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:
- Do I need a buffer? why?
- Is R14 and R13 needed, since obviously the resistors are in balance? Why?
- 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.
Answer
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: -
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