[sdiy] SSM2164 Filter Cells

[Neil Johnson]

Hi Sam,

> Thank you very much for correcting my wrong word usage,
> and much better explanation of the datasheet schematic.
> I agree with you.

Thanks!  Re-reading what I wrote, I think what I meant was not

> > So not 'essential', but perhaps 'dumb'.

but

   "essential ... using a resistor would be dumb"

The circuits using CA3080 or LM13700 work with just a capacitor on the output due to the wide output compliance of those chips.  The 2164 has a very limited compliance, but actually having an op-amp I-V converter on the output path makes a lot of sense - summing works without any pesky resistors (indeed, AD arranged all the output pins to support this easily in the PCB layout), and you invariably end up with some sort of output buffer anyway.

In a strange twist I only discovered your all-pass filter design _after_ designing my own, and coming to the same design (although slightly different resistor values).

Cheers,
Neil
--
http://www.njohnson.co.uk