I want to use your ADS131M08 for our product design, about the AGND and DGND design,
is it necessary to separate them when layout, can they be connected together?
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Is it necessary to separate [AGND and DGND] when layout, can they be connected together?
Nope, AGND and DGND do not need to be separated. Our EVM has them connected to the same node. You can take a look at the EVM user guide, where you can find the schematic, for reference: https://www.ti.com/lit/ug/sbau334a/sbau334a.pdf
In fact, there is a AGND to DGND absolute voltage spec that cannot be violated in the device, or else damage will occur, so they should attempt to be close to the same voltage potential even if split GND is used.
Can [AVDD and DVDD] use same power supply? if so, can AGND and DGND be connected to together?
Sure, AVDD and DVDD can come from the same source, and as told above AGND and DGND can be connected together. Just be sure to keep local decoupling capacitors near both pins as now the noise is applied equally to both digital and analog.
For future reference, whenever you see an IC with DGND and AGND and they're expected to be around the same voltage, the IC designers are telling you that circuitry within the IC (on the die) will be referenced to that voltage pin depending on the function of the pins near it.
In the case of the ADS131M08, you can see a lot of the Digital circuitry pins are grouped together (like SCLK, nCS, etc) and the analog pins are grouped together (AIN1P, AIN1N, etc). So we know that, for example, when the nCS pin toggles and the IC needs to sense the voltage on nCS, the local transistors for the GPIO will be referenced to the DVDD and DGND pin.