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Why dose the desk lamp make my circuit(simple voice recorder) noisy?

Sehwan Kim
Prodigy 10 points
Other Parts Discussed in Thread: OPA2373

Hi,

I made a simple voice recorder based on MSP430.
The voice-input pass through the analog MIC, OpAmp, ADC(MSP430, 12bit, 16khz) and be saved to memory(via SPI).
It works pretty well.
But when I turn on the desk lamp or switching power supply near my voice recorder while it running.
The recorded sound contains noise that may induced by the lamp and the power supply.
So, I added R1, R2, C1, C2 into my circuit to make the ADC input Low-Impedance.
And the noise has been reduced, but not removed.                                 
How do I completely remove the noise?
I need your help.
Thanks.
  • 0
    TI__Guru* 93105 points

    Hello Sehwan,

    Analog and mixed-signal  circuits can be susceptible to electromagnetic interference (EMI) generated by other electronic devices. If the switching power supply, or a switching supply inside the lamp, are dirty and producing a wide spectrum of RF signals they can be radiated from the circuits themselves, via the power cords and/or other cables connected to the devices. I suspect that if you physically move your circuit away from the EMI sources the noise decreases which would indicates that the EMI is getting to your circuit as radiated RF energy.

    You have added some series RC networks at the input and output of the OPA2373, but that doesn't necessarily mean that they provide the optimal solution for eliminating your circuit's susceptibility to EMI. Radiated EMI could require one, or more, remedies to eliminate the circuit's susceptibility to the EMI such as input filtering, correct power supply decoupling at the IC pins, a correct PC board layout that eliminates the antennas that receive the EMI and couple it to the ICs, shielding, etc. You have mentioned how you constructed the circuit and it may be that you haven't taken any steps to prevent EMI from affecting your circuit.

    There is a wealth of information about EMI and electromagnetic comparability (EMC) on line. A good place to start learning about the subject is the Henry Ott Consultants webpage. In particular see the Tech Tips and Technical Articles/Talks. A number of us here at this TI location have attended his sessions and use his book, Electromagnetic Comparability Engineering, as a reference when troubleshooting EMI issues. In my opinion he provides practical, sound advice on the subject.

    Regards, Thomas

    PA - Linear Applications Engineering