[sdiy-market] [FREE] Anti-aliasing filter PCBs for LTC chips

[Steve Lenham]

Hi,

I recently developed an alternative to the obsolete Murata AFL89 
anti-aliasing filter module (used in some early digital effects units, 
etc.) and have some bare PCBs left over from my experiments that others 
might find useful. The boards are free to anyone with a use for them; 
all I'd ask is that you cover the postage to wherever you are.

1. PCB for two LTC1563 filter ICs (12 available):

LTC1563 PCB

My first attempt! This uses two LTC1563 filter ICs in series plus a 
passive section to give a 9th order Butterworth lowpass response 
designed using LT's FilterCAD. I can supply the schematic and parts 
layout plan for a 20kHz cutoff or values can be scaled for a different 
cutoff.
The board is tiny (37 x 16mm) and includes regulators to derive +/-5V 
for the filter ICs.
I never actually built these, because I realised that a Butterworth 
response was not going to be sharp enough for my needs. But it might be 
for yours - or for just messing about with the LTC1563.

2. PCB for one LTC1562 filter IC (12 available):

LTC1563 PCB

My second attempt. This uses a single LTC1562 filter IC plus a passive 
section to give a 9th order Elliptic lowpass response designed using 
LT's FilterCAD. I can supply the schematic and parts layout plan for a 
20kHz cutoff or values can be scaled for a different cutoff.
The board is tiny (37 x 16mm) and includes regulators to derive +/-5V 
for the filter IC.
I built several prototypes using these boards (see separate posting) and 
the response was perfect - just like the AFL89. But sadly the noise 
floor of the CMOS filter IC running off +/-5V, while not bad at all, 
could not quite match the original which was built using separate opamp 
stages running off +/-15V. If that is not an issue, these are usable 
as-is, or could just be a testbed for playing with the quite versatile 
LTC1562.
If you're feeling very keen, i even have a steel paste stencil for 
populating these...

(And if you are wondering, my THIRD attempt was a direct clone using 
opamps. Both the response and the noise were spot on, and I have since 
built hundreds of them).

I know these are a bit specialist, but please get in touch with any 
interest or questions!

Cheers,

Steve L.
Benden Sound Technology



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