# Is my instrumentation amplifier's voltage offset causing problems?

I am using an MCP6N11-100 to amplify the signal out of a 2 mV/V load cell with a excitation voltage of 10 V, so the maximum output (differential) would be 20 mV. The load cell is rated for 4000 kg.

I am using the instrumentation amplifier in the following configuration. Note that at the inputs I have the outputs of the load cell connected directly. Rf = 9.9 kΩ and Rg = 55 Ω giving a gain of ~181 (both resistor values are measured). VREF is grounded.

My problem is that the gain doesn't seem to be 181 until there is sufficient difference between the inputs of the instrumentation amplifier. Suppose the difference is just 0.5 mV. The op-amp outputs 200 mV giving a gain of almost 200! However, as the difference is increased to 2 mV the output now is 366 mV to 368 mV - matching the gain much more accurately (183 vs 200). Why is this discrepancy present at low levels of input? My guess is that it's the offset voltage. The datasheet suggests that the MCP6N11-100 has a maximum Vos of 0.35 mV. Could this be the cause?

• 0.5 to 200 is a gain of 400, not 200.
– Kaz
Commented Jul 8, 2013 at 15:54
• The guess that it might be an offset requires an important clue: what is the output voltage if the inputs are tied together.
– Kaz
Commented Jul 8, 2013 at 16:29
• When you did the experiment with 0.5 mV applied, how did you measure that input voltage to know it was exactly 0.5 mV? Commented Jul 8, 2013 at 16:47
• Voltage driven bridges are progressively non-linear too. Current driven bridges have about half the non-linearity. I'm pretty sure Kaz has nailed the problem though. Commented Jul 8, 2013 at 16:49
• @Kaz This is from memory, but I think it was 2.2mV. I'll have to take a second look at work tomorrow.
Commented Jul 8, 2013 at 16:55