Datasheet

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
R
Load
DIAGNOSTIC
MENB
DGND
A1
+
-
TIA
+
-
R
TIA
LMP91002
LMP91002
SNIS163A –APRIL 2012–REVISED MARCH 2013 www.ti.com
Function Description
GENERAL
The LMP91002 is a programmable AFE for use in micropower chemical sensing applications. The LMP91002 is
designed for 3-lead not 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 I
2
C compatible interface from 2.75kΩ 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.
Figure 17. System Block Diagram
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.
Transimpedance amplifier
The transimpedance amplifier (TIA in Figure 17) 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 I
2
C interface.
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