# Shielded twisted pair balanced transmission line implementation

I want to send a signal from board 1 to 2 through 20 cm line. I have four channel in differential, First I add them, can I send them through shielded twisted pair like this:

1. Do I need a buffer? why?
2. Is R14 and R13 needed, since obviously the resistors are in balance? Why?
3. How we can create controlled impedance twisted pair? but I think it's needed if we have such small wavelength compared to transmission line.

Some note:

• R12,R3,2 is for level shifting and balancing load.
• + = 4.5 V, - = 0 V
• receivers resistors can change to be the same in top and bottom entrance.(I don't know what impedance is good, isn't it the highest?)
• transmitters resistors can be changed to determine transmitter impedance.(I don't know what impedance is good, isn't it the smallest?)
• My summing amplifier reference:

Here I want to trick and move OpAmp in receiver side, but I'm not sure is it possible to move that in other end of balanced line? also there is other question about summing.

• How are you strapping the PCB grounds together? Is all this inside a metal box? With the metal box connected to the PCB grounds? You want the case/box to be part of the electric-shielding approach. And part of the magnetic-shielding approach. Dec 13, 2018 at 15:13

Looking into the source, it is balanced regarding impedances.

Looking into the receiver it is not at all balanced because you have the differential amplifier's output (close to zero ohms impedance) connected directly to the upper wire and the 1k2 resistor is doing nothing. A balanced receiver looks like this: -

To add a little more, the source is impedance balanced but the impedances are high. In effect each signal wire has an impedance of 10k/4 + 120 ohms = 2.62 kohm and this would degrade the signal. You need to provide a buffer that then uses the 120 ohm resistors in each differential output like this: -

Above picture from here.

So, the signal entering the above diagram at the left can be regarded as the summed value of your four voltage sources (V3-V6). Summing four differential sources as you have shown is not very practical. Use a summing amplifier like this: -

• Nicely illustrated, ty. I got two error: 1.The receiver is not balance 2. transmitter have high impedance. you suggested these: 1.Use buffer in receiver 2.sum each 4 separately. Now assume: 1. I can tune receiver impedance by increasing load and changing R12 2.i can reduce 10k resistors to 1k or less (I don't know really it's effects on summing operation). Now everything will go ok or not? I will update question regarding to this assumption. Dec 13, 2018 at 12:25
• (1) Use a differential amplifier in the receiver is what I said. (2) Summing can be done with single ended inputs using the summing amplifier - the way you show it is not practical for any situation that I can think of. If you truly have 4 floating sources, put them in series to sum them. Dec 13, 2018 at 12:30
• you said(1) use feedback resistor to make balance receiver by R1||Rf=R2||Rg criteria (2) your summing is not practical (I can't understand why because it is in book "Handbook of Operational Amplifier Circuit Design" and is working on simulation) use just summing, not diff. and sum or series the sources(Good idea, they can't annoy one another?). Remained questions: (3) whats best tran. and rec. imp.? (4) Is driving STP without buffer possible? Dec 13, 2018 at 12:50
• I never said "use feedback resistor to make balance receiver"; I just showed a circuit. For balance make R1=R2 and Rf=Rg. You are missing an op-amp in your summing circuit - it's right there on your added picture. If your signal wavelength is much lower than the cable length (say at least 20 times) then Zrec can be high impedance to prevent attenuation of signal transmitted down cable. Zsend ought to be low and 50 to 150 ohms is a practical range. Providing the output impedance is the same for both wires in the STP then that is all that has to be met but Zsend still has to be low. Dec 13, 2018 at 13:06
• Finally, I must tell that actually I've tried to move summing OpAmp to receiver side, because it is easier for me to drive OpAmp in receiver side. I've tried to trick since you told everything will be fine if you using balanced transmission line. And this migration will take my life into paradise. Gratefully are there any error by taking into account what you've said before, except moving adders OpAmp for making life easier? (mking them series is new strategy that I need to think about it ) Dec 13, 2018 at 19:46