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PSoC™ 4

shge_4395051
New Contributor II

Hello,

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.

Is there a function that describes this behavior? 

thanks

Shawn

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odissey1
Honored Contributor II

Shawn,

Take a look at this thread,

Re: Trying to configure the OpAmp for Mode:OpAmp and Mode:Follower at run-time

For PSoC5LP Opamps are low impedance. PSoC 4 is likely to be similar. Try to use resistor divider of lower values (/10) .

/odissey1

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