Digital filtering technique , ADC Filter

A digital filter system usually consists of an analog-to-digital converter (ADC) to sample the input signal, followed by a microprocessor and some peripheral components such as memory to store data and coefficients of the filter, etc.

Program Instructions (software) running on the microprocessor implement the digital filter by performing the necessary mathematical operations on the numbers received from the ADC.

Here is a simple digital filter that identifies and rejects transient response signals. It returns the average of the best two samples of the last three. The third sample is discarded.

Algorithm:

1. Calculate the average of the last 3 samples

2. Identity the sample furthest from the mean value (which is the outlier)

3. Discard the outlier and average the remaining two samples

4. Returns float value of the averaged data (y(i)).

Common Mode and Differential Mode Filters to limit EMI Issues

Noise can propagate in two major ways: Differential Mode (DM) Noise occurs between two conductors carrying opposing currents, such as the positive and negative lines in a DC circuit. Common Mode (CM) Noise appears in the same direction on both conductors relative to ground. Each sort of noise behaves differently, requiring a specific filtering technique.

This EMI filter consists of a common-mode filter and a differential-mode filter. Differential mode filters typically filter noise below 30 MHz, while common mode filters filter noise between 30 MHz and 100 MHz. Both filters affect the entire frequency range where EMI needs to be limited.