Author Topic: Connecting instruments without making ground loops  (Read 1019 times)

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Offline ezalys

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Connecting instruments without making ground loops
« on: October 20, 2020, 01:27:23 am »
Let’s suppose I have a few home-made instruments I’m keeping in a rack, each in a 2U chassis. They all plug into a big analog supply at the base of the rack. How do I set up the grounding in the instruments to avoid creating ground loops through the BNC cable shields? Everything must be DC coupled.

Next to the rack is a device which can only be driven in a single-ended manner. In the end, there must be multiple connections to the ground of the DUT. The technique espoused in Ott and other books on noise is to drive the shield and center conductor on the source and then sense using a diff amp the potential between center and shield on the load, avoiding shorting the shield to the chassis. At the end of the signal chain, I can’t do this, as the device is too simplistic to sense using a diff amp. How do I work around this?
« Last Edit: October 20, 2020, 01:35:14 am by ezalys »
 

Offline bob91343

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Re: Connecting instruments without making ground loops
« Reply #1 on: October 20, 2020, 02:33:13 am »
Are you working with very small signals?  If so, you need differential connections.  If not, you probably won't have any problem, although if you do the solution should be obvious.
 

Offline ezalys

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Re: Connecting instruments without making ground loops
« Reply #2 on: October 20, 2020, 02:59:41 am »
I’d love to connect everything differentially... but I’m still not totally sure what people mean when they say that. Drive one end of a piece of coax single-ended and sense the shield and center conductor with an instrumentation amp? Do that but with shielded twisted pair? What do I do with the device itself, which can’t be built with differential connections?
 

Offline bob91343

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Re: Connecting instruments without making ground loops
« Reply #3 on: October 20, 2020, 04:14:25 am »
Don't confuse shielding with differential sensing.  Shielding is an attempt to prevent radiated energy from corrupting measurements.  Differential sensing is an attempt to ignore common mode signals, those which are present on both lines and are not desired.

The old fashioned twisted pair of early telephone lines is an example of differential operation.

A transmission line is another consideration but if your line is short compared to the wavelength of the highest frequency of interest you can treat it as a direct connection.  A line one foot long is a direct connection for 1 MHz but is a transmission line for 150 MHz.

Back to your original question, the answer is that you would benefit from learning about this stuff and making some experiments.

I was looking at a signal generator output on an oscilloscope a while back, and was distressed by the fact that the output wasn't flat, that it got bigger at the high frequency end.  Then I realized I wasn't terminating the line so there were reflections.  With a shorter cable or a resistive termination the readings were flat.  Not anything to do with ground loops, which is yet another subject.
 

Offline David Hess

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Re: Connecting instruments without making ground loops
« Reply #4 on: October 20, 2020, 04:25:49 pm »
Let’s suppose I have a few home-made instruments I’m keeping in a rack, each in a 2U chassis. They all plug into a big analog supply at the base of the rack. How do I set up the grounding in the instruments to avoid creating ground loops through the BNC cable shields? Everything must be DC coupled.

The problem is that the BNC shields, and every other "ground", which should really be called "common", are connected to one of the DC power wires, usually the negative one, and the current drawn through that wire by each instrument creates a different voltage offset from where they are all connected together.  So the answer to "How do I set up the grounding in the instruments to avoid creating ground loops through the BNC cable shields?" is that you do not, which leaves the following options:

1. Isolate the instruments at their power inputs.  This is how AC line powered instruments do it, albeit usually poorly because of large amounts of capacitive coupling.  The Tektronix TM500 series instruments do this by providing a pair of floating 25VAC transformer secondary windings to each instrument.  I sometimes include an isolated DC-to-DC converter at the 12 volt power input inside the instrument to do exactly this.  If regulation is not required, then an inverter, which has some advantages for low noise, could be used to convert 12 volts to isolated 12 volts at each power input.

2. Use differential signalling.  Audio instruments commonly do it this way with XLR connectors which provide the two differential signal pins and a common pin.  Higher performance test instrumentation may do this with a pair of coaxial connectors.  Oscilloscope differential probes which connect through a single coaxial connection to the instrument do this.

3. Remote sense the common mode signal through a single ended connection.  This is essentially the differential signaling method in 2 above but using a single coaxial connection and is also common with audio instruments.  The BNC shield connection is used to sense the remote common mode voltage.  Note that sensing could work either way with the source sensing the common mode voltage at the receiver or the reverse.  Small Signal Audio Design by Douglas Self discusses how this is done in audio applications.  Sometimes this is done with triax cable or shielded differential cable.

4. Galvanically isolate the input or output or both.  This means generating a floating power supply for each input or output and transferring the signal across the isolation barrier.  The old 10Base-2 Ethernet standard which uses BNC connectors did this.  Oscilloscopes with isolated inputs do this for each input.  Cleverscope does this for its function generator output.

Quote
Next to the rack is a device which can only be driven in a single-ended manner. In the end, there must be multiple connections to the ground of the DUT. The technique espoused in Ott and other books on noise is to drive the shield and center conductor on the source and then sense using a diff amp the potential between center and shield on the load, avoiding shorting the shield to the chassis. At the end of the signal chain, I can’t do this, as the device is too simplistic to sense using a diff amp. How do I work around this?

That is method 3 above and it can work in reverse.  Sense the remote BNC shield voltage from the transmitter and adjust the center signal accordingly, so the difference amplifier is at the transmitter side rather than the receiver side.  I have done it by driving the center signal with a current instead of a voltage and then providing a resistive load at the receiver but this requires a special cable with the resistive load built in.  Sensing the remote BNC shield voltage with a difference or instrumentation amplifier at the source is more universal and works with a standard BNC input.

Method 4 above works even better but is more complex because it requires an isolated power supply and it requires the signal to be transferred across the isolation barrier somehow.  Method 1 above with an isolated DC-to-DC converter at the DC power input is simpler.
 

Offline ezalys

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Re: Connecting instruments without making ground loops
« Reply #5 on: October 20, 2020, 06:10:39 pm »
I like method 4, but I'm quite worried about generating a bunch of common mode noise with my DC-DC converter. If I could buy faraday shielded transformers then maybe I'd be more ammenable to it. It seems easier to earth everything and then use a differential driver like a THAT1646.

Why is it exactly that method 4 beats method 3?
 

Offline ejeffrey

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Re: Connecting instruments without making ground loops
« Reply #6 on: October 20, 2020, 07:17:37 pm »
I like method 4, but I'm quite worried about generating a bunch of common mode noise with my DC-DC converter. If I could buy faraday shielded transformers then maybe I'd be more ammenable to it. It seems easier to earth everything and then use a differential driver like a THAT1646.

Why is it exactly that method 4 beats method 3?

With method 3, whichever end you are sensing the shield will have a high impedance connection to local circuit ground and shield and thus be less good at shielding.  Its common in audio gear to capacitively shunt to she shield to ground so that it is low impedance at RF where you worry about interference more than ground loops and high impedance at low frequency where the reverse is true. A ferrite bead around the cable close to the entry point can reduce the common mode noise as well.

Using a screened differential signal such as twinax, pairs of coax, or shielded twisted pair you separate the job of shielding from providing a reference voltage.  Floating transmitters use the same for both, but the isolation prevents common mode signals from flowing in the shield.
 

Offline ezalys

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Re: Connecting instruments without making ground loops
« Reply #7 on: October 21, 2020, 05:16:20 pm »
Okay! I think that makes sense. So I guess I'm now trying to think about at what level I need to be paranoid. Do I need to float each individual DAC channel? If I have three BNC cables, each three feet long, going from an instrument with a floating common to a breakout box, will I create a significant ground loop, if the source instrument shorts all the BNC shields together AND the breakout box shorts all the BNC shields together? Do I need to design a DAC where _each individual_ DAC is floating? Maybe I implement some functionality to break all the shields but one?
« Last Edit: October 21, 2020, 05:21:57 pm by ezalys »
 

Offline Electrole

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Re: Connecting instruments without making ground loops
« Reply #8 on: October 21, 2020, 06:55:16 pm »
Also, a good read about how to avoid interference is this note from NPL: http://eprintspublications.npl.co.uk/331/

Best regards
Ole
 

Online SilverSolder

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Re: Connecting instruments without making ground loops
« Reply #9 on: October 21, 2020, 07:41:25 pm »
Depending on the frequencies involved, something as simple as an audio transformer can isolate a signal generator galvanically from the DUT and thereby eliminate ground loops.

Another signal fidelity issue to worry about is voltage drops through the coaxial cable shields - if you are driving something hard enough through a long enough cable, the Common connection at the far end will not be at the same level as the Common at the generator!  (A transformer at the far end can help cure that problem too.)  Best of all is a shielded transformer, but now we are getting a little esoteric.
 

Offline ezalys

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Re: Connecting instruments without making ground loops
« Reply #10 on: October 21, 2020, 07:43:11 pm »
An isolation transformer would be a great solution but I really need everything to be DC coupled.
« Last Edit: October 21, 2020, 07:51:30 pm by ezalys »
 

Online SilverSolder

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Re: Connecting instruments without making ground loops
« Reply #11 on: October 21, 2020, 08:57:14 pm »
An isolation transformer would be a great solution but I really need everything to be DC coupled.

Understood.  Sorry for missing that in your original post... 

What sorts of DC levels of signal are you looking at?  Millivolts, microvolts?



 

Offline ezalys

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Re: Connecting instruments without making ground loops
« Reply #12 on: October 22, 2020, 11:38:06 am »
Well the DACs are all 18 bit and 10 volts full scale, and I’d like to try to not kill the ENOB. So tens of microvolts.
 

Online SilverSolder

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Re: Connecting instruments without making ground loops
« Reply #13 on: October 22, 2020, 01:02:13 pm »

Is it an option to put a battery powered chopper amplifier close to the DUT, so you don't need to run low level signals for long distances?
 

Online NANDBlog

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Re: Connecting instruments without making ground loops
« Reply #14 on: October 22, 2020, 01:50:23 pm »
Well the DACs are all 18 bit and 10 volts full scale, and I’d like to try to not kill the ENOB. So tens of microvolts.
Why do you have shielding in the first place? Do you need it? If you have signals, that are driven by a low impedance source (<50Ohm) driving a high impedance sink, you could get away without shielding, depending on your environment and your frequency range. Think about 6.5 and 8.5 digit multimeters, they are shipped with banana cables for a reason.
 

Offline jonpaul

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Re: Connecting instruments without making ground loops
« Reply #15 on: October 22, 2020, 02:57:32 pm »
Bonjour, all of this was solved decades ago. Seems the originator is not aware of the extensive work.

I Suggest the originator read the standard texts eg Henry OTT, Noise-Reduction-Techniques-Electronic-Systems

https://www.amazon.com/Noise-Reduction-Techniques-Electronic-Systems-ebook/dp/B004P1JTDE

http://k9yc.com/publish.htm

See Jim Brown and Bill Whitlock papers for AES.


Enjoy,

Jon



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Offline David Hess

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Re: Connecting instruments without making ground loops
« Reply #16 on: October 22, 2020, 05:27:55 pm »
Well the DACs are all 18 bit and 10 volts full scale, and I’d like to try to not kill the ENOB. So tens of microvolts.

That is a tough requirement no matter how it is implemented and it will be easier with a fully isolated design which does not rely on difference or instrumentation amplifiers.  If you figure a common mode voltage of 200 millivolts and 20 microvolts of allowable error, then that requires 80 dB of common mode rejection which is very practical but means using precision parts including operational amplifiers and resistors.  Common mode rejection falls with frequency so this may not be feasible except at low frequencies.

Either method will work.  I would go the precision amplifier way simply because of familiarity.  A part like the AD8421 is suitable for driving an output and sensing the remote common mode voltage.
 

Offline ezalys

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Re: Connecting instruments without making ground loops
« Reply #17 on: October 22, 2020, 09:51:53 pm »
I think now is a good time to make some practical measurements. What’s a good way to measure the common mode voltage I need to fight? I wanna say a diff probe of some kind. Maybe I just use an instrumentation amplifier!
 

Offline ezalys

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Re: Connecting instruments without making ground loops
« Reply #18 on: October 22, 2020, 09:55:32 pm »
I should also mention that I’m after precision far more than I’m after accuracy. If I’m off in an absolute scale it’s fine, but if I’m using this DAC to zoom in on an extremely fine feature, then noise is quite important. Having everything _stay_ stable is quite important as these measurements can average for a day or so, so having stable references are important... but that it is NIST traceable is unimportant and that my voltage comes out consistently 99% of what’s desired is fine by me.
« Last Edit: October 22, 2020, 09:58:31 pm by ezalys »
 

Offline ezalys

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Re: Connecting instruments without making ground loops
« Reply #19 on: October 23, 2020, 12:24:26 am »
Additionally, I’m convinced that those standards voltmeters don’t stress shielding quite as much since their detection bandwidth is so narrow. Maybe it’s something else? I suppose to start talking about bandwidth I should talk about settling time, which I haven’t done, so that’s a fair point.
 

Offline jonpaul

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Re: Connecting instruments without making ground loops
« Reply #20 on: October 23, 2020, 06:30:43 am »
Bonjour: seems to be some confusion here:

The need for isolation and differential measurement is not clear in the setup  described by ezalys.

"ground loops" are due to  a circulating current  eg  current along a neutral between 2 distribution transformers.
The resistance between two rack-mount devices in a single rack, supplied by the same distribution transformer is nil.
There is no circulating current in the rack. Thus the rack does not create a potential difference  between adjacent chassis.

For very high resolution measurements indeed a differential input is used cancel thermal EMF in connectors, eg DV V to 5-7 digits.


Just the ramblings of an old retired EE

Enjoy,

Jon







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Offline ejeffrey

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Re: Connecting instruments without making ground loops
« Reply #21 on: October 23, 2020, 03:23:40 pm »
Bonjour: seems to be some confusion here:

The need for isolation and differential measurement is not clear in the setup  described by ezalys.

"ground loops" are due to  a circulating current  eg  current along a neutral between 2 distribution transformers.

No ground loops are circulating current in the ground (not neutral).  The loop is usually a combination of the safety ground wire in the electrical wiring and the ground / shielding wires of interconnect cables.  At the consumer end, electrical wiring is usually a tree structure and does not support loops comprised of the live/neutral wires except to the extent that they are coupled to the ground such as by EMI filter capacitors.  An exception to this would be UK ring circuits

Quote
The resistance between two rack-mount devices in a single rack, supplied by the same distribution transformer is nil.
There is no circulating current in the rack. Thus the rack does not create a potential difference  between adjacent chassis.

I have measured amps of ground loop current within a single rack with a bunch of instruments connected together with coax, USB, and banana cables that had not been set up with great consideration for ground loops.
 

Offline David Hess

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Re: Connecting instruments without making ground loops
« Reply #22 on: October 24, 2020, 06:23:23 am »
I think now is a good time to make some practical measurements. What’s a good way to measure the common mode voltage I need to fight? I wanna say a diff probe of some kind. Maybe I just use an instrumentation amplifier!

Either could work.  I would start off with a differential probe on an oscilloscope unless the signal level is too small.  DC and low frequency AC measurements using a multimeter could also be relevant.
 

Offline ezalys

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Re: Connecting instruments without making ground loops
« Reply #23 on: October 24, 2020, 09:49:38 pm »
Well now I know why I paid extra for true RMS.
 


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