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I am trying to amplify the signal from a load cell using an AD623 instrumentation amplifier.

schematic diagram
(source: instructables.com)

Please refer to https://www.instructables.com/answers/Amplifying-load-cell-signal-with-AD623-problem/ for the schematic. The common mode voltage is 4.5 Volts.

1K resistor is supposed to give 100x amplification. It is instead giving me around 50. My differential voltage is 0.8mV while my output is 42mV (52.5 gain). Note that I have placed the capacitors (0.1 uF and 10 uF from Vs to ground) but are not shown in figure. What might be going on? Is there a chance the AD623 is ruined? I don't think I'm doing anything wrong here, plus I've read a lot of blogs and haven't figured it out yet. Note that when I double the differential voltage, the amplification remains around 50. And when I change the resistor to 200ohms (500x multiplication according to datasheet), I get around 250x multiplication which is also half. So amplification is quite accurate, but half of what is said in datasheet.

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  • \$\begingroup\$ Vss is specified at +/-6V max. I could never figure out if that means you can go 0-12V or not. \$\endgroup\$ Nov 12 '12 at 18:44
  • \$\begingroup\$ Try raising REF and see if the "half gain" is still there. The output may be attenuated by being so close to the rail. \$\endgroup\$ Nov 14 '12 at 1:25
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It looks like you have a bipolar differential input signal, but your output can only be unipolar because the REF pin is tied to -VS. If your input is AC, your output would be clipped at zero, so you're measuring it as half what you expect. Try moving REF up to 5V maybe.

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Your Common mode voltage is 4.5V as you mentioned. If you check the data sheet of AD623 you can find on page no.8 fig.22 that your common mode voltage is way more.enter image description here

This is the image from AD623's datasheet http://users.ece.utexas.edu/~valvano/Datasheets/AD623.pdf

You could do one thing, reduce the excitation voltage in order to bring the common mode voltage to be near 1.8V and then have more gain as per the mV/V rating of your load cell to get the desired output.

I came across the same problem but the mV/V rating of my Load cell is very less. I had to change the amplifier package. AD8420. I am still testing.

If you find a better solution please let me know.

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