10 µF

160 k

160 k

6.8 nF

CMOUT

A0

A1

LMP91050

www.ti.com

SNAS517D – NOVEMBER 2011 – REVISED MARCH 2013

FUNCTIONAL DESCRIPTION

PROGRAMMABLE GAIN AMPLIFIER

The LMP91050 offers two programmable gain modes (low/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 +/-2mV (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 +/-12mV (VINMAX_LGM).

Table 1. Gain Modes

Bit Symbol

Gain

0: 250 (default)

GAIN1

1: 42

00: 4 (default)

01: 8

GAIN2 [1:0]

10: 16

11: 32

EXTERNAL FILTER

The LMP91050 offers two different measurement modes selectable through EXT_FILT bit. EXT_FILT bit is

present in the Device configuration register and is programmable through SPI.

Table 2. Measurement Modes

Bit Symbol

Measurement Mode

EXT_FILT

0: The signal from the thermopile is being processed by the internal PGAs, without

additional external decoupling or filtering (default).

1: The signal from the thermopile is being processed by the first internal PGA and fed to the A0

pin. An external low pass, high pass or band pass filter can be connected through pins A0, A1.

An external filter can be applied when EXT_FILT = 1. A typical band pass filter is shown in the picture below.

Resistor and capacitor can be connected to the CMOUT pin of the LMP91050 as shown. Discrete component

values have been added for reference.

Figure 29. Typical Bandpass Filter

OFFSET ADJUST

Procedure of the offset adjust is to first measure the “dark signal”, program the DAC to adjust, and then measure

in a second cycle the residual of the dark signal for further signal manipulation within the µC. The signal source

is expected to have an offset component (dark signal) larger than the actual signal. During the “dark phase”, the

time when no light is detected by the sensor, the µC can program LMP91050 internal DAC to compensate for a

measured offset. A low output offset voltage temperature drift (TCVOS) ensures system accuracy over

temperature. See Figure 30 below which plots the maximum TCVOS allowed over a given temperature drift in

order to achieve n bit system accuracy.

Copyright © 2011–2013, Texas Instruments Incorporated

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