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High value resistors in AD8622 differantial mode amplifier configuration

Question asked by mustafa on Dec 27, 2015
Latest reply on Dec 28, 2015 by harryh

I'm applying two sinusoidal signals to the inputs of a AD8622 which is configured in differantial amplifier mode. The applied signals swings from -1.65V to +1.65V @1KHz. The applied signals have a phase difference of 180 degrees. I'm using amplifier to measure the difference of the input signals. The amplifier has a gain of 3 and has dual supply of +-12V. When I use resistors below 1M or around to configure the opamp, there seems no problem at the output. But when I increase the value of the resistors to say 5M or 10M, then the output signal has a component oscillates within the original signal like @100KHz with a half or one third of the amplitude of the original signal. I need to use high value resistors and cannot use instrumentation amplifier configuration due to space constraints. What may be the problem and what can I do?