How to eliminate the ground noise ?

Ground noise problem

Please help me how can I eliminate the ground noise,

I've built a preamp (see the attached schematic) and it's on the
same PCB as a MAX487 RS485 converter. Same V+, preamp uses V- too. 5V)
It's working fine, but if I start to send bytes to a client trough RS485
the preamp get's all the byte change noise. (through the GND line i think)
Iam not sure it's the byte change but it seems to be around 500 Hz on
the scope. It looks like a triangle wave , but it can't be seen
on the V+ or V- lines according to GND, but it appears on the OUTPUT of
the first stage before the balancing.
I tried a lot of things to avoid the problem FTP cable instead of UTP, separate GND etc., but just one thing worked: If I place a 100nF between
the GND and the + inputs (instead of putting them directly to the GND)
the noise gets down to half, but it's still disturbing.

What am i doing wrong?
Please help me,
thanks,
Zed

Re: Ground noise problem

Split R1 in a 2*4,7 K and put a capacitor 10 uF ( positive on the node between the 2*4,7 K, negative to ground ).
It should solve ( or at least reduce ) the noise.

Mandi

Re: Ground noise problem

Yes, and put the wire from the mic to GND instead of V-

Re: Ground noise problem

THANKS!

It solved a lot.
Meanwhile I found the source of the noise! :lol:
In the client there is a PIC after the RS485 adapter, which controls
the MIC preamps output. PIC also controls two LEDs.
I set the led ON when command started to arrive and set it to OFF
when command succesfully received. When I did a poll for 128 units
all clients received 128 commands (4 bytes -> 4*128 == ~500 Hz).

Turning the LED on/off generated the noise!
Now it's clean flat! :lol:

What kind of decoupling should I use on PIC to avoid this?
(there is a 220uF and a 100nF on V+ to GND)
Should I use LC filter? What L/C values?

Zed

Re: Ground noise problem


Two situations may have impact on the noise.
First, the circuit loop of LED crossed the amplifier circuit. The solution is isolating the two curcuit.
Second, there is a ground bound. You may need add a 10uf capacity between digital power and ground.