[sdiy] dsPIC CV input protection

Mattias Rickardsson mr at analogue.org
Tue Nov 18 19:50:58 CET 2008


2008/11/17 Seb Francis <seb at burnit.co.uk>:
> harrybissell at wowway.com wrote:
>>
>> I don't know what kind of A/D input it is... some are
>> switched capacitor in which case adding series resistance can
>> lead to serious errors... :^)
>
> The dsPIC datasheet specs a max source impedance of 1K IIRC. It has a S&H
> buffer with very small capacitor that is charged over a certain period.

1k is not easy to deal with unbuffered, I guess...
For comparison, I glanced in an Atmel AVR datasheet, which says:

  "The ADC is optimized for analog signals with an output impedance of
approximately 10 kΩ or less. If such a source is used, the sampling
time will be negligible. If a source with higher impedance is used,
the sampling time will depend on how long time the source needs to
charge the S/H capacitor, with can vary widely. The user is
recommended to only use low impedant sources with slowly varying
signals, since this minimizes the required charge transfer to the S/H
capacitor."

I guess "too high" impedances could still be useful in some
applications, but then it's important to know what one is doing.

> considering putting a gently LP filter (small series resistor, cap to
> ground) in between to help with HF noise.

HF noise is good for dithering. ;-)


/mr




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