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Unexpected Gain with AD8334 on AD9252-50EBZ eval board

Question asked by GrumpyGoat on Sep 7, 2014
Latest reply on Sep 8, 2014 by JinoL

Hello everyone, I have an AD9252-50EBZ evaluation board. The board contains two AD8334 VHF VGAs that can optionally be placed into the front-end signal path into the AD9252 ADC by changing some resistor values on the board. I've followed the directions in the AD9252 data sheet for activating the AD8834s, however, I'm seeing an unexpectedly low amount of gain at the input to the ADC. The AD8334 has been placed into HI gain mode (55.5 dB of gain) with a negative gain slope and the VG input tided to ground (maximum gain). I'm inputing a -40 dBm sinusoid (50 Ohms) at the SMA connector and probing the board at R109 (differential input to the ADC). The differential signal after the amp and into the ADC has a Vrms of 570mV (285mV single ended) which gives a power of ~8 dBm into 50 ohms for a total gain of 48 dB.

 

I don't understand why I'm not seeing the 55.5 dB of gain specified in the AD8334 data sheet. I suspect it has to do with the fact that the ADC does not have a 50 ohm input impedance, but I'm unclear on how I can predict the Vrms input to the ADC based on the known input signal and gain of the AD8334. If someone could shed some light on this problem I should be very grateful!

 

Thanks,

-Cody

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