Newmarket

Getting "BrainAche" here.

Testing (via LT Spice) Ground Cancelling Output circuit(s).
I've modelled the Test Circuit in Douglas Self's Small Signal Audio Design 2nd ed. P542 Fig 19.6
All good.

But I want to test it into a differential input ( eg 'standard 4 resistor diff amp ).  I just can't get it straight in my  head (or on the screen) how I should configure this wrt injecting CM noise.  Any thoughts ?


Audio1Man

Re: Ground Cancelling Output Test Config for Differential Input
« Reply #1 on: July 18, 2018, 12:55:34 PM »

1.   Connect the output of the signal generator to pin #2 & #3, sweep frequency through the band. This is the reference level.
2.    Connect the HOT output of the signal generator to pin #2 & #3 and common to pin #1 then sweep frequency through the band. This is the attenuation or CMRR.
3.   For the REAL WORLD CMRR, place a 10 ohm resistor in PIN #2 of the lead and run the sweep frequency through the band. Then repeat again with pin #3
I hope this answers your question.
Duke

abbey road d enfer

Re: Ground Cancelling Output Test Config for Differential Input
« Reply #2 on: July 18, 2018, 03:07:16 PM »
1.   Connect the output of the signal generator to pin #2 & #3, sweep frequency through the band. This is the reference level.
2.    Connect the HOT output of the signal generator to pin #2 & #3 and common to pin #1 then sweep frequency through the band. This is the attenuation or CMRR.
3.   For the REAL WORLD CMRR, place a 10 ohm resistor in PIN #2 of the lead and run the sweep frequency through the band. Then repeat again with pin #3
I hope this answers your question.
Duke
That's how it's been traditionally done for decades but it's not accurate in the sense it shorts the input so does not take into account the difference of CM impedance between the two inputs, which can be very significant, particularly with xfmrs at higher frequencies. Transformers are highly assymetrical when it comes to stray capacitance.
The BBC test takes care of this issue by driving the two legs each via a 100R resistor (that's for mic inputs, line inputs may use different values).
Check page 7 of
http://www.jhbrandt.net/wp-content/uploads/2014/11/Design_of_High-Performance_Balanced_Audio_Interfaces.pdf
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

squarewave

Re: Ground Cancelling Output Test Config for Differential Input
« Reply #3 on: July 18, 2018, 03:28:25 PM »
But I want to test it into a differential input ( eg 'standard 4 resistor diff amp ).  I just can't get it straight in my  head (or on the screen) how I should configure this wrt injecting CM noise.  Any thoughts ?
Just model a virtual balanced cable which could be as simple as connecting pin 2's and pin 3's together.

But I think CM performance is going to depend largely on how closely matched the two impedances are (actually 4 impedances - 2 out and 2 in).

To understand that, I would do an impedance plot. An impedance plot is like the usual AC analysis with the usual signal generator, except instead of plotting V over frequency you plot V/I over frequency (recall that Ohms law says that R = V/I and so in AC analysis R means impedance). Specifically, add a load resistor to ground on each pin and then do AC analysis, Run and click on the output pin to plot V which will show a data series label at the top like "V(n005)". Right click on that and edit the formula to read "V(n005)/I(R7)" where R7 is your load resistor to ground [1] and click Ok. Now right click on the scale on the left and select "Manual Limits". Change the drop down from "Decibel" to "Linear". Now the V/I axis should show impedance! Now click on the other output pin and edit it's formula. Now you can see the difference in impedance and decide how significant it is.

[1] If the resistor isn't grounded, you would have to use the difference in voltage across the resistor which changes the formula a little be equally useful. Actually the officially proper way to do this is to replace the signal generator with a load resistor (like 150R for a mic input for example) and attach an AC current source to the output(s) and then do like above but use a formula like "V(n005)/I(I1)". If you search "ltspice plot impedance" this is usually how it's described.

PRR

Re: Ground Cancelling Output Test Config for Differential Input
« Reply #4 on: July 18, 2018, 04:58:56 PM »
Where is the common mode noise coming from?

Two likely sources:

Longitudinal in the shield. Inject a small voltage between source and receiver "grounds".

Capacitive strays from room-crap to the signal conductors. Try 100V through 30pFd to both conductors. Try 30p one side and 29p the other side (your twist may not be exact). Being reactive you want to plot against frequency. 60Hz won't do much through 30pFd but the high harmonics of power buzz will be exaggerated.

Of course a true "Ground cancelling output" should not need a differential receiver.....

Newmarket

Re: Ground Cancelling Output Test Config for Differential Input
« Reply #5 on: July 20, 2018, 08:04:19 AM »
Thanks for the balanced (see what I did there !)  input people.

I could have been clearer that I was looking at the performance of the Ground Cancelling output stage rather than the differential receiver. But it's good to be reminded about the 'BBC' mods and intentionally unbalancing the impedances.

Basically having built some output stages with configuration options I sort of realised I wasn't quite clear how it would work into a differential input. So I thought I'd simulate it to check then was struck with doubt about whether my simulation circuit was valid.
I'll blame the heat - it's been hot here - Crawley UK -and no air con in the office  >:( Bit cooler today though  :)

It all seemed a lot clearer in my mind yesterday and was able to see sense in the sim results I was getting wrt 'Zero Impedance' output cf Impedance Balanced output into diff amp.

Thanks again.


Newmarket

Re: Ground Cancelling Output Test Config for Differential Input
« Reply #6 on: July 20, 2018, 08:09:56 AM »
Where is the common mode noise coming from?

Two likely sources:

Longitudinal in the shield. Inject a small voltage between source and receiver "grounds".

Capacitive strays from room-crap to the signal conductors. Try 100V through 30pFd to both conductors. Try 30p one side and 29p the other side (your twist may not be exact). Being reactive you want to plot against frequency. 60Hz won't do much through 30pFd but the high harmonics of power buzz will be exaggerated.

Of course a true "Ground cancelling output" should not need a differential receiver.....

Yes - I was really looking at the 'longitudinal' noise. I did in fact have a voltage between the 'grounds' but I was doubting that it was the correct way to do it.
Good ideas on the noise pick up sim.

Yes - indeed it doesn't 'need' a diff input - sort of the raison d'etre - but at some point it's going to get one !

Thanks for your help.

scott2000

Re: Ground Cancelling Output Test Config for Differential Input
« Reply #7 on: July 20, 2018, 08:11:06 AM »

I'll blame the heat - it's been hot here - Crawley UK -and no air con in the office  >:( Bit cooler today though  :)


Portable AC work really well.... If it gets too bad...... I have one and after the last hurricane here left us w/o power for a bit, it was a great thing as it was able to be used off of the generator ....

Sorry for the off topic.... It's the only experience I have to offer in this thread....lol

Newmarket

Re: Ground Cancelling Output Test Config for Differential Input
« Reply #8 on: July 20, 2018, 08:18:22 AM »
Portable AC work really well.... If it gets too bad...... I have one and after the last hurricane here left us w/o power for a bit, it was a great thing as it was able to be used off of the generator ....

Sorry for the off topic.... It's the only experience I have to offer in this thread....lol

well relying on fans and windows atm - I imagine that you need A/C for more of the year than South East England. And staying away from the soldering iron of course  :D

 

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