About the minimum aquisition time, I've measured and made a graph of the
binary value
(0-1023) from the AD conversion, charging Chold from 0 to 5V and dischargin
from 5 to
0V, using simple series resistors of 100, 470, 1K, 4K7 and 10Kohms. This was
in a
16F877, but I think this would be very simmilar in other PIC's. In real
applications,
Chold would charge and discharge faster, because the voltage in the AD pin
will be
between 0 and 5.

You can get a jpg of the graph in my ftp:

ftp://piclist:piclist@xurel.eps.cdf.udc.es/pub/tad.jpg

As you'll see, the TAD is not the same charging and discharging. There is a
constant
slope first stage, and an exponential-like second one.

Hope this helps,

Alvaro Deibe Diaz

----- Original Message -----
From: "Vasile Surducan" <vasile@S3.ITIM-CJ.RO>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Thursday, February 21, 2002 7:51 AM
Subject: Re: [PIC]: A/D input impedance > 10K


> On Thu, 21 Feb 2002, Dave Dilatush wrote:
>
> > Spehro Pefhany wrote...
> >
> > >What would the consequences be of going to 20K on the midrange 10-bit
A/D
> > >input? I've read the reference manual and the data sheet. If the input
is
> > >bypassed with a 0.1uF or something of that ilk, it seems to be
primarily
> > >limited by the worst case leakage current on the input vs. guaranteed
> > >accuracy. I'm willing to lose 1 bit or 1.5 bits in the A/D, and high
> > >temperature operation is not required.
> > >
> > >Any thoughts on this?
> > >
> > >I'd normally put an op-amp in there without much thought, but the
quantities
> > >are such that thought will be well rewarded if a few cents can be
snipped.
> >
> > The only other error I can see you getting, other than the drop across
> > your 20K resistor due to input leakage, would be a scale factor error
> > that can occur if you have the input bypassed and are sampling it at a
> > high rate.  But if you're not looking for full 10-bit accuracy I can't
> > see this as a big issue.
> >
>   You need also  ( according to datasheet ) to increase also the minimum
> required aquisition time. It's possible ( not tested yet ) to keep the
> full 10 bit accuracy. The leakage current from datasheet is +/-500nA.
> I think is a mistake or an absolute maximum rating.
> Vasile
>
> --
> http://www.piclist.com hint: PICList Posts must start with ONE topic:
> [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads
>
>
>

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads