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Methods for precision improvement

AN3137

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Doc ID 16983 Rev 3

4.2.2

Adding white noise or triangular sweep

This method combines hardware and software techniques to improve precision. From a

software point of view, this method uses averaging and from a hardware point of view, it

uses signal modification/spreading/dithering.

Averaging can be used in cases where the input signal is noisy (some signal change is

necessary in order to be able to calculate an average) and the requirement is to obtain the

mean value of a signal. A problem appears when the input signal is a very stable voltage

without noise. In this case, when the input signal is measured, each data sample is the

same. This is because the input signal level is somewhere between two ADC word levels

(e.g. between 0x14A and 0x14B). Therefore it is not possible to determine the input voltage

level more precisely (e.g. if the level is near to 0x14A or near to 0x14B level).

The solution is to add noise or some signal change (with uniform signal distribution e.g.

triangular sweep) to the input signal which pushes its level across 1-bit ADC level (so that

the signal level changes below 0x14A and above 0x14B level). This causes the ADC results

to vary. Applying software averaging to the different ADC results, produces the mean value

of the original input signal.

Example of implementation of this method is done by using a triangular generator with RC

coupling to the input signal (noise signal generation is more complicated). Care must be

taken not to modify the mean value of the original input signal (so, capacitive coupling must

be used).

A very simple implementation is to design the white noise source as a quasi-triangular

source which is generated directly by the STM8L/STM8AL microcontroller (Figure 11).

Figure 11.

Simple quasi-triangular source using a microcontroller output

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