ST72C171 datasheet(38/152 Pages) STMICROELECTRONICS

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ST72C171

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7.3 OP-AMP MODULE

7.3.1 Introduction

The ST7 Op-Amp module is designed to cover

most types of microcontroller applications where

analog signal amplifiers are used.

It may be used to perform a a variety of functions

such as: differential voltage amplifier, comparator/

threshold detector, ADC zooming, impedance

adaptor, general purpose operational amplifier.

7.3.2 Main features

This module includes:

2 rail-to-rail SPGAs (Software Programmable

Gain Amplifier), and 1 stand alone rail-to-rail

Op-Amp that may be externally connected using

I/O pins

A band gap voltage reference

A programmable eight-step reference voltage

ART Timer PWM outputs internally connected

to SPGAs input 1 and 2.

SPGAs and Op-Amp outputs are internally

connected to the ADC inputs (Channel 8, 9 &

10).

Input offset compensation

7.3.3 General description

The module contains two SPGAs (OA1 & OA2)

and 1 stand alone operational amplifier (OA3) de-

pending on the device package. OA1 and OA2

each have associated circuitry for input and gain

selection. The third operational amplifier, OA3,

without input and gain selection circuitry, is availa-

ble in some devices (see device pin out descrip-

tion).

7.3.3.1 Inputs

The non-inverting input of OA1 or OA2 may be

connected to an I/O pin, to the band-gap reference

voltage, to an 8-step voltage reference or to the

analog ground.

The eight-step voltage reference uses a resistive

network in order to generate two voltages between

1/8 VDD and VDD (in 1/8 VDD steps) that can be

connected to the non-inverting input of the two SP-

GAs. These voltages may be used as programma-

ble thresholds with the corresponding SPGA used

as a comparator or, with the SPGA programmed to

have a gain of 2, 4 or 8, they may be used for ex-

tending the ADC precision (analog zooming).

The 2 inverting inputs of OA1 or OA2 may be used

to achieve this function. The input impedance of

these inputs is around 2K.

The ART Timer PWM resistive outputs are inter-

nally connected to OA1PIN and OA2PIN pins. The

PWM outputs are enabled by the PWMCR register

and the resistive outputs are selected by Miscella-

neous register 2. Refer to Figure 28.

The inverting input of OA1 or OA2 may be con-

nected to an I/O pin, to the analog ground or may

be left unconnected (in this case the SPGA can be

used as a repeater, with the output of the SPGA

connected to this input via the resistive loopback).

7.3.3.2 Outputs

The SPGA outputs are connected either to exter-

nal pins or, internally, to the ADC input (Channel 8

& 9). The output value, digitized by a Schmitt trig-

ger, may be read by the application software or

may generate an interrupt.

The OA3 output is connected to an ADC input

(Channel 10).

7.3.3.3 Advanced features

The gain of OA1 or OA2 is programmed using an

internal resistive network. The possible values are:

1, 2, 4, 8 and 16. The internal resistive loopback

may also be de-activated in order to obtain the

open-loop gain (comparator) or to use the op-amp

with an external loopback network.

Input offset compensation

In a special calibration mode (autozero mode), the

negative input pin of OA1 or OA2 can be connect-

ed internally to the positive input pin. This mode al-

lows the measurement of the input offset voltage

of the SPGA using the ADC. This value may be

stored in RAM and subsequently used for offset

correction (for ADC conversions). Refer to Section

9.3.4.

ST72C171 Datasheet(PDF) 38 Page - STMicroelectronics

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