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PSIEVM: PSIEVM PA045B Unable to Change Offset in Bypass Mode

Omer Amir
Mastermind 30180 points
Part Number: PSIEVM

I was testing the PSIEVM PA045B output with the Precision Signal Injector tool, and I noticed that I was unable to change the offset. I verified with the user guide that the SE_VCM jumper JP4 was connected to VCM and the DIF_VCM jumper JP8 was connected to ground for single-ended mode. However, I was unable to change the offset in the tool unless I changed JP1 and JP3 to use filter instead of bypass. There are two questions I have because of this issue:

  1. Why am I unable to change the DC offset voltage for single-ended mode while using bypass? Why does using the filter mode allow a DC offset to be configured?
  2. When I read in the data using single-ended offset and filter mode, I get the below waveform on the F280039C ControlCARD (PSI tool is set up to have an amplitude of 2.1V, offset of 1.25V, and frequency of 2 kHz). However, I when I test the same PSIEVM in bypass mode the signal is clean, the only issue is that it clips below 0V. I also checked with the AG33522A wavegen at 2 kHz and the signal was clean. Is there a reason why using the filter mode results in a more noisy signal?

I have tested this offset issue on 2 PSIEVMs, so I don't believe it's an issue with just the EVM that I have.

  • +1
    TI__Mastermind 38240 points

    Hello Omer,

    1.  The final stage of the 2kHz filter is also used as the summing amplifier for the DC offset when using the single-ended SMA output.  When bypassed, the ability to add a DC offset for single-ended mode is not possible.  The differential output has an additional amplifier that is both used as the summing amplifier for the DC offset and to drive the differential SMA outputs.  In this case, you can add the DC offset when both using the 2kHz filter or when bypassed.  This is a limitation of the PSIEVM design.

    2.  This looks like an output instability due to excessive load capacitance.  When the active filter is bypassed, the single-ended output SMA is connected directly to the DAC output amplifier, which can drive a fairly heavy capacitive load.  When using the 2kHz filter, the final stage of the filter is directly connected to the output SMA, and can only drive about 50pF.  This load capacitance could be from the cable, especially if it is long, or due to input capacitance on the ADC board.  You can improve stability by adding a 50ohm output resistor on the PSIEVM (R100), or by reducing the external load capacitance.

    Regards,
    Keith Nicholas
    Precision ADC Applications