Re: Noise leaking from PSU to mic amp
From: Bob Masta (NoSpam_at_daqarta.com)
Date: 10/11/04
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Date: Mon, 11 Oct 2004 12:16:01 GMT
On Mon, 11 Oct 2004 10:46:13 +0100, "Daniel Kelly \(AKA Jack\)"
<d.kellyNOSPAM@NOSPAM.ucl.ac.uk> wrote:
>Hi,
>
>I have just built a PCB which holds a switched power supply and a microphone
>amplifier. The power supply runs a camera whilst the audio amp is powered
>directly from the 3.7v lithium polymer battery. The problem is that the amp
>picks up a lot of noise from the powersupply. So much noise that the amp is
>unusable at the moment. If I turn off the power supply then the amp runs
>perfectly. Please could I ask your advice as to how best to limit this
>noise? I've had a few ideas myself:
>
>1) The PCB is doublesided and the back of the PCB is a ground plane. This
>ground plane is continuous across the entire PCB so the powersupply and the
>amp both share the same ground plane. If I separate the ground plane by
>cutting the copper with a knife at the boundary between the two circuits,
>will this limit some of the noise that's leaking from the power supply to
>the amp?
>
>2) Use better shielding on the PSU's inducer.
>
>3) Build a separate PCB for the amp (something I don't really want to do
>because I'm quite short of time).
>
>Things I've tried already (but with no success):
>
>1) Running separate power leads from the PSU and amp to the battery
>
>2) Putting coupling capacitors all over the place.
>
>3) Shorting the length of the microphone cable to the absolute minimum.
>
>4) Shielding the entire PCB with grounded silver foil
>
>5) Shielding only one circuit with grounded silver foil.
>
>Thanks,
>Jack
>
>
Just a thought, but if you could increase the switcher
frequency to move the noise above the audio range,
you could use a simple low-pass filter on the mic amp.
Probably best used as a last resort after all the "good
design practice" stuff fails.
Best regards,
Bob Masta
dqatechATdaqartaDOTcom
D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
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