how to reduce coupling in ckt?

[uday mehta]

I am doing some test experiment using high gain amplifier as I have very low input.
but on general pcb or on bread board I am getting unwanted signal may be due to coupling. when I am applying 3 V AC from my signal generator I am getting 200 mv at oscilloscope when generator and oscilloscope probe are 2 or 3 mm apart in air ( means without any connection). I think both probes are acting as an antenna.
how to reduce this antenna effect as I have to connect generator pin 1 mm apart from amplifier input pin. due to this effect I am getting nearly 1 v AC at output of amplifier even with out any connection of ip of amplifier and generator pin.

 

[jiripolivka]

I am doing some test experiment using high gain amplifier as I have very low input.
but on general pcb or on bread board I am getting unwanted signal may be due to coupling. when I am applying 3 V AC from my signal generator I am getting 200 mv at oscilloscope when generator and oscilloscope probe are 2 or 3 mm apart in air ( means without any connection). I think both probes are acting as an antenna.
how to reduce this antenna effect as I have to connect generator pin 1 mm apart from amplifier input pin. due to this effect I am getting nearly 1 v AC at output of amplifier even with out any connection of ip of amplifier and generator pin.

You do not indicate the frequency used. Generally, oscilloscope probes are high-impedance ( 1 MOHm typ.), so coupling is easy. To prevent it use short sections of a thin coaxial cable and connect ground close to signal source.
The best way is to use 50 Ohm lines, then coupling mostly does not occur.

 

[Audioguru]

A solderless breadboard has many rows of contacts and many connecting wires that cause coupling and usually cause feedback oscillation. They also act as antennas and pickup interference.

 

[crutschow]

What is the amplifier pin normally connected to?

Pickup is normally reduced by using shielded/coax wires and shielded (coax) connectors for all connections

 

[uday mehta]

frequency is 7 khz. and generator probe is 2 mm apart in air from amplifier ip pin( means basically no connection). I am using op amp. with 10 M in feedback to inverting pin and non inverting is connected to ground.

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A solderless breadboard has many rows of contacts and many connecting wires that cause coupling and usually cause feedback oscillation. They also act as antennas and pickup interference.

then how to prevent it. I can fabricate a pcb. So in pcb designing what should I do to minimize it.

 

[Audioguru]

10M is an extremely high value for a negative feedback resistor. Then maybe the series input resistor value is also extremely high. Since the non-inverting input is grounded then the opamp is inverting and the inverting input and its resistors are picking up interference.
Why not make the input a non-inverting amplifier or follower then use an inverting opamp after it with low value resistors?

A pcb with low coupling has ground areas and traces between signal traces. It can also be double-sided with one side all ground as a ground plane.

 

[uday mehta]

sir, which configuration I should use if my ip is order of micro amp. range and I want to see the voltage change. that's why I am using high resistance with no series ip resistance.
is there any better solution to examine such low value ip? if yes plz tell me.
thanks.