Preliminary Technical Data
CN-0429
Devices Connected/Referenced
Circuits from the Lab® reference designs are engineered and
tested for quick and easy system integration to help solve today’s
analog, mixed-signal, and RF design challenges. For more
information and/or support, visit
www.analog.com/CN0429.
Precision Analog Microcontroller with
Chemical Sensor Interface
ADuCM355
Electrochemical Gas Measurement System with Sensor Diagnostics
EVALUATION AND DESIGN SUPPORT
Circuit Evaluation Boards
Electrochemical Gas Sensor Board (EVAL-CN0429-EBZ)
Arduino Shield Interface Board (EVAL-M355-ARDZ-INT)
ADICUP3029 Arduino Form Factor Ultra Low Power ARM
Cortex-M3 Development Platform (EVAL-ADICUP3029)
Design and Integration Files
Schematics, Layout Files, Bill of Materials, Software
CIRCUIT FUNCTION AND BENEFITS
Gas detection instruments are used in a wide range of
applications ranging from home air quality measurement
devices to industrial solutions for detecting toxic gases. Many of
these instruments use electrochemical gas sensors. This sensor
technology requires specialized front-end circuitry for biasing
and measurement.
By utilizing built-in diagnostics features (such as impedance
spectroscopy or bias voltage pulsing and ramping) it is possible
to inspect sensor health, compensate for accuracy drift due to
aging or temperature, and estimate the remaining lifetime of the
sensor right at the edge of the sensor network without user
intervention. This functionality allows smart, accurate sensor
replacement at the individual edge nodes. An integrated, ultra
low power microcontroller directly biases the electrochemical
gas sensor and runs onboard diagnostic algorithms.
The circuit shown in Figure 1 shows how an electrochemical
gas sensor is connected to the potentiostat circuit and how it is
biased and measured. Common 2-lead, 3-lead, and 4-lead
electrochemical gas sensors can be used interchangeably. The
integration of this signal chain dramatically reduces cost, size,
complexity, and power consumption at the sensor node.
Rev. PrA
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CN-0429
EC SENSOR CHANNEL 0
DNI
C2
0.1µF
M1
4
2
CO-A4
DGND
DE0
1
E5
2
Preliminary Technical Data
C4
30pF
DGND
SE0
E3
CE0
1
2
1
CE
AE
WE
RE
3
600Ω AT 100MHz
CAP_POT0
C1
0.1µF
600Ω AT 100MHz
AVDD
R1
100kΩ
3
G
1D
Q1
2 S MMBFJ270
E4
1
2
600Ω AT 100MHz
C3
30pF
DGND
RE0
VBIAS0 VZERO0
SW[15]
RE0
SW[2]
CE0
SW[8]
SW[10]
PA
–
SW[14]
SW[12]
+
VREF2V5
AIN4/LPF0
SW[13]
LPDAC0
ULPBUF
SW[3]
CAP_POT0
10kΩ
RE0
ULPREF
TO
CHANNEL 1
10kΩ
SW[6]
+
SW[7]
–
R
TIA
LPTIACON0
[9:5]
SW[0]
FORCE/
SENSE
TIA0
SW[5]
SW[9]
LPTIACON0
[15:13]
R
LPF
ADC
SW[4]
SW[11] LPTIACON0
[12:10]
SE0
R
LOAD
SW[1]
MUX
RC00
RC00
RC01
17302-001
Figure 1. Simplified Circuit Block Diagram
CIRCUIT DESCRIPTION
Fundamentals of Electrochemical Gas Sensors
The fundamental principle of electrochemical gas sensing is
measuring the current generated due to oxidation or reduction
at an electrode in response to a target gas. Most common
sensors have two or three electrodes. Some sensors utilize an
additional fourth electrode. In 3-electrode configuration, the
electrodes are referred to as a working electrode (WE), also
known as sensing electrode (SE), a reference electrode (RE),
and a counter electrode (CE). Figure 2 shows simplified
diagram of such an electrochemical cell.
POROUS
WORKING
ELECTRODE
MEASURED
GAS
ELECTROLYTE
COUNTER
ELECTRODE
REFERENCE
ELECTRODE
Figure 2. Electrochemical Gas Sensor—Simplified
Rev. PrA | Page 2 of 7
17302-002
Preliminary Technical Data
The target gas enters the sensor chamber through the porous
working electrode and diffuses into the electrolyte (most
commonly acid), where it is either oxidized or reduced. Electric
current generated by this reaction is then sensed and converted
to a corresponding voltage level by the external potentiostat
circuit. A continuous or pulsed bias voltage is often required to
be applied across sensor electrodes to ensure optimal performance.
In the case of a 3-electrode sensor, the bias voltage is applied
between the RE and WE. An equal but opposite reaction to the
one occurring between the RE and WE occurs at the CE. If
reduction occurs at the WE, oxidation occurs at the CE. Further
information about operation of the potentiostat circuit is
available in the
ADuCM355 Hardware Reference Manual.
CN-0429
Depending on the sensor type, bias voltage may also be
negative. The following equations explain how to configure the
DAC for both positive and negative bias voltages.
When the required bias voltage is positive, (12-bit output ≥
6-bit output),
V
VBIAS
= 0.2 V + (LPDACDAT[11:0] × 0.54 mV) + 0.54 mV
V
VZERO
= 0.2 V + (LPDACDAT[17:12] × 34.38 mV)
When the required bias voltage is negative (12-bit output <
6-bit output),
V
VBIAS
= 0.2 V + (LPDACDAT[11:0] × 0.54 mV)
V
VZERO
= 0.2 V + (LPDACDAT[17:12] × 34.38 mV)
where:
LPDACDAT
is the low-power DAC data-out control register.
0.54 mV is approximately 1 LSB of the 12-bit DAC.
34.38 mV is approximately 1 LSB of the 6-bit DAC.
The sensing/working electrode (WE) of the sensor is connected
to the LPTIAx via the inverting input pin SEx. The LPTIAx has
a programmable load resistor (R
LOAD
) and programmable gain
resistor (R
TIA
). The current flowing in/out of the sensors SE
electrode reflects the target gas in the atmosphere around the
sensor. The sensor datasheet reflects this in current/ppm. The
LPTIAx amplifier converts the current to a voltage that is then
buffered and measured via the analog-to-digital converter
(ADC). Select the R
TIA
resistor value so that it maximizes the
ADC input range of ±900 mV. The R
TIA
value is calculated using
following equation:
Electrochemical Gas Sensor Connections to ADuCM355
The data sheet for the gas sensor specifies a bias voltage
required for normal electrochemical behavior of the sensor. The
bias voltage is the voltage difference between the RE and the
SE/WE. This differential voltage is set by the outputs of a low
power, digital-to-analog converter (LPDACx). The LPDACx has
two outputs, an output with 12-bit resolution (VBIASx) and an
output with 6-bit resolution (VZEROx). The VBIASx output of
the LPDACx is internally connected to the noninverting
terminal of the power amplifier (PA). Externally, VBIASx must
be connected to the AGND pin via a 100 nF capacitor. The
output of the PA amplifier connects directly to the sensor’s CE.
The feedback to the inverting terminal of the PA amplifier is
from the RE pin of the sensor; therefore, the VBIASx voltage
determines the RE pin voltage.
The VZEROx output of the LPDACx is internally connected to
the noninverting terminal of the low power, transimpedance
amplifier, LPTIAx. Do not use this pin as a voltage source for an
external circuit. It is recommended to connect this pin to the
AGND pin.
The electrochemical gas sensor itself is only connected to the
ADuCM355
via the REx, CEx, and SEx terminals, respectively,
allowing the optional fourth terminal to be used for the
diagnostics electrode (DEx), as shown in Figure 1.
The effective sensor bias voltage is obtained using following
equation:
V
BIAS_EFF
=
V
VBIAS
−
V
VZERO
The VZERO voltage is recommended to be set to 1100 mV, and
the VBIAS voltage is then to be set with respect to the sensor
bias voltage value from the sensor data sheet.
R
TIA
½
0.9 V
Sensitivity
Max
_
Range
where:
0.9 V is the ADC input range.
Sensitivity
is defined as nA/ppm.
Max_Range
is the maximum range of the sensor in ppm.
The microcontroller can calculate the current flowing in/out of
the SEx pin and determine the ppm level of the target gas.
For more detailed information about the equations used, see the
ADuCM355 Hardware Reference Manual.
Rev. PrA | Page 3 of 7
CN-0429
Sensor Health Diagnostics and Life Expectancy
The lifespan of an electrochemical gas sensor varies between
manufacturers as well as between different target gases.
Information about expected lifetime can be found in sensor
manufacturer data sheets. The actual lifespan, however, varies
strongly depending on storage and operating conditions.
The lifetime of an electrochemical gas sensor and the regular
need for calibration are the most challenging aspects of this
type of sensor. Therefore, it is desirable to have the ability to
monitor sensor’s health directly in the instrument.
The built-in waveform generator and discrete Fourier transform
(DFT) block of the
ADuCM355
enable impedance spectroscopy
measurement by applying an ac signal sweep to the counter
electrode. This measurement indicates the quality of charge
transfer between electrodes, effectively detecting aging of the
sensor’s electrolyte. Laboratory tests show good correlation
between impedance and sensitivity of the sensor.
Other methods of detecting sensor health include pulse test and
ramp test. For these tests, a voltage pulse or ramp is applied on
top of the bias voltage to test the sensor responsivity and charge
transfer, respectively.
All of these measurements in conjunction with algorithms
running on the
ADuCM355
contribute to improving the
accuracy, performance, and lifetime of the electrochemical gas
sensor. Enabling this level of smart diagnostics and prognostics
requires that a large set of sensors be characterized by tests such
as accelerated aging. Contact the sensor manufacturer for this
type of data.
Preliminary Technical Data
Temperature and Humidity Compensation
An external temperature and humidity sensor is provided on
the sensor board. It is connected to the
ADuCM355
via an I
2
C
interface. The performances of most electrochemical sensors
varies with both temperature and humidity, and therefore
compensation for these influences is required.
COMMON VARIATIONS
The circuit was tested with 3-lead electrochemical gas sensors
(CE, RE, WE). However, it can also support 4-lead (CE, RE,
WE1, WE2) and 2-lead sensors (CE and WE).
Four-lead sensors use several electrode configurations. The
fourth electrode can be used as an additional diagnostics
electrode (DE).
Some sensors can detect two gases, in which case the fourth
electrode is configured as a working electrode (for example, a
combined CO and H
2
S sensor).
Rev. PrA | Page 4 of 7
Preliminary Technical Data
CIRCUIT EVALUATION AND TEST
This circuit uses the
EVAL-CN0429-EBZ
gas sensor board, the
EVAL-M355-ARDZ-INT
shield board, and the
EVAL-
ADICUP3029
Arduino-compatible platform board. A user
guide for this platform is available at
www.analog.com/EVAL-
ADICUP3029.
CN-0429
Sensor data acquired by the system is sent to a PC via a virtual
COM port interface, where they can be displayed and
processed.
To set up the circuit for evaluation, take the following steps:
1.
Plug the four
EVAL-CN0429-EBZ
boards into the
EVAL-
M355-ARDZ-INT
shield board, followed by the
EVAL-
ADICUP3029
circuit board.
Ensure that the switch settings are correct on both the
EVAL-M355-ARDZ-INT
and
EVAL-CN0429-EBZ
evaluation boards.
Connect the
EVAL-ADICUP3029
virtual COM USB port
to the PC.
Set up the serial terminal software to match the setting of
the
CN-0429
firmware and select the correct virtual COM
port.
Press the reset button on the
EVAL-ADICUP3029
board,
and the software displays the steps of gas measurements.
Equipment required
The following equipment is needed:
PC with a USB port and Windows® 7 (32-bit) or higher
EVAL-ADICUP3029
Arduino-compatible platform loaded
with CN-0429 firmware
EVAL-M355-ARDZ-INT
shield board
EVAL-CN0429-EBZ
gas sensor board
Suitable electrochemical gas sensor (such as the Citytech
4CF+ CiTiceL CO sensor or Alphasense CO-A4 sensor),
USB type A to USB micro cable
Serial terminal software (PuTTY, TeraTerm or similar)
Carbon monoxide alarm tester spray (optional, for CO
sensors only) or other source of target gas
2.
3.
4.
5.
System Setup
The measurement system consists of a motherboard (EVAL-
ADICUP3029),
an interposer board, and up to four gas sensors
daughter boards (see Figure 3). This four gases configuration is
widely used in both toxic gas and air quality applications.
ARDUINO SHIELD BOARD
×4
EC
GAS SENSOR
CE
RE
WE
×2
POTENTIOSTAT
DAC FOR BIASING
TIA WITH PROGRAMMABLE
GAIN AND LEAD RESISTOR
GAS SENSOR BOARD
The
EVAL-CN0429-EBZ
boards ship with dedicated firmware,
but provisions are available to program new firmware. The
EVAL-CN0429-EBZ
boards can be programmed over USB by
the debugger section of the
EVAL-ADICUP3029.
However, this
requires cutting three traces and using the included cable to
connect to
EVAL-M355-ARDZ-INT.
Other options are to use
an additional
EVAL-ADICUP3029,
or to use an external debugger.
See the
CN-0429 User Guide
for detailed instructions.
ADuCM355
LOW POWER
CORTEX M-3
MICROCONTROLLER
DISCRETE FOURIER
TRANSFORM (DFT)
ACCELERATOR
UART
ADICUP3029
ARDUINO
COMPATIBLE
I
2
C
DEVELOPMENT
PLATFORM
SWD
SPI
MVH3002D
TEMPERATURE
AND
HUMIDITY
16-BIT
ADC
WAVEFORM
GENERATOR
DIGITAL FILTERS
EC DIAGNOSTICS
Figure 3. Simplified Circuit Block Diagram
Rev. PrA | Page 5 of 7
17302-003
RF/Wirelessly
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