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AN5306_Operational Amplifier (OPAMP) usage in STM32G4 Series.pdf

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The operational amplifier (OPAMP) embedded in the STM32G4 Series device extends the analog capabilities of the microcontroller. This application note describes how to implement an OPAMP to support a number of analog applications using a minimum number of external components and outline the OPAMP configuration using the digital controls. This application note explains how to implement the OPAMP in a number of operating modes and outlines some specific practical examples.

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Introduction

The operational amplifier (OPAMP) embedded in the STM32G4 Series device extends the analog capabilities of the

microcontroller. This application note describes how to implement an OPAMP to support a number of analog applications using

a minimum number of external components and outline the OPAMP configuration using the digital controls.

This application note explains how to implement the OPAMP in a number of operating modes and outlines some specific

practical examples.

Operational Amplifier (OPAMP) usage in STM32G4 Series

AN5306

Application note

AN5306 - Rev 1 - May 2019

For further information contact your local STMicroelectronics sales office.

www.st.com

1 General information

This document applies to the STM32G4 Series Arm

-based microcontrollers.

Note: Arm is a registered trademark of Arm Limited (or its subsidiaries) in the US and/or elsewhere.

AN5306

General information

AN5306 - Rev 1

page 2/43

2 What is an OPAMP – basics.

2.1 OPAMP operation basics

Figure 1. Basic OPAMP representation

OUTPUT

Non inverting

input

Inverting

input

The operational amplifier is an analog device that amplifies differential input signals (difference between inverting

and non-inverting input) to give a resulting output voltage. Figure 1. Basic OPAMP representation illustrates the

circuit representation of an OPAMP.

The ideal operational amplifier must have the following properties:

• Infinite amplification gain (open loop)

• Infinite input impedance

• Zero output impedance

• Infinite frequency bandwidth

• Zero input offset voltage

If the OPAMP has infinite gain, then the differential signal at input must be zero (in closed loop linear operation)-

because the gain is infinite and output voltage is at finite level:

out

= U

x Gain = 0 x ∞ = finite voltage.

This basic idea (input voltage at zero) simplifies the OPAMP usage in real applications.

Real operational amplifier requires properties close to ideal requirements:

• High amplification gain (open loop) – for example: gain = 1,000,000 (120 dB)

• High input impedance – for example Z

= 100 MΩ

• Low output impedance – for example Z

out

= 10 Ω

• High frequency bandwidth – for example f

-3 dB

= f

GBW

= 10 MHz (for closed gain loop = 1)

• Low input offset voltage – for example U

offset

= 1 mV

AN5306

What is an OPAMP – basics.

AN5306 - Rev 1

page 3/43

2.2 OPAMP in practice – operation example

The ideal OPAMP has infinite gain. This means that if output needs to be set at a defined value then the input

differential signal should be zero. The output signal should influence input differential voltage through negative

feedback to balance the operation point. Figure 2. Inverting amplifier by using OPAMP below illustrates an

example of an inverting amplifier setup:

Figure 2. Inverting amplifier by using OPAMP

diff

R1 R2

out

diff

I1 I2

A voltage is applied to U

on the negative OPAMP input of U

diff

through R1 resistor. This increase on the negative

input causes a voltage decrease on U

out

– due to the OPAMP gain ratio. Then the U

out

voltage causes a

decrease of the voltage on the negative OPAMP input through R2 resistor – this is the negative feedback set up

on U

. The feedback then stabilizes the differential voltage of the OPAMP input (U

diff

) to reach zero and the U

out

voltage reaches the required level (U

out

= U

diff

x ∞). If U

diff

is zero then U

out

voltage is calculated based on U

using the following formulas:

• U

diff

= 0 V (zero differential voltage)

diff

= 0 A (zero current on the OPAMP input – because input impedance is ∞)

• I1 = U

/ R1 (current through R1, because U

diff

= 0 and non-inverting OPAMP input is grounded)

I2 = U

out

/ R2 (current through R2, because U

diff

= 0 and non-inverting OPAMP input is grounded)

• I2 = -I1 (because I

diff

= 0)

• U

out

/ R2 = -U

/ R1

out

= -Uin . (R2/R1)

The final circuit gain is calculated as a ratio between the output and the input voltage (gain = U

out

). Final gain

is given here only by external resistors values of R1 and R2 and is not depend on the OPAMP (if its gain is ∞).

AN5306

OPAMP in practice – operation example

AN5306 - Rev 1

page 4/43

3 Brief description of the OPAMP in the STM32G4 Series

The STM32G4 Series integrates OPAMPs that can be used in high speed analog to digital applications as analog

signal pre-conditioning for ADC data sampling or as standalone amplifiers. Main application areas are:

• Motor control applications (current and voltage sensing)

• Digital switched-mode power supplies – DSMPS (current and voltage sensing, control loop)

• Light control applications (current and voltage sensing)

• Analog sensors measurement applications (thermometers, low signals measurement and so on)

• Medical applications (analog signals sensing/amplification)

• Audio applications (signal amplification)

• General purpose analog signal usage (oscillators, comparators and so on).

3.1 OPAMP parameters

Basic performance parameters of the STM32G4 Series operational amplifier are:

• Input voltage offset: approx. +/- 3 mV (after built-in calibration of offset)

• Bandwidth: approx. 13 MHz

• Slew rate:

– Normal mode: approx. 6.5 V/µs

– High speed mode: approx. 45 V/µs.

• Output saturated voltage: less than 100 mV (rai-to-rail)

• Gains:

– Positive: +1, +2, +4, +8, +16, +32, +64

– Negative: -1, -3, -7, -15, -31, -63

– Typical gain error: 2%.

• Open loop gain: ~ 95 dB

• Wakeup time: 3 µs.

AN5306

Brief description of the OPAMP in the STM32G4 Series

AN5306 - Rev 1

page 5/43

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