At 17:36 1998-02-14 -0600, you wrote:
>>I am prototyping a load cell application based on PIC14000.
>>(internal single slope 16 bit A/D)
>> Works well so far.
>>I have not done the calibration routines yet, though...
>>The temperature drift is significant, so even if y don«t need exact level I
>>will have to recaculate values against values measured from internal
>>bandgap reference.
>
>Can you use the internal bandgap reference as the bridge drive?  That
>should make the measurement ratiometric, eliminating most of the
>temperature effects.

No... Yes... It is possible to let the internal MUX output connect to a pin
as well as the analog comparator, so you can select the reference and
output to that pin, if you there have a cap it can hold the reference
voltage while you are measuring another input.  You have to buffer it and
increase voltage, of course.  Since I have a very stable power supply I use
that directly to load cell.  

>Of course, that won't help if the drift is due to the tempco of the
>integration cap in the single slope converter.

Best integration cap is a polycarbonate.

The temp drift is dependent of drift in internal current generator, and
since I use internal oscillator I have that drift too.

The best way is to use soft routine to calculate the voltage using the
Reflo and Refhi voltages internally derived from bandgap. There are app
notes on that. I have not yet implemented that.

To optimize it further we can use a resistive divider to measure load cell
supply, and divide the load cell output with that.

/Morgan

>newell

/  Morgan Olsson, MORGANS REGLERTEKNIK, SE-277 35 KIVIK, Sweden \
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