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TMS320F28379D: ADC Crosstalk/Decoupling/16Bit

wolfgang gaerber
Intellectual 615 points
Part Number: TMS320F28379D
Other Parts Discussed in Thread: TMS320F28335

Hi,

After serious attempts to improve EMC issues and quality of our 512khz/16Bit measurement with built-in ADC  - we still see artefacts...

->artefacts synchronous with system processing, irq execution or external SRAM burst access

->deviation relative to input signal

->1 or 2 samples

We have seen 29LSB spikes related to XINTF access with TMS320F28335 in past projects ... extrapolating that to 16 bit - this would equal up to 500 LSB.

Any experience with external SRAM and 16Bit ADC Performance ?

rgds.

Wolfgang 

  • 0
    TI__Guru 60865 points
    Hi Wolfgang,

    The main paths for noise to couple into the ADC are through the reference (VREFHI/VREFLO) and the ADC input pins.

    Since you are using differential signaling with the 16-bit ADC, the ADC inputs should generally be reasonably robust against noise coupling since common-mode noise is rejected. This assumes that the signals are routed differentially and the noise impacts both the positive and negative components roughly equally. In the datasheet you can see that the expected CMR is about 60dB.

    Any noise that couples onto the reference pins will appear directly on the output. Because of this, it is important that you keep high-energy signals away from the VREF pins as much as possible and drive the VREFHI with a high-bandwidth buffer, as recommended.

    Noise can also couple in via the analog power supply - VDDA - but the PSR is pretty good - about 77dB as specified in the datasheet.

    For your EMIF signals that access an external SRAM, you'd want to ensure these signals have a good ground-return path that doesn't intersect with any of the ground-return loops of the sensitive signals listed above. You might also use some series resistors on theses digital signals to limit the edge-rate to try and reduce the bandwidth of the signal as a potential coupling source.