ADS7809: 7809 ADC: power sequencing/requirements

or is it better to use the quiet analog 5V supply I'll be generating for the 5V digital supply as well.  maybe put some kind of filter so the digital switching doesn't inject a bunch of noise into the quiet analog.

Can you also tell me how the +2.5V external reference should come up/down relative to the +5V analog supply?

Hello Erik, 

There are no set sequencing for the power supplies and the reference. Typically we recommend the power supplies (VANA and VDIG) are up before the reference. 

So long as no conversion is started until the supplies and the reference are up and stable. 

Is there a concern with long delays between the supplies and the reference?  

Best regards, 

Yolanda

Right now, I plan to bring VANA up first, VDIG second (may be starting up in parallel to VANA), and the ext REF will be enabled after the VANA is up.

Can you talk about how the AGND and DGND are connected inside the part.  I'm wondering if I should tie the DGND pin on the ADC to my AGND plane instead of my regular DGND plane.  I'm afraid if I connect the ADC to the DGND plane any noise will couple into the AGND of the ADC and impact performance.

I was planning on having a bridge in the layout to connect the AGND and DGND planes together at this one point right at this ADC.

Hello Erik, 

The current sequence of Vana>Vdig>ExtRef sounds good.

They should be shorted together at some point on the board, and like you mentioned, to avoid digital noise a star connection between the DGND and AGND is recommended, or the bridge configuration you spoke of should work too. 

We actually have this FAQ that goes through some Layout Guidelines and Grounding Recommendations which I think could be helpful here. 

Best regards, 

Yolanda