amplifier Amp measures board twist (unfortunately!)

Posted on Feb 6, 2014

A board design that is supposed to measure a loadcell. We have finally tracked a system accuracy fault down to the amp IC. When we twist the board, the amp IC changes its output. I get +80mV when I twist the board from its 4 corners. I`m using the amount of twist that I use to unlock my car with my car key. I get -80mV when I twist the other way. The amount

of twist is proportional to the variance in output voltage. To isolate the amp circuit, I have shorted its input and disconnected other circuits from it so that what you see in the diagram is what we`re testing with. If you can, a photo of the board would help us see physical factors that could contribute to the issue. Are the passives leaded or smt, and what size Where is the circuit located on the board, in the center or near an edge The Photon Apr 23 `13 at 5:20 I`d guess you ARE measuring a load cell - made with copper tracks, although the voltages seem far larger than reasonable. As shown your IC has no common mode restraint on the input circuit (pins 2 & 3, shorted). Look at the datasheet table 8 page 21 and the several pages of related comment preceding it and ensure you are not violating any limits there. (THey MAY just be putting common sense in a complicated way - hard to be sure without wading through it. My wife says its time to go and buy the Subway 12" special offer, so. ) Russell McMahon Apr 23 `13 at 5:58 Re: ". Good point. I should have shown on the schematic that my shorted inputs are still connected into the 350 ohm loadcell bridge. So that should take care of this problem. . " Good. Another step forward. In a few more days we may have half the circuit diagram :-). It would be `really wise` to show us the circuit in entirety at least as far as all voltages and currents are concerned. What is the value of...

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