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

(source: instructables.com)

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.

• Vss is specified at +/-6V max. I could never figure out if that means you can go 0-12V or not. Nov 12 '12 at 18:44
• Try raising REF and see if the "half gain" is still there. The output may be attenuated by being so close to the rail. Nov 14 '12 at 1:25

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.