This column is a "Part 2" and it'll make much more sense if you read "an Excelent Fit, Sir!" first. Well, I hope it will, anyway. In that column, we used Excel's 'solver' functionality to bend a filter transfer function to our will. Or, rather, our customer's will, since he is much more important than we are.

Recap: we computed a filter for use in flattening out the frequency response of a sensor with a frequency response looking like figure 1:

We 'turned the response upside down'; extended it (with orders to 'come back downwards' again); guessed some filter coefficients, and fine-tuned them with Excel's solver. The light blue curve in figure 2 shows what we got:

The 'accuracy' of our fit is shown in figure 3. It's rather wiggly; that just reflects the rather approximate way in which we eyeballed the required frequency response off figure 1 in the first place.

Now, our customer doesn't want to buy big, expensive, high quality capacitors to build an active filter with this response (good, all the more of these rare beasts for me, buwahahah). So, having established the principle of optimising in the analog domain, let's look at whether need to make any changes in order to use it to create a digital filter.