CN0357 Datasheet, PDF (3/5 Pages) Analog Devices – Devices Connected

Circuit Note
CN-0357
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.
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
2.4 V
Total Noise Free Counts =
= 64,865
(6)
18 µV p − p
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.
COMMON VARIATIONS
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.
The ADR291 precision voltage reference consumes only 12 µA
and can be substituted for the ADR3412, if lower power is
required.
More information can be found in Circuit Note CN-0234.
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 (SDP-
B) 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
+6V POWER
USB
EVAL-SDP-CB1Z
CO-AX
SENSOR
J1
PMD-SDP-IB1Z
CON A
OR
CON B
EVAL-CN0357-PMDZ
Figure 3. Test Setup Functional Block Diagram
Rev. 0 | Page 3 of 5