I'm having a few problem with a digital analyzer. When a probe is connected to an unpowered circuit. I see no reading on my digital analyzer. But once the power supply is plugged even if the circuit isn't powered.
The digital analyzer will read some pulses at around 50 Hz. This renders the digital analyzer completely useless when powering devices from an AC-DC power supply. I tried with multiple power supply at home but none help.
It's enough to connect the digital analyzer probe to the ground node of the power supply while the power supply isn't powering any load.
My guess is that the power supply are letting some AC through and aren't 100% isolated from AC.
I was wondering if there was a way to filter those 50hz spikes without messing with the power supply. I was thinking of a filter that could be connected in between the power supply and the device I'm powering.
Not sure what wasn't clear from the question. I'm not designing a power supply and not designing a digital analyzer either. I'm just trying to use my digital analyzer on something that is powered to an AC-DC power supply (those cheap wart plug).
simulate this circuit – Schematic created using CircuitLab
Best Answer
When a bridge drives a cap with some decay voltage , just before each peak the diodes force the caps back up to peak voltage. This causes EMI by the narrow current spike from the loop area of the antenna of current pulses and the rise time of current.
Even though the voltage is floating thru the transformer coupling, there is still this magneto motive force which can create some voltage into logic probes which may be say 1Meg. , enough to cross the threshold voltage of unconnected leads that are open to the transient repetitive field being radiated.
Usually the most common solution is to earth ground everything, but the last clue was that this house has no earth ground. hmm?
In that case, bonding the logic analyzer frame to the power supply return is next best bet. This reduces the amplitude of stray CM field so that it is less likely to create a differential voltage. Logic signals tend to be low impedance so that once connected to the circuit, you would find the same result, even if it was not powered up.
Conclusion: False flag signals from EMI and lack of AC earth ground to logic analyzer.