Methods for precision improvement

AN2719

12/22

3

Methods for precision improvement

3.1

Introduction

ADC converter precision is fixed by its principle, design and implementation but can be also

improved by several hardware and/or software methods. Hardware methods improve ADC

results by changing the environment around the ADC (PCB design, RC filters, ...) to obtain

more accurate ADC output. Software methods try to improve the raw ADC results by

postprocessing methods. Some CPU power is required to perform these methods, which

can be implemented in the microcontroller firmware.

3.2

Hardware methods

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select reference voltage between input signal ranges

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gives full ADC range - min. voltage per bit

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White noise added to measured signal:

–

wobbling of input signal over several bits gives the opportunity to use averaging (if

input signal is very stable)

–

white noise gives independence from sampling frequency

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Good hardware design:

–

grounding

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reference voltage filtering

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supply filtering

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preamplifier usage

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frequency independence

–…

3.2.1

Analog zooming

Analog zooming is method which improves precision by proper selection of the reference

voltage. The reference voltage is selected in the expected range of the signal to be

measured.

If the measured signal has an offset, then the reference voltage should also have a similar

offset. If the measured signal has defined maximum amplitude, then the reference voltage

should also have a similar maximum value. By matching this reference voltage to the

measurement signal range we obtain the maximum possible resolution using the full A/D

converter output range.

In the STM8 ADC the reference voltage is connected to external pins VREF+ and VREF-.

This makes it possible to match the reference voltage to the measured signal range.

To VREF- pin you can connect the minimum measured voltage and to VREF+ pin you

connect the maximum measured voltage. Take care to respect the minimum and maximum

allowed reference signal levels - these values are specified in the datasheet for the related

STM8 device type.