switching regulator's interference
Here is example of interference generated by a switchreg.

Assume the switchreg controls 1 amp input current, and switches that current in 10 nanoseconds.

Assume, nearby, is a 1cm by 1cm loop of your sensitive circuit such as ADC or opamps or radio. This loop is flat, such as on a PCB.

We model the interference as a long straight wire in the same plane as the loop.

Assume the current being switched (1 amp turning on or off in 10 nanoseconds) is in the long straight wire, located 1cm from the loop.

The equation we use is

$$Vinduce = [MU0 * MUr * Area / (2 * pi * Distance) ] * dI/dT$$

Manipulating this, we get Vinduce = 2e-7 * Area/Distance * dI/dT,
or

Vinduce = 2e-7 * 1cm * 1cm/1cm * 100Million amps/second

Vinduce = 2e-7 * 1e-2 * 1e+8

Vinduce = 2e-9 * 1e+8 

Vinduce = 0.2 volts

In an ADC with 5 volt input range, a 0.2 volt error is 4 parts per 100, 4% error.

The math also tells us, given Vinduce ~~ 1/distance, at 1 meter distance, the Vinduce drops only 100X from 0.2volts to 2 milliVolts.

Planes in your PCB will improve on this, for switching speeds at 10 nanoseconds.
Slower switching speeds ----- 100 nanoseconds ----- may be more troublesome, as Skin Effect provides little shielding but the dI/dT is 10X smaller.

[Caution] This equation is only accurate for infinitely long wire. A switchreg is not an infinite source, but when the loop dimensions are similar to the distance between loop and wire, the equation is quite useful and accurate to within a "log" factor.