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ADS 1672 input DC shift, signal and noise distortion

Peter Generic
Prodigy 30 points
Other Parts Discussed in Thread: ADS1672, THS4520

Hi!
I'm experiencing problems while registering ADS1672 input noise (differential inputs shorted). Instead of a "normal" noise waveform, I'm getting a -19.6mV DC signal with a lower half of the predicted noise. Sorry for somewhat complicated description, I hope that the picture I've attached will explain the problem I'm having.
When input signal is given, there is a -19mV DC shift, plus some clipping when registering signals with small amplitudes.

Few words about the hardware - I'm not using any evaluation boards, however all the components used (ADS1672, THS4520, REF5030) are connected according to the schematics given in the converters datasheet. Data from the DOUT output is sent to Cyclone III FPGA, which then sends all the acquired samples to the PC. I know that the amount of possible mistakes is endless, given the fact that both hardware and software were designed from scratch but most signal parameters I've measured (noise floor, signal ffts, lock-in voltage and phase measurements) were just as expected, with only the small singal performance greatly lowered. Can you suggest me any reason for the observed distortion and DC shift to occur while the inputs are shorted? All the supply and reference voltages were measured and do comply with the requirements given in the datasheets.
Best regards,
Peter

 

  • 0
    TI__Mastermind 23235 points

    Since you are probably powering the device from a single supply, make sure that you are meeting the common-mode input range specs.  You may have to adjust the common-mode on the THS device.

    You can reference the schematic for the ADS1672EVM-PDK in the user guide.

  • 0 in reply to Greg Hupp
    Prodigy 30 points

    Thank you for your reply.
    Yes, the THS4520 is powered from a single +5V supply, with a 2.5V supplied to the CM input. However, the DC shift and the effect of clipping the waveform are still observable when the amp is completly disconnected from the cicruit and the ADS1672 inputs are shorted together (as seen on the third figure form top in the TI.png file I've attached to my first post).
    When observing the converters input noise, only the values that are below the DC shift are visible - the rest is clipped to the value of -19.6mV. I have tried different reference voltage sources, with no effect. 
    Best regards,
    Peter

  • 0 in reply to Peter Generic
    Prodigy 30 points

    The issue  was of a digital matter, nothing to do with the converter. Thanks for the help, problem solved.