RE: AD8135 as input buffer for AD9235 ADC

Question

We are using AD8135 as input buffer for AD9235 80 MS/s differential input ADC. 
 The problem appeared when we try to use it as a single ended input buffer to differential output. 
 Example: Vcomm = 0.8V (Vdd+ = 3.3V, Vdss = 0V). 
 Vin = 1V, we expect: 
 +OUT to be 0.9V, 
 -OUT to be 0.7V 
 -OUT never goes below 0.8V. 
 To fix this we had to add negative supply. This is NOT mentioned in the specs!!! 
 Can you confirm, or we did something wrong?

krisf · Accepted Answer

Hi Dimoto, 
 The AD8138 cannot fully go to the rails at its output. It requires a headroom of about 1.1V from the supplies to give the correct output voltage. This, I think, is the reason why you cannot go any lower than 0.8V. The -OUT pin is already being limited by the required headroom. Please see this link . On page 4, the tables will give you the Vocm ranges for different supplies. 
 If you are only going for a differential output voltage of 1V and you really want to stick with a single supply, then I recommend that you increase your Vocm so that there is ample headroom from your supplies. Please note however, that operating at 3.3V limits the output range of the amplifier to ~1.1V to ~2.1V. I recommend that you check out DiffAmpCalc which is downloadable at this link . It enables you to play around with the Vocm voltages, gain-setting resistors and input voltages so you can accurately simulate your circuit. 
 If increasing your Vocm is not an option, could you give me the requirements of your circuit? I can try to help you find an amplifier that could be a better fit for your application. 
 I hope this helps. 
 Thanks! 
 Kris)

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RE: AD8135 as input buffer for AD9235 ADC

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