Datasheet

I2C INTERFACE
AND
CONTROL
REGISTERS
RE
VREF
VDD
AGND
CE
WE
VOUT
C1
SCL
TEMP
SENSOR
VREF
DIVIDER
C2
SDA
R
Load
VARIABLE
BIAS
MENB
DGND
A1
+
-
TIA
+
-
R
TIA
CE
WE
RE
3-Lead
Electrochemical
Cell
LMP91000
LMP91000
SNAS506H –JANUARY 2011–REVISED MARCH 2013 www.ti.com
FUNCTION DESCRIPTION
GENERAL
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
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 LMP91000 AFE works over a
voltage range of 2.7V 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 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 25. System Block Diagram
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.
Transimpedance amplifier
The transimpedance amplifier (TIA in Figure 25) 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
LMP91000 between C1 and C2 pins. The gain is set through the I
2
C interface.
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