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Thread: SPI communications leaking into DAC output

  1. #1

    SPI communications leaking into DAC output

    Hello all,

    I am using a Teensy 4.0 with SPI communications to a LTC1859 (ADC) and AD5689 (DAC). Both are powered via the 5V Vin pin on Teensy 4.0 (which is powered over USB). Both are using the same SPI channel but with different chip select pins.

    Both chips are on "chinese eval boards" with poor documentation but they are at least functioning at the moment - I am running a fixed 100us loop with a 100Hz 2Vpp sine input into the ADC and then reproducing it on the LTC1859 DAC. The output on the scope is a reproduction of the sine input except that there is a reproduction of the SPI communications on the DAC output. These unwanted signals are on the order of 200 mVpp (-100mV to 100 mV) and comprises of sub 1uS pulses).

    It is pretty obvious because it is happening at the loop frequency (every 100us) and if I shut off the ADC, part of it disappears. Even if I disconnect the ADC entirely and just get the DAC to output 0 volts every 100us , the communication to the DAC through SPI gets 'leaked' into the DAC output.

    I have tried the following:
    1. Powering the teensy via USB and powering the eval boards separately using another 5V supply
    2. Powering the teensy and the eval boards using an external 5V supply
    (the assumption of the above is that the SPI transaction "draws" current which affects the power supply)
    3. Disconnecting the ADC and just using the DAC to output a fixed voltage

    None of the above has been able to mitigate the problem, but I guess I don't really understand what is causing the problem in the first place...
    Filtering the DAC output would help since it is at such a high frequency (>1 MHz) compared to regime of interest (<10kHz).. but it is a little annoying not knowing why.

    Any thoughts/views/help on this would be appreciated!

    Thanks!

  2. #2
    Quote Originally Posted by connectivity View Post
    Both chips are on "chinese eval boards" with poor documentation!
    How cheap? Compare them to the Analog Devices eval board paying particular attention to the power supplies.

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