I have used the single slope ADC concept to consistently measure the value
of the thermistor to within 0.1 degree C and the final result worked very
well.  I was measuring the charging time of the cap, and comparing it to a
fixed reference in order to cancel supply voltage variations.  Actually it
was slightly more involved than that, the first version compared the
thermistor to the value of a digital pot by which was used to adjust the set
point of the control loop.  This worked well.  The second version had the
set point stored in software, and the digital pot replaced by a fixed
reference.  This didn't work so well!

One thing I found was that using an ICE (both ICEPIC2 and MPLAB-ICE) totaly
screwed the readings and introduced large amounts of jitter on the results.
This wasn't so bad for my development as I was dealing with a control loop
with a large time constant (several seconds) and the jittered results tended
to average out.  Using a proper PPIC on the PCB totaly cured the jitter
problems.  It looks like the ICE introduces a fair amount of noise onto the
port lines, which under normal (i.e. digital) use would probably be
unaffected.

Regards

Mike

> -----Original Message-----
> From: David VanHorn [SMTP:dvanhorn@CEDAR.NET]
> Sent: Sunday, April 29, 2001 9:13 PM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Re: [EE]: How to convert Resistance to digital data directly
>
> At 08:16 AM 4/30/01 +1200, Russell McMahon wrote:
> >  What sort of caps are you using?
> >Presumably Mylar or similar rather than eg ceramic.
> >
> >Does transferring this from the ICE to a "real" PIC produce an identical
> >result?
>
> Difficult. I've only got one board, and it's SMD.
>
> >Presumably when you say "non-linearity" you mean non linear compared to
> an
> >exponential curve.
> >
> >Might it help to eg go low, then high, and time for say 5 time constants
> >after the low to high transition and then go low again and count the time
> >taken?
>
> That's what I'm doing.  Charge for long time (verified on scope to flat)
> then discharge and time.
>
>
> >That way, even though the 5 time constants "should" pretty much have
> >the cap at top rail, driving it there for a consistent period after it
> >passes through low-high threshold may reduce variability.
>
> I'm charging it way into the flat.
> --
> Dave's Engineering Page: http://www.dvanhorn.org
> Where's dave? http://www.findu.com/cgi-bin/find.cgi?kc6ete-9
>
> --
> http://www.piclist.com hint: The PICList is archived three different
> ways.  See http://www.piclist.com/#archives for details.
>
>

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email listserv@mitvma.mit.edu with SET PICList DIGEST in the body