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Entry  Sun Oct 23 23:32:28 2011, Hao Huan, Phase Shift for ADC Readout 
    Reply  Mon Oct 24 10:30:15 2011, Stefan Ritt, Phase Shift for ADC Readout 
Message ID: 135     Entry time: Mon Oct 24 10:30:15 2011     In reply to: 134
Author: Stefan Ritt 
Subject: Phase Shift for ADC Readout 

Hao Huan wrote:

Dear Dr. Ritt,

    In the DRS 4 datasheet it is recommended to sample the analog output of the chip after 8~10 ns of the SRCLK edge for it to stablize and thus a phase shift between SRCLK and the ADC sampling clock is necessary. However in the latest version of the evaluation board firmware the phase-shifted clock was generated but not really used for the ADC interface. Is there any reason for that?

Good questions. I looked myself in the code and found:

  drs_readout_clk       <= I_CLK33;

  drs_readout_clk_ps    <= I_CLK33; -- phase shifted clock for FADC

  drs_serial_clk        <= I_CLK33;

which is apparently wrong (should be drs_readout_clk_ps <= I_CLK33_PS;) . But apparently the board is working with the unshifted clock. Could be that the PCB traces to the DRS and the ADC have different lengths, and by accident, they have just the right value.

The way to find that out is to keep the ADC clock phase variable (most FPGAs allow a +-5 ns phase adjustment inside their clock blocks), and then try different values. If the phase shift is wrong, you will see spikes every 32 samples in the readout. The spikes are always there (from some internal switching of bus segments), and you can see them with a differential probe at the DRS output. The ADC phase must then be made such that the sampling point of the ADC comes after that spike, just at the end of the settling time, barely before the next analog value shows up at the DRS output. This is a bit tricky and can be made best just by trying out different values.

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