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EMI problem with high gain amplifiers

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Advanced Member level 3
Jan 11, 2006
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I am designing a amplifier with gain of 10000 using two 1/4 lm324 lm324 has four opams i am using two for amplification and other as comparator, I am using this in PIR motion sensor.

My problem is when ever i use mobile near the ckt it gives false trigerring I know this is EMI, even when i switch loads like fan then also i got same problem.

Shielding does the job upto 90% only.
but switching of loads creates noise/transients in main which is not hindered by shield.
can any one help me on this

The layout should have a full ground plane. Make the circuit as small as possible (parts close together).

Then reduce the high frequency gain by splitting the op amp input resistor in two halves and put a shunt capacitor to ground.


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also keep in mind that :
- 10 nanoF capacitors are not effective on 900 MHz their impedance are increased, you should use also 100 picoF capacitors in parallel to 10 nano. Study the capacitors datasheets you are planning to use for 900 MHz for minimum impedance . SMD is preferred .
- Use ferrite beads on power .
if PIR is to be conneted by long wires use low pass filters which could be ferrite common mode filter and capacitors
- Use opamp with high CMRR
- Use opamp with FET on their input stage . Those are less affected by EMI.
- ...

If board is to be DIY - take a fresh pcb and make mechanical etching so all remnant fields will serve as ground. The second side also to be connected to ground in several places .

Search forum on EMI subject - you will find books and extensive recommendations .

thank you for ur suggestions and reply, i will rebuild the ckt but having full ground plane is not possible for me as i use general purpose pcb for my ckts.

You can make an impromptu ground plane by putting insulating tape on the bottom and then a layer of metal foil.


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i will try with foil also,
where it is best to connect the shield to ground or to the earth.

The connection to the foil should be to wrap it around to the top part of the board and connect it to the closest circuit ground point on each of the four sides.

You can go further after this and put insulating tape on the top of the board and wrap the whole board with foil to make a 'box" for it. You should test this for thermal problems of heating.

the solution which we are discussing here are related to shielding only, what else we can do on ckt side, like using murata emi filters or something like that but selfmade.

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