Analog Ground Plane_Determination

All,

I have an application where I will be receiving in a ground reference and two other analog signals that I am calling “Analog Ground and Signals” in the images below (left and green.) One of the pins of this connector goes directly to my analog ground reference plane (AGND).
Next I have a digital potentiometer that will take one of these analog signals, route through the potentiometer to the analog ground plane (AGND).
So far so good….

But since I want to use this digitally controlled potentiometer I will be connecting to a microcontroller that will send commands via I2C.

At this point I have introduced a second ground reference that I am calling digital ground (DGND). This will be the ground for my LDO regulator providing 3.3V and a ground for the microcontroller.

1.) At this point I was not 100% sure what to do with these ground references. Should I split these planes and connect at one point or just have one plane but keep things separated in distance? I have read different things on the web but I still was not 100% sure.

2.) What would be a recommend stackup for something like this? Two layer? Ground on the bottom and routing on the top? Then flood the top with ground?

3.) Then I was not sure which ground I should be referencing for the power supply return reference of the digital potentiometer. Should this be the digital or analog ground?

4.) Digital POT datasheet and layout recommendation:

MCP4651T-503E/ST
https://ww1.microchip.com/downloads/en/DeviceDoc/22096b.pdf
From the datasheet:
“The power source supplying these devices should be as clean as possible. If the application circuit has separate digital and analog power supplies, VDD and VSS should reside on the analog plane.”



Maybe for the digital potentiometer I should create a separate analog 3.3V (A3.3V) that powers up the potentiometers?

Thanks!

Use separate LDO for both uC and Digital Potentiometer.And use separate GND copper pours for AGND and DGND.
This is important because DGND may have very high noise signals due to switchings and it will deeply impact analog circuits.
If the cost is not an issue, use AGND and DGND separate layers then tie them together on a single main external GND point somewhere ( star connection ) for instance power supply's GND..

Hi,

Many (most) members in this forum don't use a GND plane at all. Some of them think copper_pour can replace a GND plane.
I don't recommend both. In most circuits a single, but solid GND plane (one layer GND without splits and without other signal's traces) is sufficient.

Now you use two GND planes.
In some cases two GND plane bring a benefit. Mainly at high current applications, with relatively low frequency.
(Or very low noise audio circuits. Audio is not considered to be HF)

Using two solid GND in a high frequency application often does not give the expected effect, because the plane are neext to each other ant thus there is high capacitive coupling. This means, even if they are separated, the caoacitive coupling will couple voltage noise from one plane to the other.
In such a case intelligent component placement may be more effective than two GND planes.
Thus for a detailed answer we need more informations. Component placement, GND currents, switching speed, frequencies, expected precision...

For a first try (mainly guessing):
1) Urgent: In any case, both your GND planes need to be connected at one place. (Difficult to recommend a "join location" without details.) In most applications a single GND plane combined with intelligent component placement should be sufficient.

2) We don't know your application, nor the expected layer count. For two GND planes you need at least four layers.In many cases the inner layers are GND layers.

3) for a digital potentiometer this is not critical, because it won't cause significant GND current. In detail it depends on the application.
Examples:
* For volume control of high quality audio signals --> use analog GND
* For feedback control of a SMPS (adjustable output voltage) --> use power GND

4) it depends. See 3).

Klaus