Circuit Note

CN-0357

Rev. 0 | Page 3 of 5

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.

(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

64,865

p

p

µV

18

V

2.4

=

−

=

Counts

Free

Noise

Total

(6)

The noise free code resolution becomes

(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.

EVAL-SDP-CB1Z

CON A

OR

CON B

USB

PC

USB

EVAL-CN0357-PMDZ

J1

+6V POWER

PMD-SDP-IB1Z

SENSOR

CO-AX

Figure 3. Test Setup Functional Block Diagram