Electronic – Induced power supply noise on switch contact

There is of course no single answer to what "decent" power supply noise is. That's like asking what a decent car is without telling us whether its for driving around a racetrack or back country dirt roads.

Whether the values you mention are decent depends on how that power rail will be used. What you really seem to be asking is just from the power supply's point of view whether these value seem reasonable or not. 20mV for a generic bench top supply sounds quite reasonable to me, and so does 7mV for a on-board boost converter (in fact that's actually quite good compared to a lot of them).

Your circuit, however, may have a different opinion. If the 5V supply is just powering digital circuitry, then it's a lot cleaner than it needs to be. Even 100mVpp ripple would be tolerable.

If you're powering sensitive analog circuitry, then 7mV could be large. In that case the frequency content of the ripple also matters. Most analog ICs have a power supply rejection spec. There is active electronics in the IC to make its operation somewhat independent of the power supply voltage. However, that electronics can only react to noise up to some frequency. The frequency requirements to get the specified power supply rejection ratio is rarely specified. It's a good practice to put a ferrite bead or small chip inductor followed by a ceramic cap to ground on the power leads of analog parts. This will attenuate the high frequencies of the noise, with the remaining low frequencies hopefully in the range the part can handle and reject actively.

Some parts are much more susceptible to this than others. The first time I used one of the Freescale multi-axis accellerometers there was a lot of noise on the output. The power supply noise actually seemed to be amplified onto the output. Adding the aforementioned chip inductor in series with cap to ground on the power lead helped a lot to clean up the output signal.

To answer your last question, the normal way to look at power supply noise is exactly what you did. AC couple the scope input, crank up the gain, and look at the size of the resulting mess.

Basically you want to do this:

The computer output is probably unbalanced, connected to the computer's ground, which has a lot of spiky noisy currents going through it (hard drive heads seeking, display refreshes, memory access bursts, etc). Due to the (very small, but finite) resistance of the ground traces, this means the computer's ground is at a noisy voltage relative to the power amp's ground. If the power amp measures the computer signal relative to its own ground, it will see that noisy ground difference superimposed on the signal. The power amp probably has a differential input (please specify what you're using for the power amp), which you can use to cancel that noise out.

You want the negative input of the power amp completely isolated from the power amp ground. It should connect directly to the output ground of the computer instead. That way the power amp is measuring the difference between the computer's output and the computer's ground, which will be noise-free. The grounds of the power amp and computer should otherwise be isolated from each other and connected together only at a star ground point near the power supply. You definitely don't want the ground currents from the computer going past a ground that's used as a reference by the preamp.

If the class D amp doesn't provide a differential input, you can make one with an op-amp in the differential amplifier configuration.

Rg will be connected to the ground of the power amp, V1 to the ground of the computer, V2 to the signal from the computer, and Vout to the input of the power amp.

Not shown on Wikipedia:

• You should have a compensation capacitor in parallel with Rf to avoid oscillation.
• You should add an identical capacitor in parallel with Rg to keep the common-mode rejection good at all frequencies.
• Use 1% or better resistors.