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BQ25703AEVM-732: Adding ADS1220 to reference design

Part Number: BQ25703AEVM-732
Other Parts Discussed in Thread: ADS1220, LM3480

Hello, hope you are doing well,

I am designing a custom PCB based on the BQ25703AEVM. I am adding an ADS1220 ADC to this design and I was wondering about the best practice for powering and grounding this ADS1220. I am using the ADS1220 for unipolar measurements, and I have a 3v3 buck power supply on an external board in addition to the LM3480 which is part of the BQ25703AEVM design. The BQ25703AEVM schematic has two ground symbols: PGND and GND, which should be connected in  star topology with PGND. I'm going to refer to GND as AGND for clarity.

Am I correct in thinking I should use the LM3480 and AGND to connect to AVDD and AVSS on the ADS1220? I checked the LM3480's datasheet to check for a current limit, and could only find reference to a 100mA minimum load current. I suspect that the BQ25703AEVM design draws much less than 100mA from the LM3480, which makes me think that the increased load would actually bring the LM3480 closer to nominal operation (but the ADS1220 draws so little current I don't see it making much of a difference either way).

  • Hey Tim,

    Based on the EVM, yes I would connect it to AGND to keep your signal clean from power-generated noise.

    I would also recommend you post to the Data Converter e2e forum to get more details on that device.
    e2e.ti.com/support/data-converters/f/73


    Regards,
    Joel H
  • Thanks Joel,

    What about AVDD. Would you take that from the LM3480 or would you just use the same 3.3v that I am using for DVDD (buck converter on an external board)?

  • Hey Tim

    I am not as familiar with the ADS1220 device as this is the charging forum, but I will say if you are trying to do some ADC measurements, you are better off with an LDO like the LM3480 which will effectively reduce output noise. A buck converter would be particularly noisy as the input to a signal chain device like an ADC. Just referring to the Application section of the ADS1220, you would want to isolateh the grounds for each of those pins.


    Regards,
    Joel H