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ad8331 + ad8331 + adl5511

Hi all

for a project I need to be able to amplify some signals in order to drive an ADC. I experience some problems/strange behaviour but perhaps I'm making some wrong decisions.

Setup:

My signals are three times amplitude modulated signals of max. 150uV, around 250kHz modulated at 10kHz. I'm actually only interested in the envelope (10kHz)

I was thinking of cascading for each signal two times an AD8331 to reach a gain of about 80dB, then send them through an ADL5511 in order to get the envelope of the signal and then feed them to an ADC (probably AD7476A). I've already bougth 1 AD8331 eval board and 1 ADL5511 eval board.

Questions:

  1. Is this a good practice to place two LNA/VGAs in a row to get to the 80dB?
  2. When I try connect my signal generator directly to the eval board of the ADL5511, I can probe a nice envelope (with an offset of 1.1V, yet I think this could be because of signal generator and scope in one device? That it is not taking the EREF as ground of the probe?). But when I connect the output of the AD8331 board to the input of the ADL5511 eval board, the output is attenuated a lot. Is this due to impedance mismatch?
  3. Is using an AD7476A a good option or are there better choices? I want to interface it via SPI, but am limited in port speed (approx 20MHz) so I need to limit myself in bits and must have parallel conversion...

A lot of questions, I know, sorry therefore :-)

Greetings and thanks!



bad PSU was reason I couldn't access the GAIN function of the AD8331
[edited by: wouter.devriese at 2:28 PM (GMT 0) on 18 Apr 2019]