I'm using the PSOC 4200M. I designed a circuit using an opamp in non-inverting mode. The Gain resistors are off chip. I noticed that as I increase the resistors past a gain of one, the gain appears to increase in a non-linear fashion. I'm using large value resistors to keep the current low. I am also using an Analog mux to switch in the feedback resistors.
My guess is the internal resistance to the AMUX is a function of current. Here are the resistor values I'm using:
Rin (negative terminal) = 1 M Ohm
Feedback resistors switched in (one at a time) = 1 M ohm, 2.49 M ohms, 7.2 Mohms
These values equate to gains of 2, 3.49 and 8.2. However, the actual gains calculated from measuring the input and output voltages of the opamp are 2, 4.38 and 11.59 respectively.