I have a simple circuit that converts sound level to a DC voltage - attached - for reading with a PIC ADC.
My circuit works fine on battery power. But if I use a double-insulated AC->DC adapter (wall wart) without earthing the circuit, the microphone picks up 50Hz interference (mains AC is 50Hz here in New Zealand). The opamp then amplifies the 50Hz.... If I earth the circuit, via my scope for example, the interference disappears and it works perfectly on the plug pack.
I've attached a couple of scope outputs - one with the circuit earthed and the other unearthed - green is the opamp input (x1), red is the opamp output (x10). The 50hz signal is apparent when earthed, but the extra mV or two when unearthed is causing the problem. Plus the level of interference varies a lot when unearthed.
I am a newbie, but I have looked and looked and can't find a similar problem anywhere? Common wall warts without an earth pin mean connecting signal ground to earth isn't so easy either, if that is a solution?
Thanks for any suggestions!!
My circuit works fine on battery power. But if I use a double-insulated AC->DC adapter (wall wart) without earthing the circuit, the microphone picks up 50Hz interference (mains AC is 50Hz here in New Zealand). The opamp then amplifies the 50Hz.... If I earth the circuit, via my scope for example, the interference disappears and it works perfectly on the plug pack.
I've attached a couple of scope outputs - one with the circuit earthed and the other unearthed - green is the opamp input (x1), red is the opamp output (x10). The 50hz signal is apparent when earthed, but the extra mV or two when unearthed is causing the problem. Plus the level of interference varies a lot when unearthed.
I am a newbie, but I have looked and looked and can't find a similar problem anywhere? Common wall warts without an earth pin mean connecting signal ground to earth isn't so easy either, if that is a solution?
Thanks for any suggestions!!
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