How should I layout voltage regulators (VRs) to separate Analog and Digital?

Question

I need to create two power and ground domain: Analog and Digital. I've a common power supply that I'll call VCCBattery that power 2 voltage regulators. Since each voltage regulator have decoupling Caps and GND pin, on which plane should I connect this GND pins? After that, how should this 3 planes (Battery GND, Digital GND, Analog GND) shoul be connected together? (like first Digital and Analog and next to Battery, or a star joint between them)

Here's voltage regulators that I use and their Pinout with essential capacitors:

Answer 1

Whether you separate the analog ground from the digital ground REALLY depends on how isolated the analog and digital functions are.

If they are 100% separated, then running different grounds with them only connected together at the battery terminal connection is better.

If however, there are numerous control signals between the digital and analog circuits then the long current return path through a separated grounding system causes issues and a uniform grounding system is more appropriate.

If there are only a few interconnections, then careful layout and PCB design to have those circuits as close as possible to where the grounds separate may be sufficient to allow you to continue to separate them.

Most of the time though, one really good ground plane is sufficient with the occasional localized ground "island" for a particularly delicate analog circuit.

Answer 2

They all got to Vbat- but path is explicitly used only by Analog loads or digital loads by layout and not shared.

Layout is your choice, research how ADC boards do this.

Since Inductive current noise must be minimized, and Inductance is determined by length/width ratio of tracks, and V=L di/dt, you want wide tracks or copper pour to high switched currents. Also consider stray coupling capacitance in layouts.

For high current switched loads, consider ESR of output caps and min/max range if specified in datasheet. Usually one or two caps ensures good switched load regulated noise, if there is any on these 200mA devices.

Having two separate regulators gives low Zout and excellent isolation on Vout and these parts have excellent common mode Vin noise noise rejection, but two IC'/ may not be necessary in some cases.... unless one becomes the Analog Vref for say an ADC.

Since n* CMOS gates switching at once is n* C load with low ESR/n it's Voltage Source and ground return must be as ideal as possible. Namely, for 5% ripple, 1/20th of the load ESR using a small C >yet 20x. Bulk E-Caps and batteries do not have this low ESR in the 0.35/risetime equivalent frequency range. Often high SRF high Q 100pF to 1nF caps are necessary close to the point of distribution. THis translates into 50MHz ~1GHz and depends on Cap SMD size.

The reason the above is critical is that a small T= RC load , with a transient response is effectively the ratio of Z(f) for source/load up to the frequency of 0.35/Tr.

Answer 3

If you really know why you are using split-grounds, then alright. If you don't know why and it's more of a tradition, or someone told you so without knowing, then you should wonder why you are actually splitting the grounds. IMO, placement matters a lot more.

Now, a regulator produces a voltage which is referenced to its GND pin. Say it's a 3V3 regulator, then it will output "Local GND voltage" + 3V3. The output capacitors do the same at higher frequencies. Noise on your supply is shunted to ground by the capacitors, but noise on GND is also injected into your power supply.

This is why, if you have analog and digital ground planes, and separate supplies, then the regulators for each and their output caps should be on the ground plane they power.