7 Application and Implementation

Note

Information in the following applications sections is not part of the TI component specification,

and TI does not warrant its accuracy or completeness. TI’s customers are responsible for

determining suitability of components for their purposes, as well as validating and testing their design

implementation to confirm system functionality.

7.1 Application Information

The OPAx391 are unity-gain stable, precision operational amplifier free from unexpected output and phase

reversal. The OPAx391 are optimized for full rail-to-rail input, allowing for low-voltage, single-supply operation or

split-supply use. These miniature, high-precision, low-noise amplifiers offer high-impedance inputs that have a

common-mode range 100 mV beyond the supplies and can tolerate the full supply voltage across the input. The

OPAx391 precision amplifier is designed for sensor amplification, low-power analog signal chain applications in

low or high gains, as well as a low-power discrete MOSFET or bipolar driver.

7.2 Typical Applications

7.2.1 Three-Terminal CO Gas Sensor

Figure 7-1 shows a simple micropower potentiostat circuit for use with three-terminal unbiased CO sensors.

This same design is applicable to many other type of three-terminal gas sensors or electrochemical cells.

The basic sensor has three electrodes: the sense or working electrode (WE), counter electrode (CE), and

reference electrode (RE). A current flows between CE and WE proportional to the detected concentration. The

RE monitors the potential of the internal reference point. For an unbiased sensor, the WE and RE must be

maintained at the same potential by adjusting the bias on CE. Through the potentiostat circuit formed by U1, the

servo feedback action maintains the RE pin at a potential set by VREF. R1 maintains stability as a result of the

large capacitance of the sensor. C1 and R2 form the potentiostat integrator and set the feedback time constant.

U2 forms a transimpedance amplifier (TIA) to convert the resulting sensor current into a proportional voltage.

Equation 1 calculates the transimpedance gain, and resulting sensitivity, using RF:

VTIA = (–I * RF) + VREF

(1)

RLoad is a load resistor with a value that is normally specified by the sensor manufacturer (typically, 10 Ω). The

potential at WE is set by the applied VREF.

V

REF

V

REF

OPAx391

OPAx391

CE

WE

RE

C

F

R

F

V

OUT

+

+

C

1

R

1

R

2

R

LOAD

Figure 7-1. Three-Terminal CO Gas Sensor

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OPA391, OPA2391, OPA4391

SBOS925D – DECEMBER 2020 – REVISED APRIL 2024

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