Trace Hydrogen (H2) Sensor
SGAS701
Datasheet
© 2017 Integrated Device Technology, Inc.
1
November 16, 2017
Description
The IDT SGAS701 is a solid-state chemiresistor sensor designed
to detect hydrogen in air. The sensor uses an integrated heater with
highly sensitive MOx material tailored for detection of hydrogen.
The chemiresistor sensors of IDT’s SGAS family are based upon
the principle that metal oxide materials undergo surface inter-
actions (physisorption and chemisorption) with gas molecules 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 sensing materials with
excellent sensitivity and stability.
Figure 1. Product Photo
Features
High sensitivity to low hydrogen concentrations
(<10 to 1000 ppm)
Fast response time (<15 seconds at 100ppm)
Environmental temperature range of -20°C to 50°C
Environmental humidity range of 0% to 90% RH,
noncondensing
Low dependence on flow rate
Rugged, reliable sensor based on IDT’s exclusive technology
Typical Applications
Leak Detection
Gas Concentration Detection
Breath Detection
Available Support
Evaluation Kit – SMOD701KITV1
Application Notes
Instruction Videos
Reference Design
SGAS701 Datasheet
© 2017 Integrated Device Technology, Inc.
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November 16, 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 ..........................................................................................................................................................12
7.3 Cross-Sensitivity ................................................................................................................................................................................13
8. Maximum ESD Ratings ..............................................................................................................................................................................14
9. Mechanical Stress Testing .........................................................................................................................................................................14
10. Package Drawing and Dimensions .............................................................................................................................................................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 – 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.
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November 16, 2017
List of Tables
Table 1. Pin Descriptions ...................................................................................................................................................................................4
Table 2. Electrical Specifications .......................................................................................................................................................................4
Table 3. Temperature Specifications .................................................................................................................................................................5
Table 4. Maximum ESD Ratings .....................................................................................................................................................................14
Table 5. Mechanical Stress Test Conditions ...................................................................................................................................................14
SGAS701 Datasheet
© 2017 Integrated Device Technology, Inc.
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November 16, 2017
1. Pin Assignments
Figure 2. Pin Assignments for SGAS701 – Top View
Pin 2 Pin 3
Pin 1 Pin 4
Tab
2. Pin Descriptions
Table 1. Pin Descriptions
Name
Description
Heater +
Positive input for VH heater voltage supply
Sensor +
High-side of resistive sensor element; positive input for sensing voltage VC
Heater –
Negative (ground) input for VH heater voltage supply
Sensor –
Low-side of resistive sensor element; connects to middle of resistor divider circuit to produce
sensing voltage output (VOUT)
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
PH
Heater power consumption
VH = 5.4V
600
mW
VH
Recommended heater voltage
TSENSOR = 240C
5.4
VDC
RH
Heater resistance
At room temperature
28
30
32
Ω
VC
Recommended sensing voltage
2.5
5.0
VDC
R10
Resistance in 10ppm H2
30
3000
kΩ
R100
Resistance in 100ppm H2
10
1000
kΩ
R50 / R100
Resolution: Resistance in
50 ppm / Resistance in 100 ppm
> 1.2
SGAS701 Datasheet
© 2017 Integrated Device Technology, Inc.
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November 16, 2017
Table 3. Temperature Specifications
Symbol
Parameter
Conditions
Minimum
Typical
Maximum
Units
TOP
Sensor Operation Temperature
VH = 5.4V
240
°C
TAMB
Recommended Environmental
Temperature Range
-20
50
°C
TSTOR
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
IDT’s solid-state chemiresistive sensors 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 concentration 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:
RS = A
C-α
Equation 1
where RS is resistance, C is concentration, and A and 𝛼 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 IDT’s n-type sensors follows
Equation 1. Taking the log of both sides of the equation results in Equation 2:
log (RS)
=
log(A)–
α
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:
RGAS_10ppm = 20kΩ
RGAS_100ppm = 5kΩ
AGAS
= 8.0 ∗104
αair = 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 nonlinear effects from the measurement
circuitry exacerbate these challenges and must be understood in order to account for or eliminate these effects.
SGAS701 Datasheet
© 2017 Integrated Device Technology, Inc.
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Figure 3. Typical Sensor Response Characteristic
The electronic instrumentation used to detect this 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 selected with care. For additional information, see IDT’s Application Note – 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 (VH) and circuit voltage (VC).
VH is applied to the heater in order to maintain a constant, elevated temperature for optimum sensing. VC is applied to allow a measurement of
the output voltage (VOUT) across a load resistor (RL).
Figure 4. Basic Measurement Circuit
VOUT
VH
VC
RL
GND
Sensor
(Pin 1) (Pin 2)
(Pin 3) (Pin 4)
RS
RH
Pins 1 and 3 are attached to the heater. Apply VH 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. More information can be found in IDT’s
Application Note – 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
020 40 60 80 100 120
Sensor Signal [Ohm]
Gas Concentration [ppm]
SGAS701 Datasheet
© 2017 Integrated Device Technology, Inc.
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November 16, 2017
6. Heater Driver Circuits and Control
The SGAS701 sensor contains a resistive element that is used to heat the sensor to the target operating temperature as shown in Table 3. The
SGAS701 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 applied. 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 sensitivity), 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
LM317
R1
R2
0.1µF 10µF
VSUPPLY VHEATER
VHEATER = 1.25V (1 + R2/R1) + IADJ R2
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 SGAS701 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 external control of the regulator
output voltage with a current 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.
SGAS701 Datasheet
© 2017 Integrated Device Technology, Inc.
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November 16, 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. VIN (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 VDD) across R2. An equivalent drop is imposed across
R3, and current through both R3 and RHEATER 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 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 VIN can be supplied by a fixed voltage reference (such as a divider), the flexibility of the circuit is most revealed when VIN is supplied by
a microcontroller via a digital-to-analog converter (DAC). With this type of control, the heater drive can be time-programmed 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 feedback control of bulk heater temperature is possible by raising or
lowering the heater current such that (VHEATER / IHEATER) is constant. Likewise, constant heater power can be maintained by controlling current
to keep (VHEATER
IHEATER) constant.
Figure 6. Voltage-Controlled Constant Current Circuit
VIN
VDD
R1
R2
R3
U1a
U1b
Q1
Q2
RHEATER
iHEATER = VIN
R2 / ( R1
R3 )
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 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.
SGAS701 Datasheet
© 2017 Integrated Device Technology, Inc.
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November 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 environmental 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 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). Recommendation: In these cases, operate the sensor in a feedback loop, where constant resistance at the heater is
maintained. For operation in ambient conditions above TAMB = 50°C, 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 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:
VH = -0.01 Environmental Temperature [°C] + 5.5
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.
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November 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 conditions. 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 (RAir) and the resistance in gas (RGas).
Figure 8. Typical Sensor Response to a Range of Hydrogen Concentrations 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.
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November 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 [RAir/RGas]
Concentration [ppm]
SGAS701 Datasheet
© 2017 Integrated Device Technology, Inc.
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November 16, 2017
7.2 Response and Recovery Time
The 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 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 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.
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November 16, 2017
7.3 Cross-Sensitivity
The response of the SGAS701 sensors to a range of other common 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
110 100
Sensor Signal [Ohm]
Concentration [ppm]
Air
Acetone
Ethanol
Formaldehyde
Isobutylene
Octane
R410a
Toluene
Xylenes
SGAS701 Datasheet
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November 16, 2017
8. Maximum ESD Ratings
Table 4. Maximum ESD Ratings
Symbol
Parameter
Conditions
Minimum
Maximum
Units
VHBM1
Electrostatic Discharge Tolerance – Human Body Model (HBM1)
2000
–
V
VCDM
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 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 2kHz (log variation) in > 4 minutes, 4 times in
each orientation, 50g peak acceleration
SGAS701 Datasheet
© 2017 Integrated Device Technology, Inc.
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November 16, 2017
10. Package Drawing and Dimensions
Figure 12. TO-39 Package (TO4) Outline Drawing PSC-4676
SGAS701 Datasheet
© 2017 Integrated Device Technology, Inc.
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November 16, 2017
11. Applications and Use Conditions
The SGAS701 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 liability for such
use.
12. Ordering Information
Orderable Part Number
Package
MSL Rating
Shipping Packaging
Temperature
SGAS701
4-pin TO-39 (TO4)
1
Tray
-20°C to 50°C
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
Minor correction.
October 25, 2017
Full revision.
November 9, 2016
Initial release with IDT branding.
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1-800-345-7015 or 408-284-8200
Fax: 408-284-2775
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Tech Support
www.IDT.com/go/support
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