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'12 Bit ADC for LOAD CELL APP'
1998\02\13@124017 by Kursat Guler

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Hello ,

I am looking for a 12bit ADC for PIC based load cell application.
I am currently trying to use National's ADC12030 with PIC16C74. Please
tell me If you know cheaper solution for same resolution.

Thanks.

Kursat

1998\02\14@050744 by Morgan Olsson

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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.
/Morgan

At 19:25 1998-02-13 +0200, you wrote:
{Quote hidden}

/  Morgan Olsson, MORGANS REGLERTEKNIK, SE-277 35 KIVIK, Sweden \
\  spam_OUTmrtTakeThisOuTspaminame.com, ph: +46 (0)414 70741; fax +46 (0)414 70331    /

1998\02\14@183922 by Scott Newell

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>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.

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


newell

1998\02\15@052752 by Morgan Olsson

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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 \
\  .....mrtKILLspamspam@spam@iname.com, ph: +46 (0)414 70741; fax +46 (0)414 70331    /

1998\02\15@064756 by Alberto Smulders
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Did you consider using Analog Devices AD 7730 ? No hassle, includes
excitation circuit, input amp, AD converter, programmable digital filter,
serial interface to microprocessor..... and only for about U$ 10 in 1000
quantities..... you can't build it with discrete parts for that money....
I have some (free) samples I got when I was in Europe last month, now I'm
working on a prototype for weight scale applications - but I think I'm going
to use an Atmel AT89C52 micro (sorry, no PIC), together with a serial eeprom
for storing calibration parameters, a MAX232 chip for serial communication
with a PC, and a LCD display....

Albert A. Smulders
InSAD - Encarnaci—n, Paraguay
insadspamKILLspamitacom.com.py

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