Datasheet

LMP91200
SNAS571C JANUARY 2012REVISED MARCH 2013
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FUNCTIONAL DESCRIPTION
GENERAL INFORMATION
The LMP91200 is a configurable sensor AFE for use in low power analytical sensing applications. The
LMP91200 is designed for 2-electrode sensors. This device provides all of the functionality needed to detect
changes based on a delta voltage at the sensor. Optimized for low-power applications, the LMP91200 works
over a voltage range of 1.8V to 5.5V. With its extremely low input bias current it is optimized for use with pH
sensors. Also in absence of supply voltage the very low input bias current reduces degradation of the pH probe
when connected to the LMP91200. The Common Mode Output pin (VOCM) provides a common mode offset,
which can be programmed to different values to accommodate pH sensor output ranges. For applications
requiring a high impedance common mode this option is also available. Two guard pins provide support for high
parasitic impedance wiring. Support for an external Pt1000, Pt100, or similar temperature sensor is integrated in
the LMP91200. The control of this feature is available through the SPI interface. Additionally, a user controlled
sensor diagnostic test is available. This function tests the sensor for proper connection and functionality.
pH Buffer
The pH Buffer is a unity gain buffer with a input bias current in the range of tens fA at room. Its very low bias
current introduces a negligible error in the measurement of the pH. The ph buffer is provided with 2 guard pins
(GUARD1, GUARD2) in order to minimize the leakage of the input current and to make easy the design of a
guard ring.
Common mode selector and VCM buffer
The common mode selector allows to set 7 different values of common mode voltage (from 1/8 VREF to
7/8VREF with 1/8 VREF step) according to the applied voltage reference at VREF pin. Both buffered and
unbuffered version of the set common mode voltage are available respectively at VCM pin and VCMHI pin. A
copy of the buffered version is present at VOCM pin in case of differential measurement.
Current Source and PGA
The internal current source is programmable current generator which is able to source 4 different current values
(100µA, 200µA, 1mA, 2mA) in order to well stimulate Pt100 and Pt1000 thermal resistor. The selected current is
sourced from either RTD pin (pin for thermal resistor connection) or CAL pin (pin for reference resistor
connection). The voltage across either the thermal resistor or the reference resistor is amplified by the PGA
(5V/V, 10V/V) and provided at the VOUT pin when the LMP91200 is set in Temperature measurement mode.
Output Muxes
The output of the LMP91200 can be configured to support both differential and single ended ADC’s. When
measuring pH the Output signal can be referred either to VCM or GND. When measuring temperature the Output
signal is referred to GND. The Output configuration is controlled through the SPI interface.
SERIAL CONTROL INTERFACE OPERATION
All the features of the LMP91200 (Mode of Operation, PGA Gain, Voltage reference, Diagnostic) are by data
stored in a programming register. Data to be written into the control register is first loaded into the LMP91200 via
the serial interface. The serial interface employs a 16-bit shift register. Data is loaded through the serial data
input, SDI. Data passing through the shift register is output through the serial data output, SDO_DIAG. The serial
clock, SCK controls the serial loading process. All sixteen data bits are required to correctly program the
LMP91200. The falling edge of CSB enables the shift register to receive data. The SCK signal must be high
during the falling and rising edge of CSB. Each data bit is clocked into the shift register on the rising edge of
SCLK. Data is transferred from the shift register to the holding register on the rising edge of CSB.
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