RE

CE

WE

CE

WE

RE

3-Lead

Electrochemical

Cell

I2C INTERFACE

AND

CONTROL

REGISTERS

VREF

VDD

AGND

VOUT

C1

SCL

VREF

DIVIDER

C2

SDA

RLoad

DIAGNOSTIC

MENB

DGND

A1

+

-

TIA

+

-

RTIA

LMP91002

LMP91002

SNIS163B – APRIL 2012 – REVISED OCTOBER 2015

www.ti.com

7 Detailed Description

7.1 Overview

The LMP91002 is a programmable AFE for use in micropower chemical sensing applications. The LMP91002 is

designed for 3-lead non-biased gas sensors and for 2 leads galvanic cell. This device provides all of the

functionality for detecting changes in gas concentration based on a delta current at the working electrode. The

LMP91002 generates an output voltage proportional to the cell current. Transimpedance gain is user

programmable through an I2C compatible interface from 2.75k

Ω to 350kΩ making it easy to convert current

ranges from 5µA to 750µA full scale. Optimized for micro-power applications, the LMP91002 AFE works over a

voltage range of 2.7V to 3.6 V. The cell voltage is user selectable using the on board programmability. In

addition, it is possible to connect an external transimpedance gain resistor. Depending on the configuration, total

current consumption for the device can be less than 10µA. For power savings, the transimpedance amplifier can

be turned off and instead a load impedance equivalent to the TIA’s inputs impedance is switched in.

7.2 Functional Block Diagram

7.3 Feature Description

7.3.1 Potentiostat Circuitry

The core of the LMP91002 is a potentiostat circuit. It consists of a differential input amplifier used to compare the

potential between the working and reference electrodes to a zero bias potential.. The error signal is amplified and

applied to the counter electrode (through the Control Amplifier - A1). Any changes in the impedance between

the working and reference electrodes will cause a change in the voltage applied to the counter electrode, in order

to maintain the constant voltage between working and reference electrodes. A Transimpedance Amplifier

connected to the working electrode, is used to provide an output voltage that is proportional to the cell current.

The working electrode is held at virtual ground (Internal ground) by the transimpedance amplifier. The

potentiostat will compare the reference voltage to the desired bias potential and adjust the voltage at the counter

electrode to maintain the proper working-to-reference voltage.

7.3.2 Transimpedance Amplifier

The transimpedance amplifier (TIA in Functional Block Diagram) has 7 programmable internal gain resistors. This

accommodates the full scale ranges of most existing sensors. Moreover an external gain resistor can be

connected to the LMP91002 between C1 and C2 pins. The gain is set through the I2C interface.

10

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