+

VREF

CE

WE

RE

Potentiostat (Bias Loop)

+

RF

Transimpedance Amplifier (I to V conversion)

RL

CO Sensor

VTIA

U1

U2

ISENS

R2

10

NŸ

C1

0.1µF

VREF

2.5V

Riso

49.9 k

C2

1µF

R1

10 k

15

LPV811, LPV812

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SNOSD33B – NOVEMBER 2016 – REVISED NOVEMBER 2016

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8 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. Customers should

validate and test their design implementation to confirm system functionality.

8.1 Application Information

The LPV81x is a ultra-low power operational amplifier that provides 8 kHz bandwidth with only 425nA typical

quiescent current, trimmed input offset voltage and precision drift specifications. These rail-to-rail output

amplifiers are specifically designed for battery-powered applications. The input common-mode voltage range

extends to the negative supply rail and the output swings to within millivolts of the rails, maintaining a wide

dynamic range.

8.2 Typical Application: Three Terminal CO Gas Sensor Amplifier

Figure 46. Three Terminal Gas Sensor Amplifier Schematic

8.2.1 Design Requirements

Figure 46 shows a simple micropower potentiostat circuit for use with three terminal unbiased CO sensors,

though it 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 the 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 electrodes

must be maintained at the same potential by adjusting the bias on CE. Through the Potentiostat circuit formed by

U1, the servo feedback action will maintain the RE pin at a potential set by VREF.

R1 is to maintain stability due to 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.

The transimpedance gain, and resulting sensitivity, is set by RF according to Equation 2.

VTIA = (-I * RF) + VREF

(2)

RL is a load resistor of which the value is normally specified by the sensor manufacturer (typically 10 ohms). The

potential at WE is set by the applied VREF. Riso provides capacitive isolation and, combined with C2, form the

output filter and ADC reservoir capacitor to drive the ADC.