magnetic field noise

[rocsavi]

dear all,
I worked on PHOTODETECTOR based project...In this project,input light from laser is coupled through fiber ....Near photodetector i have to place a coil near photodetector pcb ...there is no physically contact between photodetector pcb and coil...coil is driven by a function generator..when coil vibrates at desired frequency which is set by function generator..pcb also pick up same frequency signal and gives this as output..
pcb contanis LT1167 Amplifier which gives output,,,problem is that, frequency of required output signal and coil vibrating signal frequency both are same..i want to avoid coil signal that is captured by pcb...any suggestion p[lease...

 

[biff44]

schematic needed to comment. you need to use a differential amplifier to null out the coil noise...not sure if it is magnetic pickup or ground loop problems.

 

[volker@muehlhaus]

Minimize the area where magnetic flux can couple into your signal/return path.
Example: twisted wire pairs minimize the sensitivity to external magnetic fields: the area between wires is small and by twisting we try to cancel the induced voltage from an external field.

 

[vfone]

On top of using twisted pair wires, and to minimize even more the induced voltage from the external field, you can twist the wires on different layers on the PCB.
You can check the net for ideas, I've seen some granted patents on this matter.

 

[chuckey]

With no laser signal, inject a bit of the magnetic field energising source into the amplifier. Adjust its amplitude and phase to minimise the amplifiers out put. Switch on the laser, the output now comes just from the laser.
Frank

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