Electronics > Projects, Designs, and Technical Stuff

Differential pair transmission vs. single ended

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Momchilo:
Hi,
I have one control board and 3 additional boards that send and receive data from the control board over a distance of ~1m (3ft). All signals are single ended.
I have to design the boards, but I'm not sure about the best connection method.
Each of the 3 additional boards receive following signals from the control board:

* 10x PWM @12kHz
* CLK @16MHz
* 4x digital signal lines @low frequency (turn on the boards etc.)
* +15VDC @50mA
* -15VDC @50mA
* +5VDC  @200mA
* Ground
* Analog GroundAnd in the other direction, every additional board send following signals to the control board:

* 10x 1bit stream @16MHz
* 5x analog signals -10V to 10V @2mA
Now I'm unsure if I send them as single ended or use differential line drivers and receivers. What would you do?

That would be at least 34 single ended lines with one ground and one analog ground connection. Where would you put the ground lines, between each signal? Then we're talking about ~70 lines per board. And could I use a differential pair cable or should it be straight ribbon cable? Or do know a better solution? 2 cables per board would be also okay, but not ideal.

If I use differential drivers and receivers for the PWM, CLK, analog and 1 bit stream signals, we're talking about 52 signal lines (26*2) + 4 slow digital lines + power and ground per board.
3 68 pin D-Sub cables with twisted pairs should work in this scenario or not? In my opinion this is the best alternative. But also the most expensive one, because I need 15 differential line drivers and 15 differential line receivers. 3 drivers and 12 receivers (each IC 4 channels) on the control board and on every additional board 3 receivers and 4 drivers, if I convert every single ended signal to
a differential signal pair.
So is it necessary to use differential signals? Maybe just for particular signals to reduce the number of additional ICs?
Thanks for every advice and your help :)

Best regards
Momchilo

David Hess:
Two wires is not only associated with differential signaling.  What matters is the ground return for the fast signals.  With differential signalling, another driven signal becomes the differential ground return but any fast signal needs a ground return whether differential or not.  So you are already looking at pairs of wires for your fastest signals.

This also applies to low frequency analog signals when "ground" is at different potentials at two locations.  Here the second wire serves to sense the remote ground.

JagV12:
It also very much depends on the electromagnetic environment. I personally would make prototypes for various solutions and see how far I can go with each...

T3sl4co1l:
Will this ever be applied to greater length runs, or is this a one-off circumstance?

Is this connectorized and user-facing (needs to withstand hairless gorillas), or can a plain ribbon cable suffice?

What bandwidth are the analog signals?  Slow like the PWM?

Offhand I would be looking at, say, RS-422 for the clock, since it takes priority (I assume it's synchronizing everything else), at least modest filtering for the PWM and slow digital signals (assume they're being generated by a nice sharp MCU, or some kind of CMOS output pin, but that you don't need the sharp edges transmitted as well), some kind of shielded or differential cable for the bit streams, and the analog signals can run in plain multiconductor cable.

The ideal cable would be a foil-shielded twisted pair (differential clock), foil-shielded multi (single ended bit streams), and loose or foil-shielded multi (slow digital, analog), all collected inside a single insulation jacket, and braid too if desired.  All the grounds tie together (braid, foil drains), to whatever enclosure or primary system ground is.  Digital ground probably, too.  Analog ground can come along as a DC reference, assuming you don't need much analog bandwidth (otherwise if you do, consider a separate braid on that bundle, or even individual coaxes).

Making do with off-the-shelf multiconductor cable, may be possible, but crosstalk may get too bad, mainly clock getting into everything it is accidentally nearby.

If ribbon is acceptable, I would try to alternate signal and ground as much as possible (note that power supplies also serve as ground, particularly if bypass capacitors are nearby on both ends), still use a diff pair for clock, and wrap the whole thing in a foil shield, grounded at both ends.  This could even be rolled up and jacketed, but it's still not terribly robust compared to a proper cable.


If this needs to run over greater lengths as well, I would seriously consider adding an FPGA with clock divider (8MHz will be more comfortable for RS-422 transceivers), clock multiplier (up to a few 100s MHz), and serdes (to combine all the digital signals into one or a few serial streams).  ADC/DACs can be applied to the analog levels as well.  The PWM could also be moved to a local generator, that's set through a command sequence (ala I2C or SPI -- preferably, derived from this serial stream), thus reducing its bandwidth.  Then a USB cable would be a quite adequate medium, or if the serdes can be configured for DC-balanced coding, Ethernet hardware and cabling could be used.

Tim

Momchilo:
Thanks a lot for your answers :)


--- Quote from: David Hess on July 21, 2019, 11:21:05 am ---Two wires is not only associated with differential signaling.  What matters is the ground return for the fast signals.  With differential signalling, another driven signal becomes the differential ground return but any fast signal needs a ground return whether differential or not.  So you are already looking at pairs of wires for your fastest signals.

This also applies to low frequency analog signals when "ground" is at different potentials at two locations.  Here the second wire serves to sense the remote ground.

--- End quote ---
So basically twisted pair and each pair is one signal and one ground with respect to the signal?


--- Quote from: T3sl4co1l on July 21, 2019, 01:47:19 pm ---Will this ever be applied to greater length runs, or is this a one-off circumstance?

--- End quote ---
It's a prototype for a project in the university. So one off.


--- Quote from: T3sl4co1l on July 21, 2019, 01:47:19 pm ---Is this connectorized and user-facing (needs to withstand hairless gorillas), or can a plain ribbon cable suffice?

--- End quote ---
Ribbon cable would be enough. I prefer D-Sub cable, because it's more rugged, screwed in and is shielded, but it's not required.


--- Quote from: T3sl4co1l on July 21, 2019, 01:47:19 pm ---What bandwidth are the analog signals?  Slow like the PWM?

--- End quote ---
The bandwidth is around 200kHz.


--- Quote from: T3sl4co1l on July 21, 2019, 01:47:19 pm ---Offhand I would be looking at, say, RS-422 for the clock, since it takes priority (I assume it's synchronizing everything else), at least modest filtering for the PWM and slow digital signals (assume they're being generated by a nice sharp MCU, or some kind of CMOS output pin, but that you don't need the sharp edges transmitted as well), some kind of shielded or differential cable for the bit streams, and the analog signals can run in plain multiconductor cable.

--- End quote ---
The clock synchronizes 10 ADCs (Sigma-Delta modulators). And these return the results as 1bit streams to a FPGA. The clock is only for synchronizing the sample rate of the ADCs.


--- Quote from: T3sl4co1l on July 21, 2019, 01:47:19 pm ---The ideal cable would be a foil-shielded twisted pair (differential clock), foil-shielded multi (single ended bit streams), and loose or foil-shielded multi (slow digital, analog), all collected inside a single insulation jacket, and braid too if desired.  All the grounds tie together (braid, foil drains), to whatever enclosure or primary system ground is.  Digital ground probably, too.  Analog ground can come along as a DC reference, assuming you don't need much analog bandwidth (otherwise if you do, consider a separate braid on that bundle, or even individual coaxes).

Making do with off-the-shelf multiconductor cable, may be possible, but crosstalk may get too bad, mainly clock getting into everything it is accidentally nearby.

If ribbon is acceptable, I would try to alternate signal and ground as much as possible (note that power supplies also serve as ground, particularly if bypass capacitors are nearby on both ends), still use a diff pair for clock, and wrap the whole thing in a foil shield, grounded at both ends.  This could even be rolled up and jacketed, but it's still not terribly robust compared to a proper cable.

--- End quote ---
I thought maybe a 50 Pin twisted pair D-Sub cable like this one?
But of course a 50 or 60 pin ribbon cable is also a option. But they will be moved a lot. I don't think I could build a shielding for them that will last for some time. And shielded ribbon cables are very expensive.

In the attachment are two solutions, do you think one of these could work? Maybe a 68 pin shielded twisted pair D-Sub cable could be an option too, but it's more expensive than the 50 pin one.

And how would you filter the signals on the PCBs? I guess a low pass filter with a cap to ground and a resistor in line with the signals. But what should be the cutoff frequencies, 200kHz and 16MHz or other values are more practical?
And for which signals I should use a filter? Because we're talking about 3x10PWMs + 3x10bits + 3x5analog signals =75 filters + CLK.
Sorry for the many questions, but I never designed something like this and I have no idea what will work and what not.

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