noise problem with differential analogue input

[jsmith]

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

 

[martin griffith]

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

I wonder why you are using coax for a differential signal, this wont
help, twisted pair should be better.What is the required bandwidth?

What CM rejection are you getting?



martin

 

[Tom Bruhns]

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.

 

[jsmith]

I wonder why you are using coax for a differential signal, this wont
help, twisted pair should be better.What is the required bandwidth?

What CM rejection are you getting?

martin- Hide quoted text -

- Show quoted text -

50 reads/sec (averaging about 194 ADC samples[10bits] each 20
milliseconds for each read)

 

[martin griffith]

I wonder why you are using coax for a differential signal, this wont
help, twisted pair should be better.What is the required bandwidth?

What CM rejection are you getting?

martin- Hide quoted text -

- Show quoted text -

50 reads/sec (averaging about 194 ADC samples[10bits] each 20
milliseconds for each read)

Whats the ambient electrical noise like?

Ok, just had another look at the circuits, the diodes seem to be
reversed biased al the time, so they dont actually seem to do
anything.

i think you need to dump the coax, and try some screed cable, proper
screened microphone cable would do for a test, or even CAT 5.

A quick tweak would be to drop the value of R76 a bit, and put a
small pot in series, with 1% R's your CMRR will only be in the 40dB
range, the pot will get you into the 70dB or more range, but this wont
solve having he wrong cable type


martin

 

[jsmith]

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.

 

[Eeyore]

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.

What is the signal source ? Impedance, signal voltage etc. What's the source of the
interfering noise ?

Your configuration isn't a true differential input btw and that won't help if the noise
level is high.

Graham

 

[martin griffith]

What is the signal source ? Impedance, signal voltage etc. What's the source of the
interfering noise ?

Your configuration isn't a true differential input btw and that won't help if the noise
level is high.

Graham

It looks as if it's 100R per leg, but this guy needs a bit of tuition
in drawing circuits (sorry John)


martin

 

[jsmith]

It looks as if it's 100R per leg, but this guy needs a bit of tuition
in drawing circuits (sorry John)

martin- Hide quoted text -

- Show quoted text -

The main problem was fluctuation of common lines that solved with
sharing all commons with some resistors to GND of circuit. but it till
suffer from some (about +/- 2%) reading fluctuation especially at one
of the inputs. Because of some field problems.unfortunately, I have to
record scope trace of signal fluctuations in few next days.

 

[neon]

the coax cable must be grounded a the source and nowhere else. the amps must have their ground separete comming from the source.