Re: noise problem with differential analogue input

On Jun 27, 11:15 am, jsmith <John111Sm...@xxxxxxxxx> wrote:
hi

I have noise problem with my differential analogue input circuit.

Input Part #1 :http://john111smith.googlepages.com/DIOP0.JPG
Input Part #2 :http://john111smith.googlepages.com/DIOP1.JPG

All inputs wires are coaxial ones(but very long; about 15 meters)
Case/Chassis has connected to Earth of system.
Also, there is an averaging function for 50Hz noise at
microcontroller.

I will be appreciated with your comments in this

regard.

Thanks in advance
J.Smith

What exactly do you mean, "I have a noise problem..." What sort of
noise? Can you put an image of a `scope trace on a web page,
illustrating the noise? If you short the two inputs together and bias
them to an appropriate common-mode voltage (perhaps ground), do you
still have noise? What sort of diodes are those, and why are they
there? (Hint: if they are standard signal diodes as the symbol
suggests, they won't do much to clamp the signal within the common-
mode range of the amplifiers.) Have you done anything to insure the
input signals stay within the common mode range of the amplifiers?
(Do the input signals share a common "ground" with the amplifier
circuit, and are they nominally near 0V?) One common mistake in
applying this sort of amplifier is having an input signal with no
ground reference, and the amplifier inputs "float" to a DC level
that's inappropriate, outside the common mode range of the amps. A
resistor from each input to ground can often cure that problem. The
resistors in the schematic do not do that.

Like Martin wrote, it would be more usual to feed signals through a
shielded twisted-pair. Since the "gain" of the amplifier is about
-12dB, I'd be surprised if you really even needed the shielding, if
you're not in an electrically/magnetically noisy environment.

.

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