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ADS1220: Adding adc to BQ25703AEVM reference design

Tim Christman
Prodigy 90 points
Part Number: ADS1220
Other Parts Discussed in Thread: LM3480, BQ25703

Hello, hope you are doing well,

I am designing a custom PCB based on the BQ25703AEVM, and I would like to add an ADS1220 ADC to the same board. My question is about how to power and ground this device with signals present in the BQ25703EVM.

I have a 3.3v buck on an external board, and I am planning on using this supply and PGND for DVDD and DGND. I believe I should use the analog ground (GND in the BQ25703AEVM schematic) and the 3.3v LM3480 LDO for AVSS and AVDD. I was concerned about pulling too much current from the LM3480, but I checked the datasheet and couldn't find any reference to a maximum rated current, only a minimum load current of 100mA. As far as I can tell the LM3480 is supplying much less than 100mA in the BQ25703AEVM design, and adding the small supply current of the ADS1220 would actually bring the LM3480 closer to nominal operation.

  • 0
    TI__Guru** 112895 points
    Hi Tim,

    I'm not familiar with the BQ25703EVM, so I will have to investigate before I can give a good answer. As far as the LM3480, this is a 100mA device. It is a little confusing the way it is described, but it will deliver 100mA to the load.

    Best regards,
    Bob B
  • 0
    TI__Guru** 112895 points
    Hi Tim,

    As far as I can tell you should be able to use the 3.3V from the LM3480 to power the ADS1220 with no issues. What is not clear is the difference between GND and PGND. The documentation appears to use the symbols as GND for all instances. According to the schematic the LM3480 is referred to PGND. All the connectors with 3.3V show GND. I think it is the same net, but you may want to check into it further.

    Best regards,
    Bob B
  • 0 in reply to Bob Benjamin
    Prodigy 90 points

    Thanks for your help Bob. I originally posted this question in the Power Management forum, and was asked to cross-post it.

    The GND and PGND nets are supposed to be connected in a star topology. In my PCB design software I have them as separate nets connected by a net-tie. I was a little confused as to why the LM3480 was grounded to PGND, but everything it was powering was connected to GND (I find it more clear to refer to this net as AGND). I basically just tried to follow the methodology the EVM used to power analog circuits.

    To add to the confusion: the ground symbol for the LM3480 looks like the symbol used for the GND net, but is labeled PGND. I have checked the Gerbers for the EVM, and as near as I can tell the LM3480 is grounded to PGND.

  • 0 in reply to Tim Christman
    Prodigy 90 points
    I found the distinction between GND and PGND in the datasheet for the BQ25703 in the pin description section.