8 Detailed Description

8.1 Overview

The PGA855 is a monolithic, high-voltage, precision programmable-gain instrumentation amplifier. The PGA855

combines a high-speed current-feedback input stage with an internally matched gain resistor network, followed

by a four-resistor, fully differential amplifier output stage. Eight preprogrammed binary gains, ranging from 0.125

V/V to 16 V/V are selectable using gain-select pins A0, A1, and A2.

A functional block diagram for PGA855 is shown in the next section. The differential input voltage is fed into

a pair of matched, high-impedance input-current-feedback amplifiers. An integrated precision-matched gain

resistor network is used to amplify the differential input voltage. A fully differential output difference amplifier, A3,

rejects the input common-mode component and refers the output signal to the voltage level set by the VOCM

pin.

The PGA855 output amplifier bandwidth is optimized to drive high-performance analog-to-digital converters

(ADCs) with sampling rates up to 1 MSPS, without the need for an additional ADC driver. The output amplifier

uses a separate power supply that is independent of the input-stage power supply. When driving an ADC, use

a low-impedance connection from LVDD and LVSS to the ADC power supplies. This configuration protects the

ADC inputs from damage due to inadvertent overvoltage conditions.

8.2 Functional Block Diagram

+

–

IN

IN+

+

–

5 k

5 k

5 k

+

– +

–

5 k

A<2:0>

DGND

VIN

VIN+

DGND

VS+

A1

A2

A3

OUT+

LVSS

LVDD

LVSS

LVDD

OUT

FDA_IN–

FDA_IN+

VS+

VS

VOCM

500 k

500 k

VS+

VS

LVDD

LVSS

VS+

VS

VS+

VS

VS+

VS

VS+

VS

VS+

VS

PGA855

SBOSAE0 – APRIL 2023

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