Single signals to Differential signals - best methods ?

[nick mulder]

Hello all,

I have near equal amount of automation gear here that use either single or differential output and input... I'm wondering what the best and/or usual workaround is in the following situations:

single logic output >> differential logic input
single PWM output >> differential PWM input
single analog +/-10v output >> differential analog +/-10v input

differential output >> single input

Word on the street is that the cool kids are putting their single ended signal to the +ve differential input while the ground of the output device is the other twisted pair and therefore connected to the -ve differential input. The ground of the recieving device is then connected to the shield of the cable. I could have this wrong of course.

I have seen differential line drivers also but they seem to work with digital signals only. I've found it hard to find one that could do +/- 10V analog motor commnd - let alone the probable annoyance of having to build a pos/neg supply for it.

differential output >> single input - I imagine you just connect the +ve output to the single input - easy as that ... (?)

Am I getting too involved in this issue ? What is the simplest way that works to do this ?

cheers all,
nick

[nick mulder]

hmmm,

to add to my original post...

So I have a single +/-10v output here - tested its signal and can confirm it ranges correctly from -10 to +10v...

Theory goes for differential inputs that the two signals - the +ve 'original' signal and its negative mirror image signal - have their difference worked out at the differential input end... Imagine a 10v signal:

+10 - (-10) = 20v (I imagine this 20v is then halved to equate to the 10 again ??)

anyway - now do the same but with a single ended supply to +ve , ground going to -ve

+10 - (0) = 10v

Now you might think, well cool! 10v is the signal we're after so its all good - but - the differential input set up above shows us (or me, at least) that the signals are halved - which means the 10v obtained from a single-ened supply actually equates to 5v...

And it does - my oscope shows 10v signal into a differential input on a drive, but the drive is showing it as 5v input in its diagnostics. Is the solution simply that I set up the drive to multiply the signal by 2 ??

[Al_The_Man]

There are some confusing definitions used here,
There is simple ±10vdc single channel analogue where either the input is 0 to -10 or 0 to +10, ref is 0v.
Do you have the name/number of the piece of equipment that uses the type of input you are referring to?
Al.

[nick mulder]

hmmm, well lets hope I'm confused! It'd be an easier fix

but to answer your question:

Allen Bradley Ultra3000
Chapter 2 page 3 Pins 25 and 26

Granite VSD XE
Page 13/34 Pins 15 and 16

among others

[Al_The_Man]

Yes you are correct those do have differential inputs, I just happen to set the controllers I have with analogue output a bit differently.
But I am still not sure what you are aiming for?
Al.

[wildwestpat]

Firstly differential inputs are common in audio and video circuitry as well as digital control systems They are used where noise induced in the lines by stray electric or magnetic field could be a problem that shielded wire alone cannot eliminate or the earthing policy makes difficult.

The easy way is to use the appropriate integrated circuit driver to convert the single ended input signal to a pair of differential signals which swing in opposite directions in sympathy with the single ended input signal. When the balanced signal is used to feed the input of a balanced line input any noise that has been picked up affect equally both inputs so that the recieving circuit only sees the sent difference as the crud picked up is cancelled. The normal wiring would consist of a pair of loosely (lightly) twisted together insulated wires. (Normal twin twisted wire has a nominal impedance of 200 to 300 ohms which is good for data and audio transmission. To further enhance the performance in very electrically noisy environments a metal mesh screen is often added the mesh being grounded at the receive end.

If the drive signal in question is a true single ended one the proposal to connect it to a balanced input will result in an output but of half amplitude if the signal doing the driving is isolated from the zero volts of the line receiver. However the noise pick up imunity is lost and earth loop potentials may also cause problems.

The more elegant solution would be to use one of the many available balanced line driver integrated circuits. There are also available from electronic audio suppliers balanced line transformers that would work well if the telemetry is a frequency pulse train that is 5KHz. These are sold as microphone input transformers. The IC versions work with both low and high frequency as well as DC and are available opto isolated if the earth loops are a big problem.

Hope this helps. Regards - Pat

[mike_Kilroy]

AL, we used to convert Galil +/-10v analog output commands for our drives to differential in interface cards we used to make for the various DMC 700/1000/1500 etc controllers.

It was not mentioned very commonly, but there is analog differential outputs; basically it is simply an op am buffer output with a 50 ohm or so series resistor, and then part of the output tied back to the + input of the buffer op amp to act as a noise modulator thingy... I can get my old schematic out if you'd like and post here...

but it is not very important as you suggest AL. Single ended analog output is not so different from differential output but the big difference of course is on the INPUT it feeds: here it is a BIG difference. So whether one knows they have differential output or not simply can make use of the diff analog input anyway. Yes, Nick, do use the differential input with your single ended sigs if you can; rather than just tying the diff LO side to grouind or common, run a shielded pair from the source over to the diff input. Tie the common to the common on the source side: that way any common mode noise picked up by BOTH wires gets cancelled out in the differential input circuitry.

It does not have to be mirror image or one going plus one minus, just use the diff input by running the diff lo along with the diff HI wire so they both pick up noise and it gets cancelled.

[mike_Kilroy]

Originally Posted by nick mulder Hello all, I have near equal amount of automation gear here that use either single or differential output and input... I'm wondering what the best and/or usual workaround is in the following situations: 1) single logic output >> differential logic input 2) single PWM output >> differential PWM input 3) single analog +/-10v output >> differential analog +/-10v input 4) differential output >> single input nick

1) most diff receiver chips will need the other diff channel tied to 1/2 way between the real sig level; ie., if 5V TTL sig, then put 2pc 480ohm resistors in series and tie the center to the other diff input to bias it at 2.5v level: then as real sig goes hi (>3.5v) it turns on, as it goes low (<1v) it turns off. crude but gets u by in a pinch.
2) ditto 1)
3) see last post
4) just use one of the diff lines and ignore the other. rescale as necessary if u loose 1/2

[nick mulder]

Originally Posted by Al_The_Man Yes you are correct those do have differential inputs, I just happen to set the controllers I have with analogue output a bit differently. But I am still not sure what you are aiming for? Al.

I'm wondering what the best and/or usual workaround is in the following situations:

single logic output >> differential logic input
single PWM output >> differential PWM input
single analog +/-10v output >> differential analog +/-10v input


.... and to understand how this halving of voltage works

[nick mulder]

Originally Posted by wildwestpat Firstly differential inputs are common in audio and video circuitry as well as digital control systems They are used where noise induced in the lines by stray electric or magnetic field could be a problem that shielded wire alone cannot eliminate or the earthing policy makes difficult. The easy way is to use the appropriate integrated circuit driver to convert the single ended input signal to a pair of differential signals which swing in opposite directions in sympathy with the single ended input signal. When the balanced signal is used to feed the input of a balanced line input any noise that has been picked up affect equally both inputs so that the recieving circuit only sees the sent difference as the crud picked up is cancelled. The normal wiring would consist of a pair of loosely (lightly) twisted together insulated wires. (Normal twin twisted wire has a nominal impedance of 200 to 300 ohms which is good for data and audio transmission. To further enhance the performance in very electrically noisy environments a metal mesh screen is often added the mesh being grounded at the receive end. If the drive signal in question is a true single ended one the proposal to connect it to a balanced input will result in an output but of half amplitude if the signal doing the driving is isolated from the zero volts of the line receiver. However the noise pick up imunity is lost and earth loop potentials may also cause problems. The more elegant solution would be to use one of the many available balanced line driver integrated circuits. There are also available from electronic audio suppliers balanced line transformers that would work well if the telemetry is a frequency pulse train that is 5KHz. These are sold as microphone input transformers. The IC versions work with both low and high frequency as well as DC and are available opto isolated if the earth loops are a big problem. Hope this helps. Regards - Pat

Hi Pat - its does help...

You're the first person mention the halving of voltage which makes me think either we're both going mad, or there are a few people not even aware of or how setting their signalling up that way has given them that result

Digital Line drivers for logic I have found as for analog I found one IC that is built for RGB video signals - It wasn't available locally ...

How to search for IC's that can range from +/-10v ?

and PWM rates like the Mesa cards output ?

[nick mulder]

Originally Posted by mike_Kilroy 1) most diff receiver chips will need the other diff channel tied to 1/2 way between the real sig level; ie., if 5V TTL sig, then put 2pc 480ohm resistors in series and tie the center to the other diff input to bias it at 2.5v level: then as real sig goes hi (>3.5v) it turns on, as it goes low (<1v) it turns off. crude but gets u by in a pinch. 2) ditto 1) 3) see last post 4) just use one of the diff lines and ignore the other. rescale as necessary if u loose 1/2

From what I understand (which I admit is probably faulty) there might be situations where these solutions make sense - when the voltage is halved the receiving device may no longer understand the logic correctly, hence the need to fiddle it into the appropriate ranges again.

Especially relevant to analog control as every 'bit' (once its been through a ADC) is meaningful...

At the moment I think its time to get testing/playing again before I type anymore - outstanding issue:

- Finding an appropriate line driver for +/-10v analog supply

[Al_The_Man]

Originally Posted by nick mulder Hello all, I have near equal amount of automation gear here that use either single or differential output and input... I'm wondering what the best and/or usual workaround is in the following situations: single logic output >> differential logic input single PWM output >> differential PWM input single analog +/-10v output >> differential analog +/-10v input differential output >> single input nick

Nick, So far we have been dealing with generalities, It would be far easier to have details on specific equipment in order to give a productive response?
In post #4 what is the driving element?
Al.