- anti-aliasing filter. Looking at it I'm not sure whether a 7th order filter is a good idea due to phase shifts.

- single ended to differential amplifier and analog offset

- gain control block and ADC.

What's required to preserve waveform fidelity is the flattest phase delay, a Bessel-Thompson filter. I've spent a lot of time studying the subject. It's not well treated in the literature, but well documented in scopes. Of course, all high order analog filters are problematic to produce. Though with an FPGA one need only get close and the FPGA can trim the last bit.

As the order goes up the Bessel-Thompson passband approaches the Gaussian passband. So the impulse response of a 10th order Bessel-Thompson is approximately a time delayed Gaussian spike.

Tom is the reason I dropped work on hacking the Instek GDS-2000E after I blew the scope. It no longer made sense to replace it.

Have Fun!

Reg

There are a class of filters with a linear-phase (Bessel-like) passband and an equiripple stopband, originally described by Feistel & Unbehauen [1], for which Williams [2] gives some limited design tables: I have used these with success as anti-aliasing filters where waveform fidelity is important. There is a conference paper by Huard

*et al.* [3] - which I don't have a copy of, unfortunately - that describes more recent progress in similar filter designs. Huard worked at Tektronix, which may be a clue about the applications being addressed!

[1] Feistel, Karl Heinz, and Rolf Unbehauen.

*Tiefpässe Mit Tschebyscheff-Charakter Der Betriebsdämpfung Im Sperrbereich Und Maximal Geebneter Laufzeit*. Frequenz 19, no. 8 (January 1965).

https://doi.org/10.1515/FREQ.1965.19.8.265.

[2] Williams, Arthur Bernard, and Fred J. Taylor.

*Electronic Filter Design Handbook*. 3rd ed. New York: McGraw-Hill, 1995. ISBN 978-0-07-070441-1

[3] Huard, D.R., J. Andersen, and R.G. Hove.

*Linear Phase Analog Filter Design with Arbitrary Stopband Zeros*. In [Proceedings] 1992 IEEE International Symposium on Circuits and Systems, 2:839–42. San Diego, CA, USA: IEEE, 1992.

https://doi.org/10.1109/ISCAS.1992.230091.