Actually, I'm making it look a little easy. Excel's solver is prone to settling on a poor local minimum if your initial guess is too far out, so you sometimes need to intervene to help it.
This seems good in the desired frequency range, but that gain is still high outside the passband, especially at the higher frequencies. How about adding another filter section to chop off the low and high frequencies? Separate single pole lowpass and highpass filters would do. But did you realize that you can combine those single pole filters into one two-pole bandpass? It means we can reuse the formula we already created. I'll try a section with the same denominator as my first guess, but with only the s1 term in the numerator non-zero, set to match the denominator:
Let's also be more aggressive with those upper gain limits, forcing them to fall outside the band, figure 7:
We invoke the solver again, and get figure 8:
which is falls away nicely on both sides. It's still an analogue filter; in the next column we'll look at issues involved with turning it digital – in the same spreadsheet – then implementing it and analyzing it. Let me know if this technique could prove useful for your filter curve-fitting – Kendall.
This guest column is written by Kendall Castor-Perry, Principal Architect, Precision Analog at Cypress Semiconductor. Kendall is an experienced analog designer, filter wizard, audio expert, systems engineer, technology marketer and product manager. Links to his previous columns can be found below:
One giant squeak for Mankind – Revisits an audio circuit project from 'Practical Electronics'magazine, May 1969.
Use it or slew it – Provides advice on how to determine the slew rate needed from an opamp.
An E96 formula: how can you resist it? – Kendall discusses the value of spreadsheets when calculating component values.
Ping! And the accuracy is gone – Where exactly does the ringing come from when you sample the input voltage of a high-speed ADC?
Alias, damned alias and statistics – Does 'aliasing' need fixing, asks our filter wizard, and does fixing it cause problems elsewhere?
Who, what and why? – Analog DesignLine Europe's expert columnist introduces himself.
