LMP91050_15 Datasheet, PDF (12/28 Pages) Texas Instruments – Configurable AFE for Nondispersive Infrared (NDIR) Sensing Applications

LMP91050
SNAS517E – NOVEMBER 2011 – REVISED SEPTEMBER 2015
8 Detailed Description
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8.1 Overview
The input channel of the LMP91050 features two programmable gain stages that give the user flexibility in
optimizing the system gain. Access to the inter-stage connection between the gain blocks also allows the
inclusion of appropriate filtering if needed. The internal DAC allows the DC offset of the output voltage to be
adjusted independently of the common-mode voltage supplied to the sensor, enabling the sampled signal to be
centered within the ADC full-scale input range.
The following paragraphs discuss the LMP91050’s features in more detail.
8.2 Functional Block Diagram
VDD
LMP91050
CMOUT
A0
A1
G1=250,42
G2=4,8,16,32
IN
PGA1
PGA2
OUT
CMOUT
SPI
DAC
SPI
CM GEN
VREF
CSB SCLK SDIO
GND
8.3 Feature Description
8.3.1 Programmable Gain Amplifier
The LMP91050 offers two programmable gain modes (low or high) with four programmable gain settings each.
The purpose of the gain mode is to enable thermopiles with larger dark voltage levels. All gain settings are
accessible through bits GAIN1 and GAIN2[1:0]. The low-gain mode has a range of 167 V/V to 1335 V/V while the
high-gain mode has a range of 1002 V/V to 7986 V/V. The PGA is referenced to the internally generated VCM.
Input signal, referenced to this VCM voltage, should be within ±2 mV (see VINMAX_HGM specification) in high-
gain mode. In the low gain mode the first stage will provide a gain of 42 V/V instead of 250 V/V, thus allowing a
larger maximum input signal up to ±12 mV (VINMAX_LGM).
GAIN1
BIT SYMBOL
Table 1. Gain Modes
0: 250 (default)
1: 42
GAIN
12
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