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DRV8323: Built in current sense amplifier noise parameters

Hao Ye Soon
Prodigy 30 points
Part Number: DRV8323
Other Parts Discussed in Thread: ADS7853

Hi,

I am using the DRV8323RS chip with an external 14 bit ADC (ADS7853) for current sensing. Im having an issue where the signal from the current sense amplifier (CSA) is very noisy, Vpp = 30+ mV at idle. That translates to Vpp = 100mA with the current scale that I have.

Theres not much info on the CSA's analog performance in the datasheet. Hence Im thinking that its probably due to the noise from the CSA?

Thanks

  • 0
    TI__Intellectual 840 points

    Hello,

    I'm assuming you are using your own board so I had a few questions about your system in order to provide some more useful guidance. Which gain setting are you using? Would you be able to share your schematic and layout? It would be beneficial to look over these to rule out coupling. It may also help to look at how you configured the device since you are using the SPI part. A snapshot of the register map when the device is in a state where you are observing this issue would be most beneficial.

    Best regards,
    Omar

  • 0 in reply to Omar Naamani
    Prodigy 30 points

    Hi Omar,

    Im using the dafault register settings, with a gain of 20. I cant share the layout, but I think that is not the problem because I followed the layout guide as much as possible and im measuring the noise without running the motor yet. I think it would be helpful if TI can provide more information about the current sense amplifiers so that design engineers can match it properly with the ADC that they are driving.

    ADC Schematic:

    Motor Driver Schematic:

    Note that GND and PGND are connected at some point. The board size is small (40 x 40 mm)

    Thanks

    Hao Ye

  • 0 in reply to Hao Ye Soon
    TI__Intellectual 840 points

    Hi Hao Ye,

    Thanks for the info. For some reason in-line images don't post properly on E2E and the links are broken. Please resend your images using the attachment button in the toolbar. In the mean time I am going to do some digging to try and find relevant info about the CSAs, are there any specs in particular you would like me to look into? Hoping to get back to you next Tuesday at latest.

    Best regards,
    Omar

  • 0 in reply to Omar Naamani
    Prodigy 30 points

    Hi Omar,

    I have edited the post to include the attachments. I am mainly interested in the amplifier input voltage noise, current noise, Total Harmonic Distortion. It is possible that the CSA is the bottleneck here.

    I have another idea about the connection of the CSA to ADC: 

    Since I am not using phase C current sense, I can short SPC and SNC, and I would get Vref / 2 at SOC.

    I could use the signal from SOC to connect to the ADC's negative input so that it works in pseudo differential mode.

    Secondly, I could use the same voltage source for CSA's Vref and also ADC's Vref. 

    Technically this would give me much less noise and more accurate measurements.

    Do you think this will work?

    Thanks!

  • 0 in reply to Hao Ye Soon
    TI__Intellectual 840 points

    Hello,

    Using the same source for VREF and ADC reference voltage is best practice, as well as sharing the same ground. Using the SOC output for the ADCs negative input would also be a worthwhile experiment to try, especially if ground bounce is an issue. The specs requested are typically included in high performance CSAs and are not parameters that we measure during characterization and validation for this CSA, unfortunately. That being said, with no load we should not be seeing switching on the CSA, leading me to believe this is probably being caused by ground bounce or something else on the board.

    Do you happen to have any scope captures of the issue? It would be useful to see the differential voltage of the sense resistor, a single ended measurement of VREF, and a single ended measurement of the ADC input on the same scope capture. 

    Thanks,
    Omar