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BQ500412: OK to put Digital Circuitry on Analog Ground?

Seth Berggren41
Prodigy 85 points
Other Parts Discussed in Thread: BQ500412, CC2540, BQ500412EVM-584

Hello,

My Wireless Transmitter system, which uses the BQ500412, has a Bluetooth module with integrated microcontroller.

Edit: Also, the Bluetooth module is using the CC2540

Currently, my system has 3 regulators:12V Boost Reg, 5V LDO, 3.3V LDO, all of which are connected to analog ground. The Synchronous Buck NexFET (CSD97374QM) is connected to Power Ground (as per the datasheet and EVM schematic).

For the Bluetooth module, it is critical to have a solid ground plane, but, I do not want to interfere with the sensitive analog circuitry of the BQ500412.

Is it okay for the Bluetooth Module to share a ground plane with the BQ500412? Should I use a 6-layer board and implement a third (digital) ground plane?

Thanks,

Seth Berggren

  • 0
    Prodigy 85 points
    Well, I changed my design to look almost exactly like the bq500412EVM-584. Now I need to put in the CC2540 BLE Module.

    The two options I am considering are:
    1) Have the CC2540 share AGND with the BQ500412 and its related circuitry.
    2) Have separate area on the PCB with completely separate ground plane. Use this area as an isolated circuit for the BLE Module. Then tie the separate ground planes together at a single point.

    Any suggestions on my proceeding from here?

    Thanks,
    Seth
  • 0 in reply to Seth Berggren41
    TI__Guru* 75856 points
    The bq500412 EVM has a ground plane for high noise switching ckts (GND) and low noise analog ckts (AGND). The two ground are connected at one point to reduce circulating currents between the two planes.

    Also the bq500412 device needs a good ground plane. Ground problems on the device will typically show up as communications problems with the RX. Ground plane noise will reduce the quality of digital demodulation.
    During layout take care with V33D and V33A filter capacitor placement and ground returns to pin 36 AGND and Pin 32 DGND.

    You have listed two options above and the safest would be to have a separates ground planes. The shared ground plane may work but I have no info on how much noise the CC2540 will generate.