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Connecting a grounded single ended source to a differential input amplifier via coaxial cable.

Hi,

I would like to know what would be the best way to connect a grounded
single ended signal to a floating differential input amplifier?
The diff. amp is powered from an isolated power source. The chassis of
the box which contains the diff amp is grounded to the same earth as
the sensor.
The sensor output connects via coaxial cable to the diff amp. The
shield is earthed at the sensor side and the other side connects to
the diff amp negative input. The inner core of the coax connects to
the positive of the diff amp. This is done to avoid ground loop
currents in the shield.

Is it ok to connect an unbalanced cable to a diff amp? I know I lose
the advantage of common mode noise rejection.

Is this better done with a single ended input amp? What about ground
loop currents then?

With the differential input, what kind of biasing resistors are
necessary for diff amp input bias currents?

Thank you for any help!

MJ
 
J

John Larkin

Jan 1, 1970
0
Hi,

I would like to know what would be the best way to connect a grounded
single ended signal to a floating differential input amplifier?
The diff. amp is powered from an isolated power source. The chassis of
the box which contains the diff amp is grounded to the same earth as
the sensor.
The sensor output connects via coaxial cable to the diff amp. The
shield is earthed at the sensor side and the other side connects to
the diff amp negative input. The inner core of the coax connects to
the positive of the diff amp. This is done to avoid ground loop
currents in the shield.

Is it ok to connect an unbalanced cable to a diff amp?
Yes.

I know I lose
the advantage of common mode noise rejection.

Don't see why.
Is this better done with a single ended input amp?
Yes.

What about ground
loop currents then?

Ground loops? From where? The amp supply is isolated.
With the differential input, what kind of biasing resistors are
necessary for diff amp input bias currents?

If you do this, something has to establish the common-mode voltage!

What's the signal bandwidth here?

John
 
J

Joerg

Jan 1, 1970
0
John said:
Don't see why.




Ground loops? From where? The amp supply is isolated.

Worst case there is always Mini-Circuits and their little transformers :)
If you do this, something has to establish the common-mode voltage!

What's the signal bandwidth here?

Since there ain't too much happening in politics right now you could
have mentioned split grounds here, then we'd have another hot-blooded
discussion ...
 
P

PeteS

Jan 1, 1970
0
Joerg said:
Worst case there is always Mini-Circuits and their little transformers :)




Since there ain't too much happening in politics right now you could
have mentioned split grounds here, then we'd have another hot-blooded
discussion ...

Snort

Cheers

PeteS
 
Don't see why.


Ground loops? From where? The amp supply is isolated.


If you do this, something has to establish the common-mode voltage!

What's the signal bandwidth here?

John

Hi John, thanks for the reply.

Signal bandwidth is up to 100kHz, but propably a lot less.

I say I would lose the advantage with common mode noise rejection
because I connect a coaxial cable, which shields the inner core, to
the diff amp,
and hence the noise on the 2(1 shield and 1 inner core) conductors are
not the same. A twisted pair would have been better- but the customer
already has 100s
of meter of coax laid out. Or am I wrong here?

The customer wants to connect the shield to the chassis of the
measurement instrumentation, and that would create a ground loop, so
he will short isolated ground
to chassis ground. He claimes that gives better readings. He thinks
the noise on the shield needs a good return path, and connecting to
the instrument chassis on one end and earth, which is already a very
good return path, on the other end is a good idea.

I already have a diff amp, so that's why I am leaning towards it, I
also thought it would be a beter solution. The biasing resistors would
connect down to isolated ground.
What do you mean something must establish the common-mode voltage?

Why do you think the single ended amp would be better? Obviously, then
the customer should not short the shield to the chassis.
I can make my diff amp single ended by connecting the negative input
to (isolated) ground- this is done with a mosfet switch with 1ohm Ron.

Any other suggestions as to how to best measure grounded signal
sources with isolated amps?

MJ
 
Hi,

I would like to know what would be the best way to connect a grounded
single ended signal to a floating differential input amplifier?
The diff. amp is powered from an isolated power source. The chassis of
the box which contains the diff amp is grounded to the same earth as
the sensor.
The sensor output connects via coaxial cable to the diff amp. The
shield is earthed at the sensor side and the other side connects to
the diff amp negative input. The inner core of the coax connects to
the positive of the diff amp. This is done to avoid ground loop
currents in the shield.

Is it ok to connect an unbalanced cable to a diff amp? I know I lose
the advantage of common mode noise rejection.

Is this better done with a single ended input amp? What about ground
loop currents then?

With the differential input, what kind of biasing resistors are
necessary for diff amp input bias currents?


Try wrapping the coaxial cable around a ferrite toroid. This turns it
into a balun, also known as a transmission line transformer. Googling
for "balun 'transmission line transformer' " threw up a number of
links. The first one

http://www.highfrequencyelectronics.com/Archives/Feb04/HFE0204_Sevick.pdf

actually describes more complicated variations on the standard 1:1
balun as does

http://rfic.eecs.berkeley.edu/~niknejad/ee217sp05/lect10.pdf

I had to dig down quite a way to find a link that even talked about
the standard 1:1 unbalanced-to-balanced application

http://www.home.earthlink.net/~christrask/TraskTLTTutorial.pdf

My favourite reference is to Matick R E "Transmission-line pulse
transformers - theory and applications" from the Proceedings of the
IEEE volume 56, pages 47-62 (1968) but you need a university library
or a subscription to IEEE Explore to get hold of it.
 
F

Fred Bloggs

Jan 1, 1970
0
The customer wants to connect the shield to the chassis of the
measurement instrumentation, and that would create a ground loop, so
he will short isolated ground
to chassis ground.

It is not a "ground loop," it is a major line imbalance and uncontrolled
reference return...

He claimes that gives better readings.

That is probably right but not for the reason he thinks. The chassis to
isolated ground connection eliminates noise current injection into the
IA input due to potential differences between the two grounds. This will
be going on internal to the chassis.

[...snip...]
 
F

Fred Bloggs

Jan 1, 1970
0
Try wrapping the coaxial cable around a ferrite toroid. This turns it
into a balun, also known as a transmission line transformer.

That used to be called a "coaxer" and it is a very high frequency common
mode choke, not a balun...I have used this method and it works well.
Googling
for "balun 'transmission line transformer' " threw up a number of
links. The first one

http://www.highfrequencyelectronics.com/Archives/Feb04/HFE0204_Sevick.pdf

actually describes more complicated variations on the standard 1:1
balun as does

http://rfic.eecs.berkeley.edu/~niknejad/ee217sp05/lect10.pdf

I had to dig down quite a way to find a link that even talked about
the standard 1:1 unbalanced-to-balanced application

http://www.home.earthlink.net/~christrask/TraskTLTTutorial.pdf

My favourite reference is to Matick R E "Transmission-line pulse
transformers - theory and applications" from the Proceedings of the
IEEE volume 56, pages 47-62 (1968) but you need a university library
or a subscription to IEEE Explore to get hold of it.

The largest interference will most likely arise from external current
noise sources magnetically inducing a differential mode voltage into the
coax. He has no option except to voltage-to-current convert the sensor
output and terminate in a low impedance at the IA in that case, assuming
he has to use coax and maintain a wide signal bandwidth.
 
P

Paul Mathews

Jan 1, 1970
0
Hi John, thanks for the reply.

Signal bandwidth is up to 100kHz, but propably a lot less.

I say I would lose the advantage with common mode noise rejection
because I connect a coaxial cable, which shields the inner core, to
the diff amp,
and hence the noise on the 2(1 shield and 1 inner core) conductors are
not the same. A twisted pair would have been better- but the customer
already has 100s
of meter of coax laid out. Or am I wrong here?

The customer wants to connect the shield to the chassis of the
measurement instrumentation, and that would create a ground loop, so
he will short isolated ground
to chassis ground. He claimes that gives better readings. He thinks
the noise on the shield needs a good return path, and connecting to
the instrument chassis on one end and earth, which is already a very
good return path, on the other end is a good idea.

I already have a diff amp, so that's why I am leaning towards it, I
also thought it would be a beter solution. The biasing resistors would
connect down to isolated ground.
What do you mean something must establish the common-mode voltage?

Why do you think the single ended amp would be better? Obviously, then
the customer should not short the shield to the chassis.
I can make my diff amp single ended by connecting the negative input
to (isolated) ground- this is done with a mosfet switch with 1ohm Ron.

Any other suggestions as to how to best measure grounded signal
sources with isolated amps?

MJ- Hide quoted text -

- Show quoted text -

It's easy to show that common mode rejection absolutely depends on
having the impedances for both differential inputs exactly the same.
Provided that both inputs are maintained within the common mode
voltage range, the diff amp doesn't care, so to speak, whether the
signal is single-ended or what some folks call 'true' differential.
There is much practical information on this subject available at these
websites:

http://www.jensen-transformers.com/an/an003.pdf
http://www.rane.com/library.html#rnotes
www.thatcorp.com

For audio signals, the best results can be obtained using
transformers.

http://www.jensen-transformers.com/

Paul Mathews
 
It's easy to show that common mode rejection absolutely depends on
having the impedances for both differential inputs exactly the same.
Provided that both inputs are maintained within the common mode
voltage range, the diff amp doesn't care, so to speak, whether the
signal is single-ended or what some folks call 'true' differential.
There is much practical information on this subject available at these
websites:

http://www.jensen-transformers.com/....rane.com/library.html#rnoteswww.thatcorp.com

For audio signals, the best results can be obtained using
transformers.

http://www.jensen-transformers.com/

Paul Mathews- Hide quoted text -

- Show quoted text -


Thank you all for the replies- the whole problem is still unclear to
me though.

Also, this is not an RF application- it is an accelorometer sensor app
with a single ended voltage output.
The fact that the diff amp runs from an isolated supply make matters
worse, sine the customer wants to connect his
signal gnd to the diff amp chassis- which is at the same ground as his
signal gnd- but the diff amp runs off its own isolated supply.

Would it be wrong to just connect the shield to the one input of the
diff amp and the inner core to the + input of the diff amp? And not
connecting anything to the chassis.
John says a single ended amp would be better. But the single ended amp
would also be isolated (all the input channles run from an isolated
supply in this measurement system).
Most solutions show a 2 wire + shield cable- I have only 2 conductors,
the shield and the shielded inner core.

Is it so difficult to measure a ground single ended signal with an
isolated diff or single ended input amp?

MJ
 
W

whit3rd

Jan 1, 1970
0
Hi,

I would like to know what would be the best way to connect a grounded
single ended signal to a floating differential input amplifier?

Just put a resistor from the 'ground' wire of your signal cable to
the local ground. That will keep the common-mode in range if the
distant end's 'ground' connection is ever allowed to float. Any
other signal-conditioning (filters, clamps, etc.) will be the
same regardless of the wiring scheme.

Do be careful, though, that the 'ground' wire of the signal cable
doesn't connect IN MULTIPLE PLACES to any kind of building
frame or other grounded metal; minor ground-fault currents
can be guided through your cable and the resulting I*R voltage drop
can swamp a small signal. The resistor-to-ground at the amplifier
end should be chosen to be higher value than the wiring resistance
so it won't mess measurements up. Much.
 
J

Jeroen Belleman

Jan 1, 1970
0
Hi,

I would like to know what would be the best way to connect a grounded
single ended signal to a floating differential input amplifier?
The diff. amp is powered from an isolated power source. The chassis of
the box which contains the diff amp is grounded to the same earth as
the sensor.
The sensor output connects via coaxial cable to the diff amp. The
shield is earthed at the sensor side and the other side connects to
the diff amp negative input. The inner core of the coax connects to
the positive of the diff amp. This is done to avoid ground loop
currents in the shield.
[...]

Try wrapping the coaxial cable around a ferrite toroid. This turns it
into a balun, also known as a transmission line transformer. Googling
for "balun 'transmission line transformer' " threw up a number of
links. The first one
[...]

I think that if your signal comes in over a cable longer than
a meter or two, the few uH of common mode inductance added by
this kind of balun is not going to make much of a difference.

Low frequency current flowing on cable shields (common mode
current) ends up leaking in because of the cable shield
resistance, so shield currents are to be avoided. Baluns don't
help against low-frequency common mode current. Transformers
and differential amplifiers do. The choice of which is better
depends on the signal bandwidth.

Would you hire Bill or me, John? ;-)

Jeroen Belleman
 
J

Jamie

Jan 1, 1970
0
Hi,

I would like to know what would be the best way to connect a grounded
single ended signal to a floating differential input amplifier?
The diff. amp is powered from an isolated power source. The chassis of
the box which contains the diff amp is grounded to the same earth as
the sensor.
The sensor output connects via coaxial cable to the diff amp. The
shield is earthed at the sensor side and the other side connects to
the diff amp negative input. The inner core of the coax connects to
the positive of the diff amp. This is done to avoid ground loop
currents in the shield.

Is it ok to connect an unbalanced cable to a diff amp? I know I lose
the advantage of common mode noise rejection.

Is this better done with a single ended input amp? What about ground
loop currents then?

With the differential input, what kind of biasing resistors are
necessary for diff amp input bias currents?

Thank you for any help!

MJ
in cases like that, you are losing the balanced use of the input.
In my opinion, you should be tying the shield side to common.
Just use either input of the amp if polarity isn't a problem,
and tie off the other input so that it won't detect any noise.
I do think if you tie the unused input to common also, you'll
be all set.
 
in cases like that, you are losing the balanced use of the input.
In my opinion, you should be tying the shield side to common.
Just use either input of the amp if polarity isn't a problem,
and tie off the other input so that it won't detect any noise.
I do think if you tie the unused input to common also, you'll
be all set.

--
"I'm never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.http://webpages.charter.net/jamie_5- Hide quoted text -

- Show quoted text -

My 'common' is isolated and floating with respect to the sensor's
common.
Tying the shield to common, will short my isolated ground and break
the isolation barrier.
And a current might flow in the shield, inducing noise in the inner
core, because of potential differences in
the 2 'common' points.
Remember, my input stage is floating.

MJ
 
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