Chip Weller wrote:

> By the way, the ADC method covered in AN512 doesn't strike me as
> providing 16-bit result quality. If you are trying to create that sort
> of resolution you probably are fooling yourself.
>
No, I don't think 16 bit resolution is realisticaly acheivable, the schmitt
trigger in the PIC was never designed for this use and I suspect dosen't
have terribly precise switching points.  However, I am getting around 9 1/2
bits over the temperture range of interest (15 to 45 degrees C) which is
more than adequate.  Thanks for the tips on digital filtering, it's
something which I would like to investigate further.

Mike Keitz wrote:

> This is a good idea.  A little logic could be added to handle sudden
> large changes in the input.  If most of the samples are being rejected,
> the rejection process should be bypassed until the average becomes close
> to the new value.  One simple technique would use a counter that counts
> up when a sample is rejected and down if it is accepted.  If the counter
> rises above a threshold, start putting all samples into the average, but
> process everything else the same.  After enough new samples have been
> averaged, the count will reduce below the threshold and the outlying
>       sample rejection will start working again.
>
Hmm..good idea, thanks Mike.  Given another few days of continuous bashing
of my head against the lab wall, I might have come up with this, you've
maybe saved some pain :)

Lawrence Lile wrote:
        The next thing to try might simply be analog filtering of your
        signal.  Nyquist will tell you that you have to sample at more than
        twice the maximum frequency of interest, and it is a good idea to
        make sure that no frequencies above that are present.  A couple of
        caps might just solve your problem adequately.   Often we are
        looking for signals that change over seconds, and measureing them at
        megahetz.  In this case a simple decoupling cap might be all that is
        needed.

The trouble is, the adc technique outlined in AN512 tends to preclude the
use of analogue filtering, adding a cap to the thermistor would just screw
up the timing.  You wouldn't have thought that the adc would be that prone
to noise as it has a reasonable sized cap (440nF in my case) straight to
ground on the sense wire.  Goes to show I don't know what I'm talking about
I guess!  I have to agree with your comments on emulators and noise though.
I guess it's time to get out the JW parts and see what's really going to be
happening.

For anyone using AN512 for tempertaure control, can I suggest that if the
system is a closed loop, then the thermistor is linearised, either
electronically or in software.  I used an interpolated look up table and the
loop stability was improved massively.  Not surprising when you look at the
thermistor characteristics.  The slope at the lower temperature range was
around 8 times that at the top end. Trying to tune the loop was a nightmare
until that was sorted.

Another hint, be carefull with emulators, the ICEPIC 2 can leave all the pic
periperals running when the core has actually stopped (selectable option).
Trouble is I didn't actually notice until my peltier cooler had reached
about 145 degrees C.  Strangely it seems to have lost a certain amount of
efficiency :o).

Regards

Mike Rigby-Jones
mrjones@nortel.co.uk