decouplingdigital potentiometergrounding
I'm thinking of using an mcp41100 digital potentiometer to control an op amp gain.
My board will have a split ground plane – digital and analog grounds.
Unfortunately the mcp41100 doesn't have separate digital and analog power/ground rails.
How should I power and ground the chip ? Digital/Analog supply/ground ?
Sorry, my bad. I posted this image from the CY8C32 datasheet into my answer to the other question:
but didn't copy the caption. "Figure 2-8. Example PCB Layout for 100-pin TQFP Part for Optimal Analog Performance". This is for the TQFP100 part, which doesn't have the thermal pad, and doesn't apply to the QFN48 you're using.
For parts with a thermal pad the split makes no sense, and you should connect the thermal pad to digital ground.
The center pad on the QFN package should be connected to digital ground (VSSD) for best mechanical, thermal, and electrical performance. If not connected to ground, it should be electrically floated and not connected to any other signal. (page 6)
Note that when you use a thermal pad on your PCB that you shouldn't apply solder paste all over it, but use a windowed stencil to avoid the IC being pushed up by the solder paste:
"The solder paste pattern area should cover 35 % of the solder land area. When printing solder paste on the exposed die pad solder land, the solder paste dot area should cover no more than 20 % of this solder land area. Furthermore, the paste should be printed away from the solder land edges. This is illustrated in Figure 9; the solder paste pattern area lies within the boundary indicated by the red line and it is divided by the entire solder land area." (from here)
Further reading
CY8C32 datasheet
HVQFN application information, NXP application note
In general, datasheets give excellent advice that you ignore at your own peril. (Alas, like the advice of Cassandra, the advice in datasheets is often correct but misunderstood until it is too late).
However, even the writers of datasheets occasionally make mistakes.
The Burr-Brown application bulletin "Analog-to-Digital Converter Grounding Practices Affect System Performance" reports one experiment where a PCB was layed out both ways, and shows "a single ground plane" works best, even when it contradicts the "split grounds" advice given in a datasheet.
Splitting ground planes often seems to make an improvement. Making a single ground plane and fixing the actual source of the problem is even better. The Henry Ott article you linked and other "unsplit ground" articles at the MassMind explain why.
Best Answer
The MCP41100 (along with probably every other digipot I've seen) doesn't need nor use an analogue ground. It is a digitally controlled potentiometer and the three connections to the potentiometer are floating and can be connected to your target analogue circuit however you need them to be (within reason).
Here are a couple of examples from the data sheet - these examples show how the device is wired to test for frequency response and capacitance but, the diagrams serve the purpose of showing the 3 pot connections are not needed to be connected to ground (unless you want them to be): -
Note also that this particular device should not have voltages applied to the pot connections that are outside the absolute maximum ratings for the device (-0.6V to Vdd+1V). If in doubt, read the data sheet.