DavidVyhlidal

Jumps in the AD7626 output

Discussion created by DavidVyhlidal on Nov 7, 2013
Latest reply on Nov 15, 2013 by KarenNE

Hello,

I'm having issue with ADC AD7626. I'm sampling the capacitor voltage which is slowly discharging due to the leakage currents so a straight line should be seen but there are jumps in the sampled data at several locations in the whole ADC input range (see images below). In the first image the jump is roughly 16LSB (looks like a problem with 4th bit). This is the jump I'm seeing mostly. In the second image the jump is roughly 32LSB (looks like a problem with 5th bit). The jumps always appear at the same ADC output.

AD7626_output1.png

AD7626_output2.png

  • The AD7626 is operated in echoed-clock interface mode. The serial data is decoded by FPGA. To check if the issue is not caused by the FPGA I’ve used oscilloscope to decode serial data from the ADC and they matched with the data decoded by the FPGA.
  • ADC sample rate is 5.7MSPS (175ns), ADC clock frequency 200MHz (see image below)
  • Input signal to the ADC is within limits (IN± to GND … 0.1V to 4.07V)
  • I’m using buffered 4.096V external reference
  • ADC power supply sequencing is applied with one exception – due to the low pass filtering of the voltage reference, analog inputs are applied to the ADC before the full Vref voltage is reached => can this be the problem?
  • I have two channels, both showing the same jumps

 

AD7626_timing.png

 

Any advice?

 

Thank you in advance.

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