So, I am designing a PCB with the classic scenario of having both sensitive analog signals and noisy digital signals...
- Analog and digital regions are separated. the have their own gnd planes and isolated supply's.
- A flash ADC is the information bridge from the analog side to the digital side.
I have tried to find the answers to this but I keep seeing the same thing. which basically this photo in summary:
Now this shows the star ground at the ADC.
Why does it have to be close the ADC? I would think that you would want the connection between the regions as far away as possible from any kind of noise if in fact the connection is just there so the digital and analog reference don't drift apart and damage the ADC...
My thought is that depending on the ADC, like a SAR or Sigma-Delta ADC, you really need the DGND to be exactly the same as AGND for it work correctly... hence why the star is near the ADC.
But for Flash and Dual-Slope ADC, AGND does NOT need to be in reference to DGND. the only the thing going from the analog to digital portion is small leakage current on the comparator/opamp inputs (photo below shows configuration or the ADC I am using).
Do I still need to put the star near the ADC this case?
Thanks for your help,my brain hurts from thinking about this.