piclist 2001\02\17\181726a >
Thread: ETS on a PIC ??? Yes - it can be done.
face BY : Dan Michaels email (remove spam text)

Robert Rolf wrote:
>Bear in mind that the series resistance of the sampling switch (6k at
>5V) and the value of the hold capacitor (51.2pf for a '76) will band
>limit your signal to under 500khz.

Hi Robert,

Ah good, I see someone else is familiar with the PIC A/D internals.

Based upon the published info, I had expected to see a fairly good
BW in the A/D, something somewhat commensurate with

F = 1/(2*pi*7K*51pF) = 440 khz

However, I was surprised to measure only about 120 khz BW [external
influences being taken into account] for the 'C76. And the 'F876,
which has a 120 pF sampling cap, registered only about 60 khz BW.

In addition, the A/D gain internally falls off at about -60 dB/decade,
which makes a really nice anti-aliasing filter when you are doing ETS
at 1 mhz, but I was hoping for a little more BW.

Interesting that the measured values are so far off the 1st order
approximation. Got any clue as to why?

best regards,
- dan michaels

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