1. If my AVDD is unstable, will it cause the ADC conversion result to fluctuate?
2. Is there any difference in the ADC conversion value when AVDD is 4.8V and 5.2V?
3. Do you have any suggestions for AVDD circuit design?
4. Should AGND and DGND be separated in PCB circuit design? What happens if I connect these two GNDs together?
Hi YC Hsu,
Welcome to the E2E forum!
1. If my AVDD is unstable, will it cause the ADC conversion result to fluctuate?
There is a relationship between the supplies and the conversion result. For high resolution ADCs, it is important to have low-noise sources with limited output drift. Using a linear supply such as an LDO regulator is best. If a switching power supply is used, then it is best to follow up with an LDO prior to the ADC. Also any disturbance in the reference with respect to noise and drift will also appear as noise in the conversion result.
2. Is there any difference in the ADC conversion value when AVDD is 4.8V and 5.2V?
The change is less shown as noise and looks more like a drift of the conversion result. The ADS1256 datasheet specifies it as follows:
3. Do you have any suggestions for AVDD circuit design?
I'm not quite sure what you mean here. Are you referring to the overall analog input design? Or are you referring to the analog supply? For the analog supply I mentioned that in 1.
4. Should AGND and DGND be separated in PCB circuit design? What happens if I connect these two GNDs together?
You can connect both AGND an DGND together, and this is our general recommendation. The voltage between the grounds must not separate by more than 300mV as per Absolute Maximum Ratings. So if you use a star ground method back to the supply entry point, then this ground path must be low inductance. What is most important is layout. You do not want digital signals (such as clocks) to cross the domain of analog signals.
Best regards,
Bob B
