I am working on a circuit that amplifies small input voltages. The input voltage is 50 mV, and the gain should be 100. The amplifier is an instrumentation amplifier (in-amp) (AD623) (datasheet: http://www.analog.com/media/en/technical-documentation/data-sheets/AD623.pdf).

I have attached a schematic of my circuit. For reasoning behind some of the components of my schematic, see Table 5 and Figure 43, both located on Page 16 of the AD623 datasheet.

The output voltage should be 5 V, but is ~1.25 V. This output voltage is independent of the power supply voltage. That is, no matter how high or low I set the power supply voltage, the output voltage is ~1.25 V as long as the input voltage is 50 mV.

For gains less than 100 (e.g. gain of 5), with the appropriate resistor Rg, the circuit works as expected. However, for the higher gain of 100, which uses a lower Rg, there appears to be a cutoff output voltage of ~1.25 V. I hypothesize that this finding is consistent with Figure 23 on Page 11.

Please help me observe the correct amplified output voltage of (50 mV) x (100) = 5 V.