AD7705, or other ADC in loadcell application.
Mark Adcock
I have recently 'inherited' a design that involves feeding the output
of a loadcell into an AD7705. The output of the loadcell is between
+/-15mV, the reference of the AD7705 is set to 2.5V and the internal
Gain to 128. For some reason the AD7705's output seems to jump about
rather erratically every minute or so (well outside the range of the
noise levels stated in the datasheet and measured when the output was
stable), and also drifts slowly. It is unusable for any sort of
weighing application.
Has anyone had any experience of these sorts of problems with this
chip before?
As i can see it there are two solutions:
1) use an external amplifier for the signal from the loadcell and set
the AD7705 to use a lower gain. I expect that a high quality (ie
expensive) amplifier may be required to maintain the linearity of the
output of the loadcell and keep the noise to a minimum.
2) use a loadcell specific chip like the AD7730 (which costs twice as
much as using an AD7705), without an external amplifier.
the circuit must provide 1 part in 8000 accuracy on the output, in
either direction. (I make this about 14 bits)
***
also, another question (sorry), i presume that the AD7705 is a
relatively standard part. However the AD7730 appears to be just the
sort of part that would be discontinued without warning. Are there any
clones of either chips, or is it very unlikely that Analog Devices
will discontinue either? What other companies make relatively cheap
16-bit ADCs. As you may have guessed the product that the ADC is for
may be around for quite awhile (although the run is likely to be quite
small), so for any of the parts to become unavailable would be a
disaster.
many thanks,
Mark Adcock
Griffith Elder & Co. Ltd