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ADA4940-2 noise issue

Question asked by TPiggin on Nov 28, 2013
Latest reply on Oct 26, 2015 by Stvn

Hi, I am having a very similar problem to the person in this thread...


I'm using the ADA4940-2 with unity gain (Rf = Rg = 499R) to buffer 2 differential voltages from an RF demodulator. I'm using a 16-bit ADC and when i short the inputs of the adc together i get 3-4 counts of noise so around 150uVpp, which is fine. When i transfer the short to the input of the ADA4940 amp i get a similar level of noise. This is with the short connected to ground though (not the case for when the ADC was shorted). I then connected an LDO output to the input of the amp and the noise on my adc inputs is now 250 counts! still centred around zero counts due to Vdiff = 0, but with noise in the region of +/-125 counts (9.5mVpp). Any suggestions why the noise has increased dramatically even though the differential voltage is still zero and the LDO output is very stable as this was tested directly on the ADC with the results being very similar to when a short to ground was used...