AD ADR3412 Devices connected Datasheet

Circuit Note
CN-0357
Devices Connected/Referenced
Circuits from the Lab® reference designs are engineered and
tested for quick and easy system integration to help solve today’s
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information and/or support, visit www.analog.com/CN0357.
ADA4528-2
5.0 V, Ultralow Noise, Zero Drift, RRIO,
Dual Op Amp
AD5270-20
1024-Position,1% Resistor Tolerance Error,
50-TP Memory Digital Rheostat
ADR3412
Micropower, 0.1% Accurate,1.2 Voltage
Reference
AD8500
Micropower, RRIO, Op Amp
AD7790
Low Power, 16-Bit Sigma-Delta, ADC
Low Noise, Single-Supply, Toxic Gas Detector, Using an Electrochemical Sensor with
Programmable Gain TIA for Rapid Prototyping
EVALUATION AND DESIGN SUPPORT
The circuit shown in Figure 1 uses the ADA4528-2, dual auto
zero amplifier, which has a maximum offset voltage of 2.5 µV at
room temperature and an industry leading 5.6 µV/√Hz of
voltage noise density. In addition, the AD5270-20 programmable
rheostat is used rather than a fixed transimpedance resistor,
allowing for rapid prototyping of different gas sensor systems,
without changing the bill of materials.
Circuit Evaluation Boards
CN-0357 Circuit Evaluation Board (EVAL-CN0357-PMDZ)
SDP to Pmod Interposer Board (PMD-SDP-IB1Z)
System Demonstration Platform (EVAL-SDP-CB1Z)
Design and Integration Files
Schematics, Layout Files, and Bill of Materials
The ADR3412 precision, low noise, micropower reference
establishes the 1.2 V common-mode, pseudo ground reference
voltage with 0.1% accuracy and 8 ppm/°C drift.
CIRCUIT FUNCTION AND BENEFITS
The circuit shown in Figure 1 is a single-supply, low noise,
portable gas detector, using an electrochemical sensor. The
Alphasense CO-AX carbon monoxide sensor is used in this
example.
For applications where measuring fractions of ppm gas
concentration is important, using the ADA4528-2 and the
ADR3412 makes the circuit performance suitable for interfacing
with a 16-bit ADC, such as the AD7790.
Electrochemical sensors offer several advantages for
instruments that detect or measure the concentration of many
toxic gases. Most sensors are gas specific and have usable
resolutions under one part per million (ppm) of gas
concentration.
3.3V
VREF
U4
ADR3412
VIN VOUT
C10
0.1µF
GND
R4
1.2V 12.4kΩ
C8
0.1µF
AVCC
R3
12.4kΩ
U2-A
U1
ADA4528-2
CO-AX
CE WE
C3
0.02µF
RE
C4
0.02µF
Q1
MMBFJ270
D
S
R6
12.4kΩ
C9
10µF
C2
0.02µF
G
W
A
VDD
REF(+)
U5
1.2V
AD8500
R12
150Ω
AIN1(–)
DOUT/RDY
AIN1(+)
U8
R9
3.3kΩ
C14
5.6nF
SCLK
AD7790
CS
GND
AGND
TO
PROCESSOR
DIN
REF(–)
AGND
U3-B
R1
1MΩ
R10
3.3kΩ
R8
100kΩ
R2
33Ω
3.3V
R5
12.4kΩ
U2-B
ADA4528-2
AD5270-20
12332-001
AVCC
Figure 1. Low Noise Gas Detector Circuit (Simplified Schematic: all Connections and Decoupling not Shown)
Rev. 0
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CN-0357
Circuit Note
CIRCUIT DESCRIPTION
Table 1. Typical Carbon Monoxide Sensor Specifications
Figure 2 shows a simplified schematic of an electrochemical
sensor measurement circuit. Electrochemical sensors work by
allowing gas to diffuse into the sensor through a membrane and
by interacting with the working electrode (WE). The sensor
reference electrode (RE) provides feedback to Amplifier U2-A,
which maintains a constant potential with the WE terminal by
varying the voltage at the counter electrode (CE). The direction
of the current at the WE terminal depends on whether the
reaction occurring within the sensor is oxidation or reduction.
In the case of a carbon monoxide sensor, oxidation takes place;
therefore, the current flows into the working electrode, which
requires the counter electrode to be at a negative voltage (typically
300 mV to 400 mV) with respect to the working electrode. The
op amp driving the CE terminal should have an output voltage
range of approximately ±1 V with respect to VREF to provide
sufficient headroom for operation with different types of sensors
(Alphasense Application Note AAN-105-03, Designing a
Potentiostatic Circuit, Alphasense, Ltd.).
Parameter
Sensitivity
IWE
+
RE
VREF
CE
RF
Overrange Limit (Specifications Not
Guaranteed)
The output voltage of the transimpedance amplifier is
VO = 1.2 V + IWE × RF
IWE
The maximum response of the CO-AX sensor is 100 nA/ppm,
and its maximum input range is 2000 ppm of carbon monoxide.
These values result in a maximum output current of 200 μA and
a maximum output voltage determined by the transimpedance
resistor, as shown in Equation 2.
VO = 1.2 V + 2000 ppm × 100
IWE
VOUT
SENSOR
Figure 2. Simplified Electrochemical Sensor Circuit
The current into the WE terminal is less than 100 nA per ppm
of gas concentration; therefore, converting this current into an
output voltage requires a transimpedance amplifier with a very
low input bias current. The ADA4528-2 op amp has CMOS inputs
with a maximum input bias current of 220 pA at room
temperature, making it a very good fit for this application.
The ADR3412 establishes the pseudo ground reference for the
circuit, which allows for single-supply operation while consuming
very little quiescent current (100 µA maximum).
Amplifier U2-A sinks enough current from the CE terminal to
maintain a 0 V potential between the WE terminal and the
RE terminal on the sensor. The RE terminal is connected to the
inverting input of Amplifier U2-A; therefore, no current flows
in or out of it. This means that the current comes from the
WE terminal and it changes linearly with gas concentration.
Transimpedance Amplifier U2-B converts the sensor current into
a voltage proportional to the gas concentration.
The sensor selected for this circuit is an Alphasense CO-AX
carbon monoxide sensor. Table 1 shows the typical
specifications associated with carbon monoxide sensors of this
general type.
(1)
where IWE is the current into the WE terminal, and RF is the
transimpedance feedback resistor (shown as the AD5270-20
U3-B rheostat in Figure 1).
WE
12332-002
–
VREF
Response Time (t90 from 0 ppm to 400 ppm CO)
Range (ppm) CO, Guaranteed Performance)
Value
55 nA/ppm to
100 nA/ppm
(65 nA/ppm
typical)
<30 seconds
0 ppm to
2,000 ppm
4,000 ppm
nA
× RF
ppm
VO = 1.2 V + 200 µA × RF
(2)
Applying 1.2 V to VREF of the AD7790 allows a usable range of
±1.2 V at the output of the transimpedance amplifier, U2-B.
Selecting a 6.0 kΩ resistor for the transimpedance feedback
resistor gives a maximum output voltage of 2.4 V.
Equation 3 shows the circuit output voltage as a function of
ppm of carbon monoxide, using the typical response of the
sensor of 65 nA/ppm.
VO = 1.2 V + 390
μV
ppm
(3)
The AD5270-20 has a nominal resistance value of 20 kΩ. There
are 1024 resistance positions, resulting in resistance step sizes of
19.5 Ω. The 5 ppm/°C resistance temperature coefficient of the
AD5270-20 is better than that of most discrete resistors, and its
1 µA of supply current is a very small contributor to the overall
power consumption of the system.
Resistor R4 keeps the noise gain at a reasonable level. Selecting
the value of this resistor is a compromise between the magnitude of
the noise gain and the sensor settling time errors, when exposed
to high concentrations of gas. For the example shown in
Equation 4, R4 = 33 Ω, which results in a noise gain of 183.
Warning: carbon monoxide is a toxic gas, and concentrations
higher than 250 ppm can be dangerous; therefore, take extreme
care when testing this circuit.
Rev. 0 | Page 2 of 5
NG = 1 +
6.0 kΩ
33 Ω
= 183
(4)
Circuit Note
CN-0357
VOUTPUTNOISE = 97 nV × NG = 18 µVp-p
(5)
Because this is a very low frequency 1/f noise, the noise is very hard
to filter. However, because the sensor response is also very slow,
it is possible to use a very low frequency, low-pass filter (R5 and
C6) with a cutoff frequency of 0.16 Hz. Even with such a low
frequency filter, its effect on the sensor response time is
negligible, when compared to the 30 second response time of
the sensor.
The noise free code resolution of the system is determined from
the peak-to-peak output noise. The maximum output of the
ADA4528-2 is 2.4 V, so the noise free number of counts is
Total Noise Free Counts =
2.4 V
18 µV p − p
= 64,865
(6)
The noise free code resolution becomes
Noise Free Code Resolution = log2(64,865) = 15.9 bits
(7)
To take advantage of the entire ADC range available (±1.2 V),
the AD8500 micropower, rail-to-rail input/output amplifier is
chosen to drive the input of the AD7790. If the entire range is
not necessary, the AD8500 can be removed and the internal
buffer of the AD7790 can be used in its place.
One important characteristic of electrochemical sensors is their
very long time constant. When first powered up, it can take several
minutes for the output to settle to its final value. When exposed
to a midscale step in concentration of the target gas, the time
required for the sensor output to reach 90% of its final value can
be in the order of 25 seconds to 40 seconds. If the voltage between
the RE terminal and the WE terminal has a sudden change in
magnitude, it can take several minutes for the output current of
the sensor to settle. This long time constant also applies when
cycling power to the sensor. To avoid very long start-up times,
P-channel JFET Q1 shorts the RE terminal to the WE terminal
when the supply voltage drops below the gate-to-source
threshold voltage (~2.0 V) of the JFET.
CIRCUIT EVALUATION AND TEST
The circuit shown in Figure 1 uses the EVAL-CN0357-PMDZ
circuit evaluation board, the PMD-SDP-IB1Z interposer board,
and the EVAL-SDP-CB1Z system demonstration platform (SDPB) controller board. In addition, the EVAL-CN0357-PMDZ
comes in a Pmod form factor, making it possible to connect the
board to any Pmod controller board, allowing for rapid
prototyping.
The CN-0357 Evaluation Software communicates with the SDP
board to capture data from the EVAL-CN0357-PMDZ circuit
evaluation board.
Equipment Needed
The following equipment is needed for evaluating the
CN-0357 circuit:
•
PC with USB port and Windows® XP, or Windows Vista
(32-bit), or Windows 7 (32-bit)
EVAL-CN0357-PMDZ evaluation board
PMD-SDP-IB1Z interposer Board
EVAL-SDP-CB1Z controller board
CN-0357 Evaluation Software
Calibration gas (less than 250 ppm)
EVAL-CFTL-6V-PWRZ or equivalent 6 VDC power
supply
•
•
•
•
•
•
Getting Started
Load the evaluation software by placing the CN-0357
Evaluation Software CD into the CD drive of the PC. Using My
Computer, locate the drive that contains the evaluation
software CD and run the setup.exe. Follow the on screen
prompts for installing and using the evaluation software.
Functional Block Diagram
A functional block diagram of the test setup is shown in Figure 3.
There is a complete schematic of the evaluation board
contained in the CN-0357 Design Support Package, including
printouts of the gerber files, as well as a bill of materials.
PC
USB
COMMON VARIATIONS
+6V POWER
SENSOR
PMD-SDP-IB1Z
120-PIN SDP
J1
CO-AX
EVAL-SDP-CB1Z
120-PIN SDP
The ADR291 precision voltage reference consumes only 12 µA
and can be substituted for the ADR3412, if lower power is
required.
USB
12-PIN
Electrochemical sensors operate with very small amounts of
current, making them well suited for portable, battery-powered
instruments. If lower power is required, the ADA4505-2
amplifier has a maximum input bias current of 2 pA and
consumes only 10 µA per amplifier. However, the noise of the
ADA4505-2 is greater than that of the ADA4528-2.
EVAL-CN0357-PMDZ
More information can be found in Circuit Note CN-0234.
Rev. 0 | Page 3 of 5
Figure 3. Test Setup Functional Block Diagram
CON A
OR
CON B
12332-003
The input noise of the transimpedance amplifier appears at the
output, amplified by the noise gain. For this circuit, only low
frequency noise is of interest because the frequency of operation
of the sensor is very low. The ADA4528-2 has a 0.1 Hz to 10 Hz
input voltage noise of 97 nV p-p; therefore, the noise at the
output is 18 µV p-p, as shown in Equation 5.
CN-0357
Circuit Note
Setup
Connect the electrochemical sensor to the socket on the EVALCN0357-PMDZ circuit evaluation board.
Connect the EVAL-CN0357-PMDZ to the interposer board and
ensure that the jumper on the interposer board selects +3.3 V as
the configuration. Next, connect the SDP-B board to the
interposer board and apply power to the interposer board via
the DC barrel jack.
Figure 4 shows the circuit response after rapidly removing the
sensor from the 175 ppm CO atmosphere and is a better
representation of circuit performance.
A photo of the EVAL-CN0357-PMDZ board is shown in
Figure 5.
Connect the USB cable supplied with the SDP-B board to the
USB port on the PC and to the SDP-B board.
The software can communicate with the SDP board if the
Analog Devices System Development Platform driver appears
in the Device Manager. When USB communications are
established, the SDP board can now send, receive, and capture
serial data from the EVAL-CN0357-PMDZ circuit evaluation
board.
Navigate to the installation location of the CN-0357 Evaluation
Software and open the CN0357.exe file. (The file should be located
in a folder named Analog Devices in the start menu of the
operating system.)
12332-004
Test
Figure 4. Response to a 175 ppm to 0 ppm Step of Carbon Monoxide
12332-005
When the application opens, the software automatically connects to
the SDP-B board. If multiple SDP-B controller boards are
connected, the selected board will have a light emitting diode
(LED) blinking.
The CN-0357 Evaluation Software User Guide contains
information and details regarding how to use the evaluation
software for data capture.
Figure 5. EVAL-CN0357-PMDZ Evaluation Board
The input signal for this board is gas concentration; therefore, a
calibration gas source is required. When testing with carbon
monoxide, consider that 250 ppm is the maximum short term
exposure limit.
The software is designed to be used with any electrochemical
sensor, so it is important to input the correct specifications for the
chosen sensor.
The maximum sensor sensitivity and sensor range are needed to
calculate and set the resistance of the AD5270-20 digital rheostat.
This value is a signed value. Positive values are used to describe
sensors than sink current, whereas negative values are used to
describe sensors that source current.
The typical sensor sensitivity is used to calculate the system
conversion coefficient of ppm/mV.
Pressing the Run button begins acquisition of concentration data at
1 second intervals.
Rev. 0 | Page 4 of 5
Circuit Note
CN-0357
LEARN MORE
Data Sheets and Evaluation Boards
CN-0357 Design Support Package:
http://www.analog.com/CN0357-DesignSupport
CN-0357 Circuit Evaluation Board (EVAL-CN0357-PMDZ)
System Demonstration Platform (EVAL-SDP-CB1Z)
AN-1114, Lowest Noise Zero-Drift Amplifier Has 5.6 nV/√Hz
Voltage Noise Density, Analog Devices.
ADA4528-2 Data Sheet
MS-2066 Article, Low Noise Signal Conditioning for SensorBased Circuits, Analog Devices.
ADR3412 Data Sheet
AD5270-20 Data Sheet
AD8500 Data Sheet
MT-035 Tutorial, Op Amp Inputs, Outputs, Single-Supply, and
Rail-to-Rail Issues, Analog Devices.
AD7790 Data Sheet
CN-0234 Circuit Note, Single Supply, Micropower Toxic Gas
Detector Using an Electrochemical Sensor, Analog Devices.
Alphasense Application Note AAN-105-03, Designing a
Potentiostatic Circuit, Alphasense Limited.
REVISION HISTORY
7/14—Revision 0: Initial Version
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CN12332-0-7/14(0)
Rev. 0 | Page 5 of 5