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THS3217EVM: -VCC tied to GND on THS3217EVM?

Robin Coxe
Prodigy 10 points
Part Number: THS3217EVM
Other Parts Discussed in Thread: THS3217,

I ordered two THS3217 EVMs to prototype a DC-coupled DAC driver circuit for a Xilinx RFSoC driving an I/Q mixer.   I would like to connect +VCC to 6V and -VCC to -6V.  However, it appears that -VCC is shorted to GND on both of the boards.   I discovered this issue after the -6V rail of the lab supply immediately went into overcurrent mode upon power-up.

The manual does not seem to indicate that this should be the case.  Do any components on the eval board need to be modified from the factory configuration?

  • 0
    TI__Mastermind 22480 points

    Hello Robin,

      The THS3217EVM accepts both single and bipolar supplies. There shouldn't be any necessary modifications for the device's EVM to accept +/-6V supplies. Are you applying an input to the EVM, or is this occurring at device powerup with floating inputs/outputs? 

      What are the settings to the other pins? 

    Thank you,
    Sima 

  • 0
    Guru 47545 points

    Well Robin, the board of course was not intended to have that, have you tried a DVM ohmeter from -Vcc to ground with nothing applied? If that is shorted, some defect is present. 

  • 0 in reply to Michael Steffes
    Prodigy 10 points

    OK, I don't think it's an issue with the PCB.  I disconnected both the differential inputs and the single-ended output and the -6V rail no longer shorts to ground.  I reconnected the inputs (from the Xilinx RFSoC DAC in 30 mA mode, so DAC_AVTT = 3 V and V_DAC_CM = 2.07 V), still fine.   However, when I reconnect the output to the RF chain (the first element is a Mini-circuits PMA3-83LN+ LNA), at that point the -VCC rail shorts to ground.  Hmm.  It smells like a common mode voltage problem.

  • 0 in reply to Robin Coxe
    TI__Genius 14206 points

    Could you capacitively couple the output via the R38 footprint? Or do you need the low frequency range down to DC?

  • 0 in reply to Sean Cashin
    Prodigy 10 points

    We need to DC couple the DAC because we subsequently upconvert the RF pulse to ~10 GHz with an I/Q mixer and we apply DC voltages for to compensate for I/Q imbalance.  The LNA upstream of the mixer is biased at +5V (single-ended).   We have considered AC-coupling the DAC output with a transformer and operating the mixer in SSB mode.   

  • 0 in reply to Robin Coxe
    TI__Genius 14206 points

    It sounds fine to DC couple the DAC to the THS3217 inputs, but it sounds like there may be a DC short circuit at the THS3217 output into the RF chain. Looks like the PMA3-83LN+ already has a 0.5GHz low frequency minimum, excluding DC, so AC coupling with ~100pF should be invisible to the rest of the circuit.