What is a good decent power supply noise?
Let me expand, two cases, I have a bench top PSU, I put my scope in AC coupling and look at the ripple it is around 20mV. Is this a good number for a decent PSU? (I am fiddling with ANalog circuits so 20mV noise is big deal)
Second case is my on board regulator, I have a booster that takes 2V to 5V. I look at the 5V without any load and I see a 7mV ripple (saw). Is this normal? I have all the decoupling caps there, so I would have expected much less especially without a decent load.
Bonus question, what is the best way of measuring power supply noise? I guess especially with small currents such as this, there has to be more than touching with a probe?
Best Answer
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.