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sberner66
Observer
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Registered: ‎10-06-2018

ZCU111 loopback ADC overvoltage triggering

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I am working on a high bandwidth baseband application and have a loopback in place, DAC differential output pair to ADC differential input pair. 

The DAC works at 20 mA current, for the ADC AC coupling is enabled in the GUI.

The loopback works with 3 dB attenuators between. However, the SNR is too low to support long packets at high QAM orders and coding rates.

When I connect the DAC directly to the ADC, the ADC overvoltage protection triggers immediately as indicated by the real time status outputs from the ADC. The DAC is driven with zeros for this test. After reseting the ADC (FSM restart) the overvoltage protection triggers again immediately once the ADC is up.

What can be the reason ? Is this loopback connection "legal" ?

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sberner66
Observer
Observer
726 Views
Registered: ‎10-06-2018

root cause found.

I ran into this problem of OV triggering again when trying to connect real hardware to the ADC input. It turned out that the cause were 50 ohm terminations to GND, means each differential line had a 50 ohm resistor to GND. Therefore, is seems terminations to GND are not allowed.

When looking at the mini-circuit lowpass filters which come with the ZCU111, VLFX-1300+, the data sheet says 50 ohm, DC to 1300 MHz, so I would expect to measure 50 ohm to GND, however there is no connection. Can somebody explain this to me ? Are those filters modified for Xilinx ? Or is my understanding wrong ? If the filter is specified 50 ohm, including DC, I would expect to measure 50 ohm at DC ?

 

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sberner66
Observer
Observer
727 Views
Registered: ‎10-06-2018

root cause found.

I ran into this problem of OV triggering again when trying to connect real hardware to the ADC input. It turned out that the cause were 50 ohm terminations to GND, means each differential line had a 50 ohm resistor to GND. Therefore, is seems terminations to GND are not allowed.

When looking at the mini-circuit lowpass filters which come with the ZCU111, VLFX-1300+, the data sheet says 50 ohm, DC to 1300 MHz, so I would expect to measure 50 ohm to GND, however there is no connection. Can somebody explain this to me ? Are those filters modified for Xilinx ? Or is my understanding wrong ? If the filter is specified 50 ohm, including DC, I would expect to measure 50 ohm at DC ?

 

View solution in original post

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