all of idts n - type sensors follows equation 1. taking the log of b oth sides of the equation results in equation 2: log ( r s ) = log ( a ) C ? log(c) equation 2 this shows that log resistance versus log concentration is linear. an immediately observable consequence of equation 1 is that sensor resista nce will change rapidly at low concentrations and much less at high concentrations. this is illustrated in the following example: r gas_10ppm = 20k r gas_100ppm = 5k a gas = 8.0 ? 10 4 air = 0.602 the non - logarithmic response plot shown in figure 3 illustrates the fundamental challenge that must be addressed when measuring the resistance of chemiresistor sensors and relating these measurements to gas concentrations. additional non - linear effects fro m the measurement circuitry exacerbate these challenges and must be understood in order to account for or eliminate these effects.
sgas707 datasheet ? 2017 integrated device technology, inc . 5 october 2 5 , 2017 figure 3 . typical sensor response characteristic the electronic instrumentation used to detect thi s change in resistance influences the quality and accuracy of the gas sensing result. in particular, the choice of the analog front - end used to measure resistance can ultimately have a significant effect on overall measurement characteristics and must be s elected with care. for additional information, see idts application note C resistance measuring circuits for sgas sensors. 5. basic measurement circuit the sensor can be operated using a simple voltage divider. this requires two voltage supplies: the heater voltage (v h ) and circuit voltage (v c ). v h is applied to the heater in order to maintain a constant, elevated temperature for optimum sensing. v c is applied to allow a measurement of the output voltage (v out ) across a load resistor (r l ). figure 4 . basic measurement circuit pins 1 and 3 are attached to the heater. apply v h across these pins. pins 2 and 4 are attached to the resistive sensor element. connect these pins in the measuring circuit. idt supplies basic measurement circuitry for many of our sensors. m ore information can be found in idts application note C resistance measuring circuits for sgas sensors . 0.0e+00 2.0e+04 4.0e+04 6.0e+04 8.0e+04 1.0e+05 1.2e+05 0 20 40 60 80 100 120 sensor signal [ohm] gas concentration [ppm] v o u t v h v c r l g n d s e n s o r ( p i n 1 ) ( p i n 2 ) ( p i n 3 ) ( p i n 4 ) r s r h
sgas707 datasheet ? 2017 integrated device technology, inc . 6 october 2 5 , 2017 6. heater driver circuits and control t he sgas707 sensor contains a resistive element t hat is used to heat the sensor to the target operating temperature as shown in table 2 . the sgas707 voc sensor uses a purely resistive element that is nominally 30 ? at all temperatures. 6.1 constant voltage drive the simplest method of applying heater power is use of a constant vo ltage drive . because heaters draw a relatively large amount of current in normal operation, a method of current amplification is required . additionally, because relatively small changes in voltage levels will affect the temperature of the heater (and conse quently gas sensitivity), voltage regulation is required. an easily implemented control circuit utilizes a three - terminal vol tage regulator, with the lm317 serving as an example as shown in figure 5 . figure 5 . three - terminal voltag e regulator r1 and r2 (one of these can be a potentiometer) are selected to provide the target heater driv e voltage for the sensor. the example for the lm317 is capable of regulating voltages down to 1.25v and is thus suitable for sgas707 sensors . however, a wide variety of more advanced three - terminal voltage regulators are available from componen t manufacturers . circuits of this type are relatively efficient, particularly if a switching regulator is used . 6.2 constant - current drive the constant - current drive is more complex and costly than the constant voltage drive, but the added capabilities justify the expen se for many applications. a dditionally, the circuit is microcontroller friendly because heater curr ent is directly controllable with by an input vo ltage signal. the constant current heater drive circuit is shown in figure 6 . v in (supplied by an exte rnal source) is forced across r1, thus providing a predictable current through both r1 and r2 with a predictable voltage drop (relative to v dd ) across r2. an equivalent drop is imposed across r3, and the current through both r3 and r heater is thus controll ed independently of the load resistance according to the equation in figure 6 . the heater current is controllable to below 1ma. however, the circuit is inefficient compared to others, as power is dissipat ed in r3 and q2 as well as the heater. limiting the supply voltag e to several hundred mv above the highest required drive voltage will help increase circuit efficiency. while v in can be supplied by a fixed voltage reference (such as a divider), the flexibility of the circuit is most revealed when v in is supplied by a mi crocontroller via a digital - to - analog converter (dac). with this type of contro l, the heater drive can be time - programmed to allow pulsing of the heater with a variable amplitude. determination of the heater power or resistance is possible by reading the v oltage level at the heater. l m 3 1 7 r 1 r 2 0 . 1 f 1 0 f v s u p p l y v h e a t e r v h e a t e r = 1 . 2 5 v ? ( 1 + r 2 / r 1 ) + i a d j ? r 2
sgas707 datasheet ? 2017 integrated device technology, inc . 7 october 2 5 , 2017 figure 6 . voltage - controlled constant - current circuit 6.3 pulse - width modulation pulse - width modulation (pwm) is a very efficient method of providing con trollable drive to the h eater. however, this method has not undergone sufficient testing at idt to allow idt to recommend it for any sensors in the sgas family . pwm heater drive design should keep the following in mind: ? voltage to the heater should not exceed the maximum voltage allowed for a given heater family. ? a low - pass filter should be considered as part of the sensor signal circuit path to reduce noise from the heater pwm. v i n v d d r 1 r 2 r 3 u 1 a u 1 b q 1 q 2 r h e a t e r i h e a t e r = v i n ? r 2 / ( r 1 ? r 3 )
sgas707 datasheet ? 2017 integrated device technology, inc . 8 october 2 5 , 2017 6.4 operating the sensor at temperature extremes this sensor is intended for indoor air quality; however, t here may be some applications where it is desirable to measure voc levels at low and high temperatures. however, the relative response of the sensors to differing vocs will be a function of environmental temperature when the sensor is operated with a const ant voltage or current applied to the heater. this behavior is readily explained by considering that large shifts in ambient temperatures affect the operating temperature at the sensor surface, in turn altering the kinetics and thermodynamics of the intera ction of the sensing surface with vocs. this alters the electrical conduction of the sensor element (the basis of metal - oxide sensor operation). recommendation: in these cases, operate the sensor using an adjustment of the heating voltage to a predetermin ed setting based o n the environmental temperature. a graphical representation of the recommended temperature set - point voltage versus the environmental temperature is shown in figure 7. the mathematical description for the curve is given in equation 3: v h = - 0.01 ? environmental temperature [c] + 3.8 equation 3 figure 7 . recommended applied heater voltage as a function of environmental temperature 3.0 3.2 3.4 3.6 3.8 4.0 4.2 -30 -20 -10 0 10 20 30 40 50 60 70 applied heater voltage [v] environmental temperature [ c]
sgas707 datasheet ? 2017 integrated device technology, inc . 9 october 2 5 , 2017 7. sensing characteristics the following graphs show the typical responses that are to be expected from the sgas707 sensors on exposure to a variety of test conditions. for sensor specifications, refer to table 2 . 7.1 sensitivity the typical sensitivity of the sgas707 sensor to a range of organic chemicals is shown in figure 8 . figure 8 . typical sensor response to a variety of organic chemicals
sgas707 datasheet ? 2017 integrated device technology, inc . 10 october 2 5 , 2017 figure 9 . typical sensor sensitivity to a variety of organic chemicals 1 10 100 1 10 100 1000 sensitivity [r air /r gas ] concentration [ppm] acetone ethanol formaldehyde isobutylene octane toluene xylene
sgas707 datasheet ? 2017 integrated device technology, inc . 11 october 2 5 , 2017 the typical response of the sensor to changes in humidity is shown in figure 10 . figure 10 . effect of different humidity levels on the sensor signal at ambient temperature 1e+05 1e+06 1e+07 1e+08 20 200 sensor signal [ohm] concentration (ppm) air at 15% rh air at 45% rh air at 65% rh ethanol at 15%rh ethanol at 30%rh ethanol at 45%rh ethanol at 65%rh
sgas707 datasheet ? 2017 integrated device technology, inc . 12 october 2 5 , 2017 7.2 cross - sensitivity the response of the sgas707 sensor s to a range of other common gases is shown in figure 11 . figure 11 . response of the sgas707 se nsor to other industrial gases 1e+04 1e+05 1e+06 1e+07 1e+08 0.1 1 10 100 1000 sensor signal [ohm] concentration [ppm] air ammonia carbon monoxide ethanol hydrogen sulfide methane nitrogen dioxide
sgas707 datasheet ? 2017 integrated device technology, inc . 13 october 2 5 , 2017 8. maximum esd ratings table 4 . maximum esd ratings symbol parameter conditions minimum maximum units v hbm1 electrostatic discharge tolerance C human body model (hbm1) 2000 C v v cdm electrostatic discharge tolerance C charged device model (cdm) on packaged module 500 C v 9. mechanical stress testing the qualification of the sgas707 is based on the jedec standard (jesd47). after subjection to the mechanical shock and vibration testing conditions give n in table 5 , the sgas707 sensor will meet the specifications given in this document. for information on constant acceleration test conditions and lim its, contact idt (see contact information on last page). table 5 . mechanical stress test conditions stress test standard conditions mechanical shock jesd22 - b104, m2002 y1 plane only, 5 pulses, 0.5 ms duration, 1500 g peak acceleration vibration variable frequency jesd22 - b103, m2007 20hz to 2 k hz (log variation) in > 4 minutes, 4 times in each orientation, 50g peak acceleration
sgas707 datasheet ? 2017 integrated device technology, inc . 14 october 2 5 , 2017 10. package drawing and dimensions figure 12 . to - 39 package (to4) outline drawing psc - 4676
sgas707 datasheet ? 2017 integrated device technology, inc . 15 october 2 5 , 2017 applications and use conditions the sgas7 07 sensor is designed for measurement of ppm levels of volatile organic chemicals . the sensor is not intended, recommended, or approved for use in safety or life - protecting applications or in pote ntially explosive environments. idt disclaims all liability for such use. for sensor storage, idt str ongly recommends a dust - free and voc - free atmosphere ; e.g. , in synthetic air. 11. ordering information orderable part number description and package msl rating shipping packaging temperature sgas707 4 - p in to - 39 (to4) 1 tray 0c to +40c SMOD707KITV1 smod707 evaluation kit, including the smod707 smart sensing module (includes the sgas707 sensor), and mini - usb cable. the smod7xx application software is available for download at www.idt.com/smod707 . 12. revision history revision date description of change october 2 5 , 2017 full r evision . november 9 , 2016 changed to idt branding. corporate headquarters 6024 silver creek valley road san jose, ca 95138 www.idt.com sales 1 - 800 - 345 - 7015 or 408 - 284 - 8200 fax: 408 - 284 - 2775 www.idt.com/go/sales tech support www.idt.com/go/support disclaimer integrated device technology, inc. (idt) and its affiliated companies (herein referred to as idt) reserve the ri ght to modify the products and/or specifications described herein at any time, without notice, at idt's sole discretion. performance specifications and operating parameters of the described products are d etermined in an independent state and are not guaranteed to perform the same way when installed in customer pro ducts. the information contained herein is provided without representation or warranty of any kind, whether express or implie d, including, but not limited to, the suitability of idt's products for any particular purpose, an implied warranty of merchantabil ity, or non - infringement of the intellectual property rights of others. this document is presented only as a guide and does not convey any license under intellectual property rights of idt or any third parties. idt's products are not intended for use in applications involving extreme environmental conditions or in life support systems or similar devices where the failure or ma lfunction of an idt product can be reasonably expected to significantly affect the health or safety of users. anyone using an idt p roduct in such a manner does so at their own risk, absent an express, written agreement by idt. integrated device technology, idt and the idt logo are trademarks or registered trademarks of idt and its subsidiaries in the united states and other countries . other trademarks used herein are the property of idt or their respective third party owners. for datasheet type definitions and a glossary of common terms, visit www.idt.com/go/glossary . all contents of this document are copyright of integrated device technology, inc. all rights reserved.

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