I am designing a circuit with a very sensitive analog part. In order to eliminate SMPS noise from a regular power supply, the device has its own non-SMPS power supply followed by an ultra low noise linear regulator (LT3042). However even after this, the device still sees some noise coming from mains socket if a noisy equipment is connected on the adjacent socket.
Usually, simply choosing a socket on a different wall, or moving the equipment power cables apart, significantly reduces noise. I see this as free noise reduction over and above what can be done in the circuit.
I would like to have some mechanism in the device that can inform the user about noise on the line. I don't need exact ripple voltage measurements just something good enough to tell that cable arrangement B is better than cable arrangement A. This would allow the user to try different sockets and choose one that has the least noise.
Alternatively, the device can simply have a noise threshold below which a "Power Good" indication will be provided to the user.
Just FYI, this is for an industrial application and the position of the device or its wiring won't change very often. If during operation the noise level rises above threshold, say due to some other equipment connected nearby, the device can notify the master via RS485.
Best Answer
Well duplicated here and on the web are imperfect desires for low noise with no specs on: signal {V,I,Z, BW} or Noise {...}. The answers do not depend so much on choice of PS , rather an understanding that noise ingress into high Z signals can be induced by every mode; conducted , radiated, common mode impedance ratio from source that converts to differential noise.
The answers are everywhere, Baluns, twisted pairs shielding, CM chokes. Pi filters , Cable orientation and proximity , decoupling caps. Each of many solutions depends on specs (a priori)
The expertise and wisdom comes from simple impedance divider relations from source to destinations using CM Impedance., Zcm and differential Impedance Zdm, from unbalanced lines. CM chokes serve to raise Zcm over some limited spectrum to create more balance and greater attenuation. Twisted pairs and shielding serves to lower Zdm(f ) from stray coupling either pF from 240Vac rms E fields or nH from current nearby induced power cords.
You will need many articles or a good book (Henry Ott) on EMC, EMI reduction, to fully comprehend.