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Methods for precision improvement

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3.2.2

Adding white noise

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.

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 can appear when input signal is a very stable voltage

without noise. In this case, when the input signal is measured, each 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 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. This method is also called oversampling and/or dithering.

Adding a white noise signal gives 0.5LSB more each time you double the number of

samples. By adding triangle noise, you get +1LSB, which is more efficient. The noise source

can be the microcontroller itself.

This can be implemented using a noise generator with RC coupling of this noise to the input

signal. Care must be taken to not modify the mean value of the original input signal - so

capacitive coupling is used.

A very simple implementation is to design the white noise source as a square (or triangle)

source which is generated directly by the STM8 microcontroller (Figure 11).

Figure 11.

Simple white noise source using a microcontroller output

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