I2C INTERFACE

AND

CONTROL

REGISTERS

RE

VREF

VDD

AGND

CE

WE

VOUT

C1

SCL

TEMP

SENSOR

VREF

DIVIDER

C2

SDA

VARIABLE

BIAS

MENB

DGND

A1

+

-

TIA

+

-

CE

WE

RE

3-Lead

Electrochemical

Cell

LMP91000

LMP91000

www.ti.com

SNAS506I – JANUARY 2011 – REVISED DECEMBER 2014

7 Detailed Description

7.1 Overview

The LMP91000 is a programmable AFE for use in micropower chemical sensing applications. The LMP91000 is

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

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

LMP91000 generates an output voltage proportional to the cell current. Transimpedance gain is user

Ω to 350 kΩ making it easy to convert current

ranges from 5 µA to 750 µA full scale. Optimized for micro-power applications, the LMP91000 AFE works over a

voltage range of 2.7 V to 5.25 V. The cell voltage is user selectable using the on board programmability. In

addition, it is possible to connect an external transimpedance gain resistor. A temperature sensor is embedded

and it can be power cycled through the interface. The output of this temperature sensor can be read by the user

through the VOUT pin. It is also possible to have both temperature output and output of the TIA at the same

time; the pin C2 is internally connected to the output of the transimpedance (TIA), while the temperature is

available at the VOUT pin. Depending on the configuration, total current consumption for the device can be less

than 10 µA. 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 LMP91000 is a potentiostat circuit. It consists of a differential input amplifier used to compare the

potential between the working and reference electrodes to a required working bias potential (set by the Variable

Bias circuitry). 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.

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