#### elsalty

##### Newbie level 4

https://www.analog.com/static/imported-files/data_sheets/AD8651_8652.pdf

indicates the GBW is 50MHz, the input bias current is 10pa and on page 9 shows the closed loop gain for G=1 out to beyond 50Mhz. The load is the non inverting input of a second AD8651 (i.e. very high impedance). I wanted to use it with several gain options so the feedback resistor (Rf) is 10k and the resistor to Vmid (2.5V) is either 1)open for unity gain, 2) 10k for gain of 2, or 3) 4.99k for gain of 3. I have just been testing with the unity gain and inputting a 0.5 Vpp sine wave centered about Vmid (2.5V). When I increase the frequency to beyond about 50khz the output magnitude starts to increase significantly. By the time I am at 1MHz the output signal is 1.5x the input signal. I have found that if I replace the 10k feedback resistor with a 0.0 ohm resistor this problem goes away.

I was just wondering if anyone could help me figure out why this is happening? If I had to guess I would assume it is something with the input capacitance (6pF) but I am not sure. Thanks for any suggestions.