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Old 16th October 2012, 11:20 PM   #1
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Default Differential Line Driver - Impedance Questions

Looking for some help from you experienced guys out there. I need a simple differential line driver to take a single ended, line level signal to a balanced signal. It is only passing audio signals, but I would like it to be flat up to about 100 kHz.

After reading the specs on the OPA1632, I dug into the TI white paper on differential amplifiers, SL0A54D. Early in the paper, they show this circuit for converting single ended to differential signals.

Click the image to open in full size.

I have had it drummed into my head that "balanced" means balanced impedance. At audio frequencies, the influence of the opamp and feedback on reducing impedance makes this look pretty balanced, even if looking past the opamp to the input is clearly unbalanced.

A bit later in the paper, they show this source termination technique for high frequencies.

Click the image to open in full size.

Question 1

Do I need to do a source termination to stay balanced? If I use a buffer for Vs, then Rs is very small. R3 + Rt ~= R1. If R1 = R3, Rt goes to zero and the circuit starts to look like the first one.

I have also been thinking about putting a small value (200 pF) sliver mica cap in parallel with the feedback resistors. That would kill the gain above 100 kHz, so that high frequency balance wouldn't be an issue.

Question 2

What value resistors should I use in this circuit? It has been suggested that 5 kOhms is a good zone to be in for crossovers. I plan on using equal input and feedback resistors. I'm tempted to use 1 kOhms, but that might increase the need for a source termination. Any thoughts or experience here would be welcomed.

Jac
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Old 17th October 2012, 12:07 AM   #2
dmills is offline dmills  United Kingdom
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Dont know that I would bother with source termination at anything like audio frequencies (It is mostly an RF tequnique), but a 50 ohm or so resistor in series with each output is a good idea as it both defines the output impedance and decouples the cable capacitance from the opamp feedback loop which helps stability.
Make these resistors close tolerence to maximise low frequency CMRR when run with a poor following stage.

Also some RFI filtering at the output is always a good plan.

Have a look at some of the THAT corp app notes, they are quite useful.

Regards, Dan.
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Old 17th October 2012, 10:37 AM   #3
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Thanks Dan,

I appreciate it. Your suggestion of output resistors makes good sense and I will include it.

Jac
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Old 17th October 2012, 11:45 AM   #4
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balanced refers to a differential vs single ended signal. impedance matching is secondary and comes necessarily into play at HF.
source termination is only required for mic pre-amps for noise considerations and HF circuits for reflection issues. note that R1=R3+Rt||Rs and R2=R4 for differential gain and no common mode gain. before shunting the feedback resistor with a capacitor check the manufacturer specs for the phase margin else oscillations may arise.
The min. value of resistors is also dependent on the driving capability of the amp. the max. is limited by the bias currents and offset voltages. a few KOhms is the norm for such audio circuits.
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Old 17th October 2012, 12:12 PM   #5
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by demeterart View Post
balanced refers to a differential vs single ended signal. impedance matching is secondary
This is completely wrong.

You are under the impression that the two voltage signals of a balanced connection must be equal voltage and of opposite phase. This condition is completely unnecessary.
the necessary condition for balanced connection is that the source impedances and the receiver impedances must exactly match and not be influenced by or connected to ground. A transformer did/does this very well. All the electronically derived balanced connection corrupt this by having some reference, albeit usually very small, to ground.

It's the voltages that are secondary or unnecessary.
One voltage can be zero and the other voltage is the signal in a balanced connection. From that you can then add or subtract signal to both phases. i.e. start with 1Vac on cold (-IN) and 0Vac on hot (+IN). Then add 4Vac to both to give 5Vac on cold and 4Vac on hot (still a differential signal of 1Vac), or subtract 500mVac from both to result in -500mVac on hot and +500mVac on cold to again have a dif = 1Vac (- & + indicating 180 degrees of phase difference of the signal).
The opener correctly identified this !
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Last edited by AndrewT; 17th October 2012 at 12:20 PM.
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Old 17th October 2012, 12:24 PM   #6
AndrewT is offline AndrewT  Scotland
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Leh,
any components that you add for improved stability, or Rf attenuation, or coupling should all be sensibly matched for impedance. the main resistor feeds need < 0.1%, whereas you can probably manage with ~1% for the "other" components.
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Old 17th October 2012, 12:27 PM   #7
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i did not specify the voltage conditions. this is your impression. differential signaling is the difference of voltages. the impedance matching statement though was unfortunate. i should have said that it follows and not simply secondary. i stand corrected.
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Old 17th October 2012, 01:40 PM   #8
sreten is offline sreten  United Kingdom
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Hi,

Is this circuit needed at all ? Unless the cable is very long converting
to balanced has very little advantage if the single ended source has
decent cable driving capability, see the arrangements here :

Balanced Line Technology

rgds, sreten.
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Old 17th October 2012, 01:49 PM   #9
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by AndrewT View Post
.......................the necessary condition for balanced connection is that the source impedances and the receiver impedances must exactly match and not be influenced by or connected to ground. A transformer did/does this very well. All the electronically derived balanced connection corrupt this by having some reference, albeit usually very small, to ground................
this is my interpretation of a number of papers from Jung & Self & Whitlock.

If I have misunderstood something as stated above, then please correct me. I would welcome a better understanding of Balanced.
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Old 17th October 2012, 02:19 PM   #10
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you may have misunderstood my posting. i was referring to differential signals which is slightly different from balanced signals. differential signals require difference of potential and impedances are of secondary importance. on the other hand if someone wants to use a differential amplifier for a balanced system then it indeed it follows that impedances must also be matched. i should have clarified it better from the beginning but i thought it was simple and clear.
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