Load Cell to Arduino

I put an AD7730 chip on the breadboard, got it interfaced to the Arduino, 
and am getting numbers from a load cell.  This took some playing with to get 
working almost correctly but I'm getting readings that look good, 5 grams on 
a 30 Kg load cell is producing about 25 counts from the A/D value / 128. 
Should be able to do a little better with actual calibration instead of 
internal reference calibrations.

Next I'll write some calibration and tare functions plus conversions for 
lbs, oz, grams, Kg's, grains, etc.

One concern I'm having so far is sometimes the weight reading go down with 
the addition of a little weight, not sure if this is because of a bad load 
cell or if there is some misreading from the AD7730 chip.  I tried using all 
SPI modes from the arduino library, I couldn't get mode 0 to work at all, 
mode 1 works most reliable, mode 2 and 3 both work but the chip locks up 
after a few readings.

RogerN

On 2012-03-14, RogerN <regor@midwest.net> wrote:

> One concern I'm having so far is sometimes the weight reading go down with 
> the addition of a little weight, not sure if this is because of a bad load 
> cell or if there is some misreading from the AD7730 chip.  I tried using all 
> SPI modes from the arduino library, I couldn't get mode 0 to work at all, 
> mode 1 works most reliable, mode 2 and 3 both work but the chip locks up 
> after a few readings.

That part is based around a sigma-delta DAC and so has no reason not
to be monotonic.  I'd try substituting a different source that you
know to be monotonic - eg: a pot in series with a big resistor -
and repeating your measurements. bracketing the discontinuity may give
some information as to what the cause is.



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