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trace hydrogen (h 2 ) sensor sgas701 datasheet ? 2017 integrated device technology, inc . 1 n ovember 16 , 2017 description the idt sgas701 is a solid - state chemiresistor sensor designed to detect hydrogen in air. the s ensor uses an integrated heater with highly sensitive mox material tailored for detection of hydrogen. the chemiresistor sensors of idts sgas family are based upon the principle that metal oxide materials undergo surface inter - actions (physisorption and chemisorption) with gas molecule s at elevated temperatures, resulting in a measurable change in electrical resistance. as these materials are polycrystalline (i.e., composed of multiple grains with distinct grain boundaries), the adsorbed gases have significant electronic 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 nanostructured gas sens ing materials with excellent sensitivity and stability. figure 1 . product photo feature s ? high sensitivity to low hydrogen concentrations ( < 10 to 1000 ppm) ? fast r esponse time ( < 15 seconds at 100 ppm) ? environmental temperature rang e of - 20c to 50c ? environmental humidity range of 0 % to 90% rh, noncondensing ? low dependence on flow rate ? rugged, reliable sensor based on idts exclusive technology ? leak detection ? gas concentration detection ? breath detection available support ? evaluation k it C smod70 1 kitv1 ? application notes ? instruction videos ? reference design
sgas701 datasheet ? 2017 integrated device technology, inc . 2 n ovember 16 , 2017 contents 1. pin assignments ................................ ................................ ................................ ................................ ................................ ........................... 4 2. pin descriptions ................................ ................................ ................................ ................................ ................................ ............................ 4 3. sensor specifications ................................ ................................ ................................ ................................ ................................ ................... 4 4. sensor characteristics ................................ ................................ ................................ ................................ ................................ ................. 5 5. basic measurement circui t ................................ ................................ ................................ ................................ ................................ ........... 6 6. heater driver circuits and control ................................ ................................ ................................ ................................ ................................ 7 6.1 constant voltage drive ................................ ................................ ................................ ................................ ................................ ........ 7 6.2 constant current drive ................................ ................................ ................................ ................................ ................................ ........ 8 6.3 pulse - width modulation ................................ ................................ ................................ ................................ ................................ ....... 8 6.4 operating the sensor at temperature extremes ................................ ................................ ................................ ................................ . 9 7. sensing characteristics ................................ ................................ ................................ ................................ ................................ .............. 10 7.1 sensitivity ................................ ................................ ................................ ................................ ................................ .......................... 10 7.2 response and recovery time ................................ ................................ ................................ ................................ .......................... 12 7.3 cross - sensitivity ................................ ................................ ................................ ................................ ................................ ................ 13 8. maximum esd ratings ................................ ................................ ................................ ................................ ................................ .............. 14 9. mechanical stress testing ................................ ................................ ................................ ................................ ................................ ......... 14 10. package drawing and d imensions ................................ ................................ ................................ ................................ ............................. 15 11. applications and use conditions ................................ ................................ ................................ ................................ ................................ 16 12. ordering information ................................ ................................ ................................ ................................ ................................ ................... 16 13. revision history ................................ ................................ ................................ ................................ ................................ .......................... 16 list of figures figure 1. product photo ................................ ................................ ................................ ................................ ................................ ...................... 1 figure 2. pin assignments for sgas701 C top view ................................ ................................ ................................ ................................ ......... 4 figure 3. typical sensor response characteristic ................................ ................................ ................................ ................................ ............. 6 figure 4. basic measurement circuit ................................ ................................ ................................ ................................ ................................ .. 6 figure 5. three - terminal voltage regulator ................................ ................................ ................................ ................................ ...................... 7 figure 6. voltage - controlled constant current circuit ................................ ................................ ................................ ................................ ....... 8 figure 7. recommended applied heater voltage as a function of environmental temperature ................................ ................................ ...... 9 figure 8. typical sensor response to a range of hydrogen concentrations in a background of 30% rh at room temperature ................ 10 figure 9. typical sensor sensitivity to a range of hydrogen concentrations in a background of 30% rh at room temperature ................ 11 figure 10. typical sensor response to step changes in hydrogen concentration for four sgas701 sensors ................................ .............. 12 figure 11. typical sensor response to other common gases ................................ ................................ ................................ .......................... 13 figure 12 . to - 39 package (to4) outline drawing psc - 4676 ................................ ................................ ................................ ........................... 15
sgas701 datasheet ? 2017 integrated device technology, inc . 3 n ovember 16 , 2017 list of tables table 1. pin descriptions ................................ ................................ ................................ ................................ ................................ ................... 4 table 2. electrical specifications ................................ ................................ ................................ ................................ ................................ ....... 4 table 3. temperat ure specifications ................................ ................................ ................................ ................................ ................................ . 5 table 4. maximum esd ratings ................................ ................................ ................................ ................................ ................................ ..... 14 table 5. mechanical stress test conditions ................................ ................................ ................................ ................................ ................... 14
sgas701 datasheet ? 2017 integrated device technology, inc . 4 n ovember 16 , 2017 1. pin assignments figure 2 . pin assignments for sgas701 2 top view 2. pin descriptions table 1 . pin descriptions pin number name description 1 heater + positive input for v h heater voltage supply 2 sensor + high - side of resistive sensor element; positive input for sensing voltage v c 3 heater negative (ground) input for v h heater voltage supply 4 sensor low - side of resistive sensor element; connects to middle of resistor divider circuit to produce sensing voltage output (v out ) 3. sensor specifications note: all measurements were made in dry gas at room temperature. specifications are subject to change. table 2 . electrical specifications symbol parameter conditions minimum typical maximum units p h heater p ower c onsumption v h = 5.4 v 600 mw v h recommended h eater v oltage t sensor = 240 ? p i n 2 p i n 3 p i n 1 p i n 4 t a b
sgas701 datasheet ? 2017 integrated device technology, inc . 5 n ovember 16 , 2017 table 3 . temperature specifications symbol parameter conditions minimum typical maximum units t op sensor operation temperature v h = 5 . 4 v 240 c t amb recommended environmental temperature range - 20 50 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 ,'7?vvrolg - state chemiresistive sensors are an advanced type of gas - sensitive resistor; i.e. t hey sense the presence 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 chemiresi stive sensors show a reduced resistance with increasing gas concentration according to equation 1 : r s = a ? c - equation 1 where r s c is concentration, and a ? 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 resistance will change rapidly at low concentrations and much less at high concentrations. this is illustrated in the following example: r gas_10ppm r gas_100ppm a gas ? air
sgas701 datasheet ? 2017 integrated device technology, inc . 6 n ovember 16 , 2017 figure 3 . typical sensor response characteristic the electronic instrumentation used to detect this change in resistance influences the q uality 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 selected with care. for additional infor pdwlrqvhh,'7?v 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 circu it 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 measurement circuit. idt supplies basic measurement circuitry for many of our sensors. m ruhlqirupdwlrqfdqehirxqglq,'7?v application note C resist ance 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
sgas701 datasheet ? 2017 integrated device technology, inc . 7 n ovember 16 , 2017 6. heater driver circuits and control th e sgas701 sensor contains a resistive element that is used to heat the sensor to the target operating temperature as shown in table 3 . the sgas7 0 1 sensor uses a thermistor heater element with a positive temperature coefficient, i.e. the heater resistance increases from the cold (room temperature) resistance as power is applie d. this provides the opportunity for constant power and constant resistance (closely related to constant temperature) control of the heater. 6.1 constant voltage drive the simplest method of applying heater power is the use of a constant voltage 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 consequently gas sensitiv ity), voltage regulation is required. an easily implemented c ontrol circuit utilizes a three - terminal voltage regulator, with the lm317 serving as an example as shown in figure 5 . figure 5 . three - terminal voltage regulator r1 and r2 (one of these can be a potentiometer) are selected to provide the target heater drive voltage for the sensor. the example using the lm317 is capable of regulating voltages down to 1.25v and is thus suitable for sgas7 0 1 sensors. however, a wide variety of more advanced three - terminal voltage regulators is available from component manufacturers. constant voltage circuits of this type are relatively efficient, particularly if a switching regulator is used. adding extern al control of the regulator output voltage with a curr ent sensing resistor would allow feedback control of the sensor heater power and temperature, but the required circuitry is somewhat complex. applications requiring feedback control are better implemented with the constant current circuit that is described in section 6.2 . 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 ? r 2 / r 1 ) + i a d j ? r 2
sgas701 datasheet ? 2017 integrated device technology, inc . 8 n ovember 16 , 2017 6.2 constant current drive the constant current drive is more complex and costly than the constant voltage drive, but the added capabi lities justify the expense for many dssolfdwlrqv$gglwlrqdoo\wkhflufxlwlv3plfurfrqwuroohuiulhqgo\ehfdxvhwkhkhdwhufxuuhqwlvgluhfwo\frqwuroodeohyld an input voltage signal, and feedback control of the heater is possible through a simple measu rement of the resultant voltage on the heater. the constant current heater drive circuit is shown in figure 6 . v in (supplied by an external 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 current through both r3 and r heater is thus controlled 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 compa red to others, as power is dissipated in r3 and q2 as well as the heater. limiting the supply voltage to several hundred mv above the highest required drive voltage will help incr ease 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 microcontroller via a digital - to - analog converter (dac). with this type of contro l, the heater drive can be time - pr ogrammed to allow pulsing of the heater with variable amplitude. determination of the heater power or resistance is possible by reading the voltage level at the heater. since the heater temperature directly correlates with heater resistance, direct feedbac k control of bulk heater temperature is possible by raising or lowering the heater current such that ( v heater / i heater ) is constant. likewise, constant heater power can be maintained by controlling current to keep ( v heater ? i heater ) constant. figure 6 . voltage - controlled constant current circuit 6.3 pulse - width modulation pulse - width modulation (pwm) is a very efficient method of providing controllable drive to the heater. 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 shoul d 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 )
sgas701 datasheet ? 2017 integrated device technology, inc . 9 n ovember 16 , 2017 6.4 operating the sensor at temperature extremes when this sensor is used outdoors, the relative response of the sensor to the target gases will be a function of environmenta l temperature when the sensor is operated with a constant 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 thermodynam ics of the interaction of the sensing surface with flammable g ases. this alters the electrical conduction of the sensor element (the basis of metal - oxide sensor operation). recommendation: in these cases, operate the sensor in a feedback loop, where constant resistance at the heater is maintained. for operation in am bient conditions above t amb = 50c, additional compensation of the sensor signal may be required and should be developed by the user with the specific application and operating conditions in mind. a graphical representa tion of the recommended tempera ture s et - point voltage versus environmental temperature is shown in figure 7 . the mathematical description for the curve is given in equation 3: v h = - 0.01 ? equation 3 figure 7 . recommended applied heater voltage as a function of environmental temperature 4.8 4.9 5.0 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 -30 -20 -10 0 10 20 30 40 50 60 70 applied heater voltage [v] environmental temperature [ c]
sgas701 datasheet ? 2017 integrated device technology, inc . 10 n ovember 16 , 2017 7. sensing characteristics the following graphs show the typical responses that are to be expected from the sensors on exposure to a variety of test conditi ons. for sgas701 sensor specifications, refer to table 4 . 7.1 sensitivity the typical response and sensitivity of the sgas701 sensor to hydrogen is shown in figure 8 and figure 9 . sensitivity is defined as the ratio between the resistance in air (r air ) and the resistance in gas (r gas ). figure 8 . typical sensor response to a range of hydrogen c oncentrations in a background of 30% rh at room temperature 1e+03 1e+04 1e+05 1e+06 1e+07 10 100 1000 sensor signal [ohm] concentration [ppm] air hydrogen
sgas701 datasheet ? 2017 integrated device technology, inc . 11 n ovember 16 , 2017 figure 9 . typical sensor sensitivity to a range of hydrogen concentrations in a background of 30% rh at room temperature 1 10 100 1000 10 100 1000 sensitivity [r air /r gas ] concentration [ppm]
sgas701 datasheet ? 2017 integrated device technology, inc . 12 n ovember 16 , 2017 7.2 response and recovery time t he typical response and recovery time of a sampling of seven sgas701 sensors is shown in figure 10 . the response and recovery time can be strongly aff ected by the sensor location within the system, including any barriers to diffusion of gas to and from the sensor surface, an d whether the sensor is operated with or without a pump or other source of external flow . figure 10 . typic al sensor response to step changes in hydrogen concentration for four sgas701 sensors 1e+03 1e+04 1e+05 1e+06 1e+07 0 5 10 15 20 25 30 sensor signal [ohm] time [min] air 50ppm 100ppm 250ppm 490ppm air
sgas701 datasheet ? 2017 integrated device technology, inc . 13 n ovember 16 , 2017 7.3 cross - sensitivity the response of the sgas701 sensors to a range of other co mmon gases is shown in figure 11 . figure 11 . typical sensor response to other common gases 1e+03 1e+04 1e+05 1e+06 1e+07 1 10 100 sensor signal [ohm] concentration [ppm] air acetone ethanol formaldehyde isobutylene octane r410a toluene xylenes
sgas701 datasheet ? 2017 integrated device technology, inc . 14 n ovember 16 , 2017 8. maximum esd ratings table 4 . maximum esd ratings symbol parameter conditions minimum maximum units v hbm1 electrostatic discharge tolerance human body model (hbm1) 2000 v v cdm electrostatic discharge tolerance charged device model (cdm) on packaged module 500 v 9. mechanical stress testing the qualification of the sgas701 is based on the jedec standard (jesd47). after subjection to the mechanical shock and vibration testing conditions given in table 5 the sgas701 sensor will meet the specifications given in this document. for information on constant acceleration test conditions and limits, contact idt (see contact informa tion 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
sgas701 datasheet ? 2017 integrated device technology, inc . 15 n ovember 16 , 2017 10. package drawing and dimensions figure 12 . to - 39 package (to4) outline drawing psc - 4676
sgas701 datasheet ? 2017 integrated device technology, inc . 16 n ovember 16 , 2017 11. applications and use conditions the sgas70 1 sensor is designed for gas leak detection and measurement of ppm levels of hydrogen. the sensor is not intended, recom - mended , or approved for use in safety or life protecting applications or in potentially explosive environments . idt disclaims all liabi lity for such use. 12. ordering information orderable part number package msl rating shipping packaging temperature sgas701 4 - pin to - 39 (to4) 1 tray - 20c to 50c SMOD701KITV1 smod701 evaluation kit, including the smod701 smart sensing module (includes the sgas701 sensor), mini - usb cable, and wall - mounted 9v power supply. the smod7xx application software is available for download at www .idt.com/smod701 . 13. revision history revision date description of change november 16, 2016 m inor correction. october 2 5 , 2017 full r evision . november 9 , 2016 initial release with 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 ',6&/$,0(5,qwhjudwhg'hylfh7hfkqrorj\,qf,'7dqglwvdiiloldwhgfrpsdqlhvkhuhlquhihuuhgwrdv3,'7uhvhuyhwkhul ght to modify the products and/or specifications described herein at any time, without notice, at idt's sole discretion. performanc e specifications and operating parameters of the described products are determined in an independent state and are not guaran teed to perform the same way when installed in customer products. the information contained herein is provided without representati on or warranty of any kind, whether express or implied, including, but not limited to, the suitability of idt's products for any particular purpose, an implied warranty of merchantability, 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 malfunction of an idt product can be reasonably expected to significantly affect the health or safety of users. anyone using an idt product in such a manner does so at their own risk, absent an expres s, 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 res pective 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 r ights reserved.

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