Balanced Outputs

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OutSmarts with Added Build-out

Postby mediatechnology » Sat Nov 12, 2011 11:55 am

Thanks Doug and John.

Here's a current-boosted THAT1646 using two devices in parallel with OutSmarts enabled.
Comparative results both with and without added build-out resistors are also shown.


Image
Current-boosted THAT1646 using two devices in parallel with OutSmarts and added build-out resistors.

A few notes...

The THAT1646 has internal build-outs of ~25R. Paralleled, these serve as ballasting resistors and the native Rout/leg is 12.5 Ohms.
Adding 36R to 12.5R provides a Rout of ~48.5 Ohms and a ZoutDiff of ~ 97 Ohms.
Csense capacitors should be used to reduce each THAT1646's common mode offset.
Without Csense, VosCM could be as high as 250 mV and any output current gained by adding the second device would be wasted by it tugging against the other output.
Using Csense, VosCM is typically <<5 mV.
The value of Csense is doubled because it is driving two OutSmarts bridge networks.

Comparison of waveform fidelity vs. build-out values.
Note that in every example, the actual drive voltage is twice the recovered output.
A recovered signal of 10V p-p is actually 20V p-p differentially.


No external build-out, small-signal response, 2V p-p recovered. ZoutDiff ~ 25 Ohms.

Image
THAT1646X2, 0R Build-out, Small Signal 1V/div, 10kHz, 400ft 9451, Recovered -6dB


36 Ohm external build-out, small-signal response, 2V p-p recovered. ZoutDiff of ~ 97 Ohms.

Image
THAT1646X2, 36R Build-out, Small Signal 1V/div, 10kHz, 400ft 9451, Recovered -6dB


No external build-out, large-signal response, 10V p-p recovered. ZoutDiff ~ 25 Ohms.

Image
THAT1646X2, 0R Build-out, Large Signal 5V/div, 10kHz, 400ft 9451, Recovered -6dB


36 Ohm external build-out, large-signal response, 10V p-p recovered. ZoutDiff of ~ 97 Ohms.

Image
THAT1646X2, 36R Build-out, Large Signal 5V/div, 10kHz, 400ft 9451, Recovered -6dB


36 Ohm external build-out.
Single-ended termination, one leg shorted to the shield at the far end.


Image
THAT1646X2, 36R Build-out, Large Signal 5V/div, 10kHz, 400ft 9451, Recovered -6dB, Single-ended.

This all seems like it's worth the extra two resistors, or if one is rolling their own output, making the resistors the optimum value.
A ZoutDiff of about 90-100R seems about right for this cable and the majority of 2C shielded based on published characteristic impedances.
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Re: Balanced Outputs

Postby JR. » Sat Nov 12, 2011 12:48 pm

http://www.ti.com/lit/ds/slos481a/slos481a.pdf

check the several images on page 15 for impact of build out R on ringing for a LM833 opamp, driving a capacitive load.

FWIW...

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Re: Balanced Outputs

Postby mediatechnology » Sat Nov 12, 2011 1:08 pm

Thanks John. Getting used to seeing NSC parts on TI datasheets is going to take awhile.

The highest Cload they showed was 1000 pF and 35R Rbuild.

I estimate our Cload (at 67 pF/ft conductor-conductor) to be 27 nF and ~14 nF conductor to shield.

Yes, the ringing looks to be capacitive, the "hook," cable reflection.
The hook can be shifted up into the rounded edge when Zout << than Zcharactersitic making it look like, or masking, the Cload ringing.

In some of my earlier tests the 5532/34 runs out of current pretty early compared to the 1646.
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Re: Balanced Outputs

Postby JR. » Sat Nov 12, 2011 1:18 pm

FWIW the National data sheet for that part does not include those images.

I just coincidentally came across that the other day...

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Re: Balanced Outputs

Postby mediatechnology » Sat Nov 12, 2011 1:22 pm

What I haven't shown are intermediate values of Zout...

By varying Rbuild-out I can slide the step up or down....

Image

I can tell when Zout = Zcharacteristic when the "hook" is at midpoint. (Balanced)

If Zout << Zcharacteristic the hook is > mid-point on the rising edge. (It also rings.)

If Zout >> Zcharacteristic then the recovered waveform is overdamped and the hook is < mid-point on the rising edge.

By varying Zout, the location of the hook is moved. This is not pure capacitance.

There is a sweet-spot where Zout ~ Zcharactersitic. This gives the cleanest overall response in the recovered waveform.
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Re: Balanced Outputs

Postby ricardo » Sat Nov 12, 2011 4:19 pm

Thanks for these great measurements Wayne.

Have you got a simple way to test comparative EMI immunity? Maybe just a mobile phone?

Best would be a Calrec CB radio but that wouldn't endear you to your neighbours. :lol:
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Re: Balanced Outputs

Postby JR. » Sat Nov 12, 2011 4:44 pm

My initial though that R0=Rz was a little too neat (convenient), and the mechanism for the glitch sliding was probably some delay related to RxC, but another possibility is a simple divider formed by the output R and characteristic cable R that would indeed give a -6dB step function for =R.

Note: This is just a guess so take it with a grain of salt.

It seems logical for events or frequency components that are slower/lower frequency than the transit time down and back the wire run, any reflected energy just looks like phase shift or simple delay. or not...

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Re: Balanced Outputs

Postby JR. » Sat Nov 12, 2011 4:53 pm

ricardo wrote:Thanks for these great measurements Wayne.

Have you got a simple way to test comparative EMI immunity? Maybe just a mobile phone?

Best would be a Calrec CB radio but that wouldn't endear you to your neighbours. :lol:


Attempting to be scientific, about the best you can do is look for rectification, at valid input voltages. If it doesn't rectify for any valid input voltage that is generally considered good enough, Some mic preamps may be expected to reject out of band input larger than the valid signal, but for line level interfaces not slew limiting or rectifying for valid input levels seems fair.

Note: there are topologies that are benignly rise time limited so it is possible the harmlessly pass or ignore infinite input edge rates up to a given voltage level.

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Re: Balanced Outputs

Postby mediatechnology » Sat Nov 12, 2011 4:59 pm

Thanks ricardo.

Have you got a simple way to test comparative EMI immunity? Maybe just a mobile phone?


Yes.

I don't think I'm through in that area since I believe some network like the one you have shown or THAT's would be appropriate. Maybe just a pair of ferrite beads and shunt Cs.
Having said that I've not seen problems without it.

Though both similar in some ways, yet different in others, I was curious about exploring the use of a common mode choke that would provide some differential series L along with a big (effective) common mode L.
The Cooper VersaPac inductors are very cool: http://www.ka-electronics.com/images/pdf/Coilcraft_VersaPac_Inductors.pdf
They first piqued my curiosity as mic preamp common-mode chokes (and switching PSU inductors) but they might be usable here.

I'm in a high RF 820 kHz environment already. I had the spectrum analyzer hooked up recently and it showed 820 in the mV level.
The Motorola 2600 I have also has the tracking generator option so I'm good from 400 kHz to 1 GHz.

My 4G Wimax modem (CLEAR) gets rectified by my phone handset cord/instrument so I have a good source of 100Hz pulses in the 2 GHz region.

So, FWIW, I'm not seeing any problems yet.

I do have another long piece of Brand-Rex cable I got from SSL.
The spool is damaged and I can't get at one end.
I may have to uncoil that one to re-spool it and when I do may test it laid-out as a long wire antenna.

John wrote regarding the step:

but another possibility is a simple divider formed by the output R and characteristic cable R that would indeed give a -6dB step function for =R.


John, that's exactly it.
You may also not recall it earlier but I did test it with the cable terminated in it's Zchar of ~100 Ohms.
(The drive level of course was reduced significantly due to the low Zterm.)
The step goes away and the final value is 1/2V.

For an unterminated cable with Rsource = Zcharacteristic:

0, 1/2V, then 1V - 1/2V, then 1/2V + 1/2V = 1/2, then 1/2, then 1.
At t=0, t, 2t, then t final.

The explanation:

About those "hooks:" Some call it time domain reflectometry. Take a look at the expanded waveform at 500 nS/division.

Image
THAT1646 feeding 400 feet of 9451 recovered by a THAT1246. 10 kHz, 1 V P-P, expanded 500 nS/division. Line out top, recovered bottom.

At the top we see the THAT1646 output. Almost immediately we see the signal rise to a plateau at about 60% of it's final value.

This tells us one thing from the beginning: The output impedance is not exactly equal to the characteristic impedance of the cable. If it were, the plateau would be at 50%. But, we're close.

On the bottom trace we see the 1246 start to respond approximately 600 ns later. This is the cable delay. With the delay of 9451 being 1.5 nS/foot it is apparent we have about 400 feet of cable.

The incident wave from the 1646 output is initially terminated by the cable characteristic impedance. Our cable delay is about 600 nS. But the plateau is twice that. Why?

Because the incident wave has to travel 400 feet down the cable and back another 400 feet. The delay of the reflected component is 2T because it has to make a round trip. Thus, the plateau is approximately 1.2 us.

After 1.2 us the reflected wave sums with the incident to converge upon the final value.

Though I have not proved it yet by testing the single-ended case, we have visual evidence to support:

Use of differential circuits at both ends can insure signal integrity at least wrt those two ends.

Fully balanced transmission and reception permit greater time-domain waveform fidelity. Though we usually think of music as sinewaves, we occasionally have to transmit timecode...

What we see in the time domain is also in the frequency domain. It's just another way of looking at things. Just my 2 cents.


Please visit "Transmission Lines at Audio Frequencies, and a Bit of History," Jim Brown, Audio Systems Group, Inc.:

http://www.audiosystemsgroup.com/TransLines-LowFreq.pdf

Image
"Transmission Lines at Audio Frequencies, and a Bit of History," Jim Brown, Audio Systems Group, Inc.

Our 10 kHz sqaurewave has components >100 kHz where the line reaches it's characteristic impedance.
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Re: Balanced Outputs

Postby ricardo » Sun Nov 13, 2011 1:53 am

JR. wrote:Note: there are topologies that are benignly rise time limited so it is possible they harmlessly pass or ignore infinite input edge rates up to a given voltage level.
I'd be really interested in these, especially if they can be implemented with little extra real estate & bits.
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