grounding consideration for high precision ADC

I have one 4 layer board with high accuracy Analog 18bit ADC on board.

 

Bottom left is USB with Power input, going up is the switching power supply and analog opamp and ADC,

Right side are all digital ucontroller and FPGA.

 

I need to get the lowest noise on the ADC, how should I manage the ground plane, Now I have the 2nd layer as ground plane.

The switching power supply ( I have both boost and charge pump in the lower left section) generate a lot of noise on the ground plane.

 

 

I was thinking star ground the PS ground at the USB ground. Use the USB ground as the star.

Separate the analog and digital ground, but there is few digital lines for analog conversion and SPI interface.

 

Need you comments.

 

Thank you

Andrew

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    •  Analog Employees 
    on Jan 15, 2019 8:35 AM over 2 years ago in reply to ahgu

    Hi Ahgu,

          Normally and also as mentioned on the AD7667 datasheet, there should be a separate section on the PCB of the analog and digital grounds. The two should be connected only at single point. This is to isolate the one from the other. The placement of components can be factor. A correct placement would also help in designing the PCB plane including the ground planes. There are PCB designs that would isolate a noisy circuit then connect to plane by just a point. This would help lower the noise contribution of that circuit.

          You may also wanted to check the layout of the evaluation board as this design has been proven to work. You may check the EVAL-AD7667 user guide. There is a layout section of the document that you may use as reference.

    Regards,

    Jonathan

  • Many people and literature says use single ground plane. but use slot to control the ground current flow. 

    I thought about the separate ground plane for power supplies, if the power supply ground plane is closer to the power input, would it make a difference to have them separate or one piece?

    Regarding 7641, if  I am getting 1-2 RMS on DC signal with 16bit, would it worthwhile to go to 18bit, I assume I would get all noise for the last 3bits. Also, many CPU don't support 18bits for SPI interface, how do you handle this issue?