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AD4020 eval board external supply


I wanted to externally power the AD4020 eval board. After going through the datasheet of the eval board I figured out that I had to do the following changes.

Switch SL9 for +Vs

Switch SL8 for -Vs

Switch SL7 for Vdd

Switch LK2 for Vref

On the eval board it looks like

+Vs = 7V

-Vs  = -2.5V

Vdd = 1.8V

Vref = 5V

I provided the following voltages

+Vs = 5V

-Vs = -5V

Vdd = 1.8V

Vref = 5V

After doing these changes I connected it to the ADI-SDP board to test it. I gave a 40kHz sine wave as the input to the ADC. However this is the output I got 


The output is varying between two codes (0 to -1) when the input is a sine wave of 200mV amplitude and 40kHz frequency. However the frequency of the output is correct. So I thought I blew up the ADC while doing the changes to the board. So I replaced the ADC and tested it again. The same problem persisted, only this time the output was a square of the right frequency but was varying between 0 and -500000 instead of 0 and -1. 

Any idea why this is happening? What am I doing wrong? I also tested to see if the signal was actually reaching the ADC properly. And it is. So I am clueless as to what is wrong. Please help me debug this.


Mohan Prabhakar 

Adding tags for internal tracking purposes
[edited by: tschmitt at 3:53 PM (GMT 0) on 25 Feb 2020]
  • Hi Moran,

    Thanks for your patience as we continue to try to solve this.

    I think this is my last "analog" question - does the code output issue occur only when your signal generator is connected to the EVAL-AD4020FMCZ inputs (i.e. the J7/J11 SMA connecotrs)? Or does it happen even when J7/J11 are not connected to anything? In the EVAL-AD4020FMCZ default configuration, if J7/J11 are not connected to any external source, then the AD4020 inputs will just be driven to VREF/2, and it should return a mean code of close to 0.

    The reason I ask is that it's possible that depending on the output stage of your signal generator, it's possible that the ADA4807-1 inputs are getting dragged down to ground or some other voltage when it's connected. I know you said you verified the AD4020 IN+/IN- inputs are at 2.5V already but I just want to double check that this isn't the problem before we start debugging on the digital side.

    In the meantime, you could also probe the SDO output of the AD4020 to see if it really is outputting the codes you're seeing in the eval software, or if there's some issue with the FPGA board as you suggested. I recommend probing CNV, SCK, and SDO and seeing what the state of SDO is on each SCK falling edge to see if the results displayed in the eval software match reality.

  • Hi taylor,

    When the signal generator is not connected to the eval board, I typically see the output code to be 0. But before this whole issue started showing up, I would see a noise worth 60-70 codes around 0. However, after this issue, I just see a straight line of code 0.

    I tried even that. The SDO is definitely toggling the way it's supposed to. But since the clock is not triggering properly (i.e, instead of 20 pulses, it's kind of showing up as one big pulse) we are getting an ENOB of 2 (0 and -500000). Hence, the output is following the frequency but not able to capture the wave. This was my conclusion after probing the SPI pins. Does this make sense?

  • Hi Moran,

    Thanks for the info. If you're seeing this issue even with a brand new EVAL-AD4020FMCZ board when the signal generator is connected, then you're probably correct about it being an issue with the SPI. More specifically, with the SDP-H1 board for some reason. It's possible it was damaged when switching from the on-board supplies to the external ones, though I can't say for certain.

    Just out of curiosity, when you say "SDO is definitely toggling the way it's supposed to," do you mean that the output codes you see on SDO match the 0 and -500000 you see in the AD4020 software, or that they are roughly code 0 with some noise around them?



  • Hi Moran,

    I've moved your query over to the SDP Support thread to make sure you get support from the SDP team, since we've narrowed down the problem to your SDP-H1 board. Someone should be able to help you out.



  • No they are toggling between 0 and -500000