ADS131M08IPBSR datasheet(21/118 Pages) TI2

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8.3.3 Programmable Gain Amplifier (PGA)

Each channel of the ADS131M08 features an integrated programmable gain amplifier (PGA) that provides gains

of 1, 2, 4, 8, 16, 32, 64, and 128. The gains for all channels are individually controlled by the PGAGAINn bits for

each channel in the GAIN1 register.

Varying the PGA gain scales the differential full-scale input voltage range (FSR) of the ADC. Equation 3

describes the relationship between FSR and gain. Equation 3 uses the internal reference voltage, 1.2 V, as the

scaling factor without accounting for gain error caused by tolerance in the reference voltage.

FSR = ±1.2 V / Gain

(3)

Table 8-1 shows the corresponding full-scale ranges for each gain setting, assuming the internal reference is

used.

Table 8-1. Full-Scale Range

GAIN SETTING

FSR

±1.2 V

±600 mV

±300 mV

±150 mV

±75 mV

±37.5 mV

±18.75 mV

128

±9.375 mV

The input impedance of the PGA dominates the input impedance characteristics of the ADS131M08. The PGA

input impedance for gain settings up to 4 behaves according to Equation 4 without accounting for device

tolerance and change over temperature. Minimize the output impedance of the circuit that drives the

ADS131M08 inputs to obtain the best possible gain error, INL, and distortion performance.

330 kΩ × 4.096 MHz / fMOD

(4)

where:

• fMOD is the ΔΣ modulator frequency, fCLKIN / 2

The device uses an input precharge buffer for PGA gain settings of 8 and higher. The input impedance at these

gain settings is very high. Specifying the input bias current for these gain settings is therefore more useful. A plot

of input bias current for the high gain settings is provided in Figure 6-5.

8.3.4 Voltage Reference

The ADS131M08 has two reference voltage options: the internal reference and an external reference. The

internal reference uses a low-drift band-gap voltage to supply the reference for the ADC. The internal reference

has a nominal voltage of 1.2 V, allowing the differential input voltage to swing from –1.2 V to 1.2 V. The reference

circuitry starts up very quickly to accommodate the fast-startup feature of this device. The device waits until after

the reference circuitry is fully settled before generating conversion data. Do not place any capacitance on the

REFIN pin if the current-detect mode is used. See the Current-Detect Mode section for more details about the

current-detect mode.

The device also allows for an external reference voltage, which can be input at the REFIN pin. The nominal

external reference voltage is 1.25 V. The full-scale range of the ADC is 1.2 V / gain when the reference voltage is

1.25 V. The ADC full-scale range maintains the ratio of 1.2 to 1.25 for all recommended reference voltages. For

example, the ADC full-scale range is 1.248 V / gain if 1.3 V is applied to the REFIN pin instead of the nominal

1.25 V. The startup time for the external reference is governed by the external circuitry.

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ADS131M08

SBAS950B – OCTOBER 2019 – REVISED FEBRUARY 2021

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