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flammable gas sensor sgas711 datasheet ? 2017 integrated device technology, inc . 1 october 24, 2017 description the idt sgas711 is a solid - state chemiresistor sensor designed to detect flammable gases in air. the sgas711 sensor uses an integrated heater with highly sensitive mox material tailored for detection of flammable gases, such as methane, propane, hydrogen, liquefied petroleum gas (lpg), and more. in addition to detecting flammable gases, the sensor is also sensitive to other hydrocarbons, such as pentane and r410a, and it can be used for leak detection. the chemiresistor sensors in the idt sgas family are based on the principle that metal - oxide materials undergo surface inter - actions (physisorption and chemisorption) with gas molecules at elevated temperatures, resulting i n 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 electronic effects on the individual grains. these gas - soli d 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 develo ped a set of nanostructured gas - sensing materials with excellent sens itivity and stability. figure 1 . product photo features ? high sensitivity to a wide range of flammable gases ? non - specific response; capable of being calibrated to detect a wide range of flammable gases ? long lifetime (several yea rs in typical applications) ? typical response time < 30 seconds to 90% of full scale ? environmental temperature range : - 20c to 50c ? minimal response to environmental humidity over the range of 0% to 95%, non - condensing ? rugged, reliable sensor based on idt s exclusive technology ? to - 39 package typical applications ? leak detection ? gas concentration detection ? process control examples of t arget gases ? hydroc arbons (hc) ? hydrogen ? liquid petroleum gas (lpg) ? methane ? natural gas ? propane ? pentane ? r410a available support ? evaluation kit C smod711 smart sensing module ? application notes ? instruction videos ? reference d esigns
sgas711 datasheet ? 2017 integrated device technology, inc . 2 october 24, 2017 contents 1. pin assignments ................................ ................................ ................................ ................................ ................................ ........................... 4 2. pin descriptions ................................ ................................ ................................ ................................ ................................ ............................ 4 3. sensor specifications ................................ ................................ ................................ ................................ ................................ ................... 4 4. sensor characteristics ................................ ................................ ................................ ................................ ................................ ................. 5 5. basic measurement circuit ................................ ................................ ................................ ................................ ................................ ........... 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 ................................ ................................ ................................ ................................ .......................... 11 7.3 cross - sensitivity ................................ ................................ ................................ ................................ ................................ ................ 12 8. maximum esd ratings ................................ ................................ ................................ ................................ ................................ .............. 13 9. mechanical stress testing ................................ ................................ ................................ ................................ ................................ ......... 13 10. package drawing and d imensions ................................ ................................ ................................ ................................ ............................. 14 11. applications and use conditions ................................ ................................ ................................ ................................ ................................ 15 12. ordering information ................................ ................................ ................................ ................................ ................................ ................... 15 13. revision history ................................ ................................ ................................ ................................ ................................ .......................... 15 list of figures figure 1. product photo ................................ ................................ ................................ ................................ ................................ ...................... 1 figure 2. pin assignments for sgas711 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. sensor response to a variety of flammable gases ................................ ................................ ................................ ......................... 10 figure 9. effect of different humidity levels on sensor signal at ambient temperature ................................ ................................ ................ 11 figure 10. typical sensor response to step changes in methane concentration for seven sgas711 sensors ................................ ............ 11 figure 11. typical sensor response to other common gases ................................ ................................ ................................ .......................... 12 figure 12. to - 39 package (to4) outline drawing psc - 4676 ................................ ................................ ................................ ........................... 14
sgas711 datasheet ? 2017 integrated device technology, inc . 3 october 24, 2017 list of tables table 1. pin descriptions ................................ ................................ ................................ ................................ ................................ ................... 4 table 2. electrical specification s ................................ ................................ ................................ ................................ ................................ ....... 4 table 3. temperature specifications ................................ ................................ ................................ ................................ ................................ . 5 table 4. maximum esd ratings ................................ ................................ ................................ ................................ ................................ ..... 13 table 5. mechanical stress test conditions ................................ ................................ ................................ ................................ ................... 13
sgas711 datasheet ? 2017 integrated device technology, inc . 4 october 24, 2017 1. pin assignments figure 2 . pin assignments for sgas711 C top view 2. pin descriptions table 1 . pin descriptions note: see figure 4 for the connections described below. 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 C negative input for v h heater voltage supply (ground) 4 sensor C low - side of resistive sensor element; connects to middle of resistor divider circuit to p r oduce sensing voltage output (v out ) 3. sensor specifications note: a ll measurements were made i n dry gas at room temperature. specifications are subject to change. table 2 . electrical specification s symbol parameter conditions minimum typical maximum units p h h eate r power consumption v h = 7.0v 900 mw v h recommended heater voltage t sensor = 300 ? c 7.0 vdc r h heater resistance at room temperature 28 30 32 ? v c recommended sensing voltage 2.5 5.0 vdc r 10 00 resistance at 1000ppm methane (ch 4 ) 10 1000 k? r 1000 /r 2500 resolution : resistance in 100 0 ppm/ resistance in 25 00 ppm 1.2 t a b p i n 4 p i n 3 p i n 2 p i n 1
sgas711 datasheet ? 2017 integrated device technology, inc . 5 october 24, 2017 table 3 . temperature specification s symbol parameter conditions minimum typical maximum units t op sensor operation temperature v h = 7.0v 300 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 environm ental temperature specification , but exposure for short durations will not will not harm the sensor. 4. sensor characteristics idts solid - state chemiresistive sens ors are an advanced type of gas - sensitive resistor; i.e. they sense the presence of a target gas through a change in resistance of the sensing element. most sensors exhibit reduced resistance as gas con centration increases, typically over several orders of magnitude across the sensing range. solid - state chemiresistive sensors show a reduced resistance with increasing gas concentration according to equation 1 : r s = a ? c - equation 1 where r s is resistance, ? 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 concentrati on relationship for all of idts n - type sensors follows equation 1 . taking the log of both 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 resistance will chang e rapidly at low concentrations and much less at high concentrations. this is illustrated in the following example: r gas_10ppm = 20k r gas_100p pm = 5k a gas = 8.0 ? 10 4 air = 0.602 th e 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 from measurement circui try exacerbate these challenges and must be understood in order to account for or eliminate these effects.
sgas711 datasheet ? 2017 integrated device technology, inc . 6 october 24, 2017 figu re 3 . typical sensor response characteristic the electronic instrumentation used to detect this change in resistance influences the quality and accuracy of the gas sensin g result. in particular, the choice of the analog front - en d used to measure resistance can ultimately have a significant effect on overall measurement characteristics and must be selected 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: 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 s ensing. 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 measurement 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
sgas711 datasheet ? 2017 integrated device technology, inc . 7 october 24, 2017 6. heater driver circuits and control this sensor contains a resistive element that is used to heat the sensor to the target operating temperature as shown in table 3 . the sgas711 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 appl ied. 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 . becau se heaters draw a relatively large amount of current in normal operation, some method of current amplification is required. additionally, because relatively small changes in voltage levels will affect the temperature of the heater (and consequently gas sen sitivity), voltage regulation is required. an easily implemented control 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 which can be a potentiometer) are selected to provide the target heater drive voltage for the sensor. the example for the lm317 is capable of re gulating voltages down to 1.25v and is thus suitable for sgas711 sensors. however, a wide variety of more advanced three terminal voltage regulators is available from component manufacturers . constant voltage c ircuits of this type are relatively efficient, particularly if a switching regulator is used. a dding external control of the regulator output voltage with a current sensing resistor would allow feed back control of the sensor heater power and temperature, but the required circuit ry 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 ? ( 1 + r 2 / r 1 ) + i a d j ? r 2
sgas711 datasheet ? 2017 integrated device technology, inc . 8 october 24, 2017 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 expense for many applications. additionally, the circuit is microcontroller friendly because the heater current is directly controllable via an input voltage signal, and feedback control of the heater is possible through a simple measurement 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 cur rent through both r1 and r2, along with a predictable voltage drop (relative to v dd ) across r2. an equivalent drop is imp osed across r3, and the current through both r3 and r heater is thus controlled independently of the load resistance according to the equation in figure 6 . the h eater curre nt is controllable to below 1ma . howev er, the circuit is inefficient compared 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 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 micro controller via a digital - to - analog converter (dac) . with this type of control, the heater drive can be time programme d to allow pulsing of the heater with variable amplitude. determination of heater power or resistance is possible by reading the voltage level at the heater. since the heater temperature directly correlates with heater resistance, direct feedback control o f 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 te sting at idt to allow us 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 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 )
sgas711 datasheet ? 2017 integrated device technology, inc . 9 october 24, 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 environme ntal 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 tu rn altering the kinetics and thermodynamics of the interaction of the sensing surface with flammable gases. this alters the electrical conduction of the sensor element (the basis of metal - oxide sensor operation). in these cases, it is recommended to operat e the sensor in a feedback loop, where constant resistance at the heater is maintained. for operation in ambient conditions above t amb = 50 c, additional compensation of th e sensor signal may be required and should be developed by the user with the specifi c application and operating conditions in mind. a graphical representation of the recommended temperature set 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 ? environmental temperature [c] + 7.2 equation 3 figure 7 . recommended applied heater voltage as a function of environmental temperature 6.5 6.6 6.7 6.8 6.9 7.0 7.1 7.2 7.3 7.4 7.5 -30 -20 -10 0 10 20 30 40 50 60 70 applied heater voltage [v] environmental temperature [ c]
sgas711 datasheet ? 2017 integrated device technology, inc . 10 october 24, 2017 7. se nsing characteristics the following graphs show the typical response s that are to be expected from the sensors upon exposure to a variety of test conditions. for sensor specifications, refer to table 2 . 7.1 sensitivity the typical sensitivity of the sgas711 sensor to a range of flammable gases is shown in figure 8 . sensitivity is defined as the ratio between th e resistance in air (r air ) a nd the resistance in ga s (r gas ) . figure 8 . sensor response to a variety of flammable gases
sgas711 datasheet ? 2017 integrated device technology, inc . 11 october 24, 2017 the typical response of the sensor to changes in humidity is shown in figure 9 . figure 9 . effect of different humidity levels on sensor signal at ambient temperature 7.2 response and recovery time t he typical response and recovery time of seven sensors is shown in figure 10 . the response and recovery time can be strongly affected by the sensor location within the system, including any barriers to diffusion of gas to and from the sensor surface and whether the sensor is operated with or without a pump or other source of external flow. figure 10 . typical sensor response to step changes in methane concentration fo r seven sgas711 sensors 1e+05 1e+06 1e+07 0 300 600 900 1200 1500 sensor signal [ohm] time [s] air 10ppm 50ppm 100ppm air 1e+04 1e+05 1e+06 1e+07 1000 10000 sensor signal [ohm] . concentration [ppm]
sgas711 datasheet ? 2017 integrated device technology, inc . 12 october 24, 2017 7.3 cross - sensitivity the response of the sgas711 sensors to a range of other common gases is shown in figure 11 . figure 11 . typical sensor response to other common gases 1e+04 1e+05 1e+06 1e+07 1 10 100 1000 sensor signal [ohm] concentration [ppm] air co nh3 formaldehyde xylenes
sgas711 datasheet ? 2017 integrated device technology, inc . 13 october 24, 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 sgas711 is based on the jedec standard (jesd47). after subjection to the mechanical shock and vibration testing conditions given in table 5 , t he sgas711 sensor will meet the specifications given in this document . for information on constant accelerat ion test conditions and limits, 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, 0.5ms duration, 1500 g peak acceleration vibration variable frequency jesd22 - b103, m2007 20 hz to 2 k hz (log variation) in > 4 minutes, 4 tim es in each orientation, 50g peak acceleration
sgas711 datasheet ? 2017 integrated device technology, inc . 14 october 24, 2017 10. package drawing and dimensions figure 12 . to - 39 package (to4) outline drawing psc - 4676
sgas711 datasheet ? 2017 integrated device technology, inc . 15 october 24, 2017 11. applications and use conditions the sgas711 sensor is designed for gas leak detection and measurement of ppm levels of flammable gases. the sensor is not intended, recommended , or app roved for use in safety or life - protecting applications or in potentially explosive environments . idt disclaims all liability for such use. for sensor storage , idt strongly recommends a dust and voc free atmosphere, e.g. in synthetic air. 12. ordering information orderable part number description and package msl rating shipping packaging temperature sgas711 4 - pin to - 39 (to4) 1 tray - 20c to +50c SMOD711KITV1 smod711 evaluation kit, including the smod711 smart sensing module (includes the sgas711 sensor) , mini - usb cable, and wall - mounted 9v power supply. the smod7xx application software is available for download at www.idt.com/smod711 . 13. revision history revision date description of change october 2 4 , 2017 full revision. november 4 , 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. 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 suppor t 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 express, wri tten 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 respectiv e 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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