Lets say I have a circuit which involves an OpAmp and a comparator. They're both single supply running from a noisy USB power supply.
Each component has local bypass capacitors, a big and a small one (10µF and 0.1µF, here simplified as one capacitor).
The comparator drives a load of +20mA and during switching I see the signal feeding back into the OpAmp over the power supply line. This cause my signal to degrade quite a bit and I need more hysteresis that I would like to.
I thought about adding some kind of noise isolation between the two components by placing inductors between them like this:
Inductors are in the range of 10µH and have series resistance around 2Ohm.
Simulation shows that the interaction over the shared power supply is almost non existent afterwards. Also the improved filtering reduces the noise from the USB supply a lot. This is to be expected because it is a CLC or Pi-filter.
The question: Is this a good or bad idea? I'm specifically asking because I've never seen this done in commercial circuits so far.
Best Answer
The obvious solution is to use a linear Low-Drop Out regulator (LDO) to feed both the Vdd opins on the op-amps and comparitors as well as any reference voltages needed.
You will continue to power your load directly from the Vusb supply but the op-amps & comparitors should be nicely insulated from noise on the USB supply.
The total current that the LDO needs to supply will be very low so long as it is not required to power the load.