Re: noise problem with differential analogue input

On Jun 27, 10:37 pm, Tom Bruhns <k...@xxxxxxx> wrote:
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.- Hide quoted text -

- Show quoted text -

Thanks for your post and Martin ones,

I try to put an image of a scope trace, tomorrow.
All signals (6) are DC voltages between 0 to 10 volts. In fact, they
are from resistors of voltage dividers that show voltage of some High
Voltage capacitors.
Ground of all capacitors (6) have connected to same point (earth and
my boards chassis) that connect to G inputs. 5 capacitors (signals)
have posetive charges and 1 capacitor (signal) has negative charge.
If I short the two inputs together, my system shows (true) Zero
voltage.
Diodes are 13 Volts/1Watt Zener for opamp protection from possible
high voltage inpouts.

.

Relevant Pages

  • Re: Update on laser distance meter project (master thesis) and question
    ... > measure signals lower then noise. ... total capacitance at the detector and the amplifier input noise, ... frequcncy instabliity wich you could not see on your scope. ...
    (sci.electronics.design)
  • Re: linear model Hioki - Telecommunications
    ... > the intelligence and noise such that. ... there is a concept that two or more signals can be analyzed ... amplifier noise is almost always referred to its input. ... short the input (no generator) and measure the noise and put ...
    (sci.electronics)
  • Re: Noob question about distortion
    ... That defines the ability of the amplifier ... intermodulation of signals applied to the input - they do not have the ... be considered 'noise' or hum (although hum is usually mentioned to be ...
    (rec.audio.tech)
  • Re: Noob question about distortion
    ... That defines the ability of the amplifier ... to reject external interfering signals which appear on the two input ... Now the noise portion of THD+N should include *everything* within the ... be considered 'noise' or hum (although hum is usually mentioned to be ...
    (rec.audio.tech)
  • Laser range finder optoelectronic mixing noise problem
    ... inject a frequenty into the bias voltage of the avalanche photodiode. ... give sufficient signal at the input of my amplifier. ... resistor the better for the signal-noise ratio(for thermal resistor ... noise) (as noise increases with root of the resistance and the signal ...
    (sci.electronics.design)