> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On
Behalf
> Of Jinx
> Sent: 13 August 2008 04:46
> To: Microcontroller discussion list - Public.
> Subject: [PIC] A/D impedances
> 
> Just out of curiosity (and a little $$$ saving), is a capacitor
> acceptable as a low-impedance "buffer" for inputs ?
> 
> d/s says, for Vref for example
> 
> "In order to maintain the A/D accuracy, the voltage reference source
> impedances should be kept low to reduce voltage changes. These
> voltage changes occur as reference currents flow through the reference
> source impedance. The maximum recommended impedance of the
> VREF+ and VREF- external reference voltage sources is 250R"
> 
> I'm revising a circuit that currently uses op-amps and voltage
reference
> ICs, and wonder if much of that can be dispensed with. The signals to
> be measured are fairly steady DC voltages, on a minutely basis
> 
> The ADC sampling is over in a few microseconds, so if the answer to
> my first question is "yes", then what calculation is needed for the
value
> of C ? Or would one simply increase the acquisition time ?
> 
> I've already trialled a 48V input using a 910:10k divider (FSD is
> 61.44V) followed by a 47nF capacitor, and can't see any difference
> between that and with the existing op-amp buffer in place, even with
> the very short acquisition time of a 40MHz PIC

A capacitor can help with the Vref transient current demands during
acquisition and conversion, but it won't help if the low source
impedance requirements are driven by quiescent and/or leakage currents.

Regards

Mike

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