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.
The ideal regulator keeps the output voltage constant as long as:
- The input voltage is within the valid range specified for the device,
- and the output current draw is within the allowable range.
Of course no regulator is perfect. For voltage regulators, there are two main specs that tell you how much the output voltage varies as a result of operating conditions. Power supply rejection is actually a unusual term applied to voltage regulators. This term makes more sense for a part with some analog output, like a opamp. However, a voltage regulator can be viewed that way so it's not wrong. More commonly though you'd see the term input rejection ratio for voltage regulators.
In any case, this is telling you how much variations on the input of the regulator get onto the output. Ideally none of them would, but in the real world some fraction of input voltage variation is going to appear on the output voltage. Let's say that the input voltage has 1 Vpp ripple on it. If the resulting output voltage has 1 mVpp ripple on it, then the gain from input to output is 1/1000, and the rejection ratio is 1000.
This rejection ratio is often expessed in dB. Keep in mind that dB expresses a ratio of powers, and that power is proportional to the square of the voltage. A dB is 10Log10(power ratio), which is 10Log10((voltage ratio)²), which is more easily expressed as 20Log10(voltage ratio). Therefore, the 1000:1 rejection ratio from the example above could be expressed as 60 dB.
I mentioned there are two main specs used to describe the dynamic performance of a voltage regulator. So far we have talked about the input rejection ratio, which is what you asked about. Sooner or later you'll bump in to some kind of output rejection spec, although that can have different names. This is a measure of how much the output voltage changes for changes in the output current. If you work out the units, you will see this is in Ohms, although it is often not expressed as such explicitly. You can think of this as the resistance in series with the output of a regulator that does not vary its output at all as a function of current.
Best Answer
What you are expecting is never shown as this is the product of the spectral density of the load current times the spectral density of the source impedance.
Vs(f)= I(f)*Zs(f)
It is more useful to examine impedance ratios, Q and f-3dB and use a filter simulator to examine problems. I have found this way to be very effective ( Falstad's filter+Bode plots) for component selection and effects of ESR on various C values with CLC Pi filters to attenuate and improve decoupling with both differential for conducted noise and CM chokes for radiated noise.
At DC this reduces to simply DC impedance ratio source/load which is also known as Load Regulation error which is often in the 1% range.
In the audio frequency range , the inverse of this is called damping ratio referred to a standard load speaker/ source impedance with closed loop gain.