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ADA4930 oscillating with a feedback capacitor

Question asked by Gerlinde on Nov 23, 2017
Latest reply on Jan 30, 2018 by JinoL

I'm currently designing a ADC driver with the ADA4930. However I find the differential amplifier oscillating when adding a feedback capacitor in parallel with the feedback resistor Rf. How can I prevent this?

Above is a view of the diff-amp calculator including the supply voltages,  input levels & frequency and used resistor values.

The input is a +/-square wave of 40MHz base frequency. The 1st, 3rd, 5th and 7th harmonic must be amplified. The gain is set to 1. The gain and feedback resistor are 301 Ohm. It is important that the amplifier doesn't add to much noise to the signal. So the parallel feedback capacitor has a value of 3.3pF. Creating a 3dB point above 140MHz.

 

The load is a transmission line of +/- 20cm in to a AD9257. There is no low pass filter in front of the ADC. Only a 100Ohm resistor to terminate the transmission line. Not shown above are 2 10Ohm resistors in series with the both outputs. I have found that the shape of the output signal is bad if the output series resistor and the termination resistor aren't present. (This measurement was done without the feedback capacitor)

The input are 2 ADA4855 opamps out of the same package. The signal shape at the output of these single ended opamps is ok.

 

Can I prevent the ADA4930 from oscillating while limiting the bandwidth of the noise?

Would this issue be resolved by removing the stray capacitance at the input nodes? I must admit that in the current board design the planes are not removed underneath the input. 

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