[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. :tired:

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]

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]

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]

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]

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]

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.

[nick mulder]

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.

The links I provided above are the specific input equipment - the driving element is a Mesa 7i33 PWM to analog daughter card for a Mesa 5i22

But it could be a desktop supply or anything really - the output of the 7i33 is solid....

[wildwestpat]

Hi Nick

The differential input for an analogue balanced receiver is an operational amplifier using both inputs. Hence the output deflection is definitely halved as it is best thought of as the inputs being parted and brought together by the input driver. The differential signal is one that is balanced so that one line goes positive and the other negative by the same amount. This makes the lead linking the transmitter to the receive almost bomb proof in terms of interference rejection provided the wires take the same physical route. Hence the use of twisted pairs. The screen in this case adds a bit more protection.

Can you give me a bit more information on the signal that is to be converted and I as there should be IC's out there that are readily obtainable. Much of this sort of thing works on standard plus 15 volts zero and minus 15 volts but these voltages are not critical provided the required signals are less than say 10volts dpa (double peak amplitude).

The balance to unbalance microphone transformer is a simple answer if there is no steady DC component this is not usual - but - never say never in electronic interfacing is a golden rule!

It is probable that the input receiver takes the differential signal and uses a voltage comparator function to clean up the edges of the wave form in which case it will not mind if it is fed with half the voltage it will still give the same output as it is the transitions that are being sort in latter bit of the circuit. The use of comparators can also introduce noise if fed with an unclean signal. This is a further good reason to try and convert the unbalanced to a balanced drive as the noise immunity is greatly improved.

Give me a bit more detail on voltage swing and frequency if known as well as the voltage rails you have available for an interface converter. A simple quad operational amplifier LM741 (nice and cheap) should be configurable to do the job if you are up for a bit of soldering! Here is a link to a typical circuit to give you the idea of what is involved http://www.fivefish.net/diy/balanced/default.htm - and here is a link to an audio transformer http://www.alliedelec.com/search/pro...px?SKU=9672506

Regards - Pat

[nick mulder]

Pat,

Signal would swing from -10v to +10v

I'm designing the system so voltages are up to me...

I'm sure I'll have a 12v supply at least (what ever W is required)
Prob/maybe a 24v same deal.
5v maybe but hopefully just tapped and regulated off the 12v (7805's for instance) - not drawing much...

I'd love to avoid negative supplies unless they can be cheaply 'synthesized' out the options above. If I recall floating grounds can be confusing in that you think you've got the right supplies for your op-amps but they aren't at the right potententials with the rest of the system.

These drivers should be individual units at the signal origins huh, or it's wasted time I guess?

[wildwestpat]

Hi Nick

There are DC to DC converters that would give you a minus rail. Also available are small switch mode power supplies that are intended for the micro PC building. Here is a link to one supplier mini-itx.com - store - power supplies. that has both types. I would not recommend the mini ITX boards as the video circuits tend to be non standard. I use the the mains input mini ITX power units for interface circuits and powering MPG and breakout boards as this makes earthing very simple. Here is the manual for the PSU I use http://www.mini-itx.com/store/inform...atx-manual.pdf and I power this from an external wall wart type 12 volt DC mains plug. I have also used the mains input power unit also from mini ITX which by the way I have no connection other than as a customer for bits I can not get else where for less!

At the design stage try and see if you can use balanced drive because of the increased noise immunity. The important thing is to keep the signalling current reasonably high i.e. low impedance drive and low impedance input on the receiver for both balanced and unbalance interfacing.

Regards - Pat

[mike_Kilroy]

The differential input for an analogue balanced receiver is an operational amplifier using both inputs. Hence the output deflection is definitely halved as it is best thought of as the inputs being parted and brought together by the input driver. The differential signal is one that is balanced so that one line goes positive and the other negative by the same amount. This makes the lead linking the transmitter to the receive almost bomb proof in terms of interference rejection provided the wires take the same physical route. Hence the use of twisted pairs. The screen in this case adds a bit more protection.

OK,but why do this?? The gain will be so small why bother? I assume you would take your +/-10v sig and convert it to -- +/-10v differential so go from 0-10v to 0-20v sig. this is a voltage gain of 2 or only a small additional level of protection against noise.

I think you should ask yourself 'where does the noise immunity come from with a differential input?' Answer above: any noise added to the signal is added in the wires BETWEEN the source and the diff input amp; the differential amp does not care if it is equal and opposite on both wires or one goes +/-10v and the other is at 0 volts! It will reject any COMMON noise picked up on BOTH wires anyway! so the 'bombproof-ness' of it is that, not the fact that they are equal and opposite. that only potentially increases the voltage by 2, giving you a bigger signal compared to the potential noise

That said, you better relook at the differential input specs before you do this; this is not video equipment where larger signals may be common, this is drives stuff: all drives I have ever seen have 10v peak to peak max input range! We even typically kept the input voltage to about 8v max for TWO reasons: 1) to give us room to goose it higher if need be in a heavy cut or accel, & 2) to keep away from the op amps unlinear area close to its supply rail - on our +/-12v supply op amps in our drives we said the input differential sig became a tad unlinear at about 9.7v max.

No drive diff input I ever saw "halved" the input sig either since it was a 10v max sig and the input was typically 0 to +10 or 0 to -10v in; this gave 10v sig to the drive innards....

So at the end of the day, you probably will NOT gain a factor of 2 more in voltage as you would have to REDUCE the +/-10v sig to +/-5v to make it pure differential. You would gain nothing.

If you still insist on - in my opinion - wasting your time and resources on making a differential output op amp stage, here is a device:

Circuit Note | CN0032 | Converting a Single-Ended Signal with the AD7982 Differential PulSAR ADC | Analog Devices

Attached is a pix showing how to use the present +/-10v sig so you gain all the benefit of the differential input.

[wildwestpat]

Hi Mike

I am sorry to have to say I do not agree with your views for the following reasons:-

1. The whole reason for using balanced drivers is to get the very large decrease in interference picked up on the input leads. The balance input receiver has two inputs one drives the output in the oposite direction to the other. This means that any signal interference is induced equally in both inputs. The result is that the interference on one input is cancelled by the input on the other input. In electronic engineering terms this is called 'common mode rejection'.

2. The wanted signal is sent as a two lines that are of equal magnitude BUT opposite sign. That is when one moves to +3 volts the other moves to - 3 volts.

3. Disconnecting one input of the differential amplifier immediately halves the gain. Consider my point 2 above the input seen by the receiver is the difference between its inputs which would not be 6 volts and removing one input causes the input to fall to 3 volts hence the gain is reduced. However many digital balanced line receivers contain extra stages that further clean up the signal using voltage comparators so that the output will be a square wave.

4. The conceptual problem arises when a single ended output is connected to the differential receiver. One input is supplied with the active signal and the other input is connected to the common return of the driving signal. As in 3. above the gain is halved. BUT More importantly the noise picked up on the input lead is amplified. Also the source earth being connected to the other input has the potential to add noise as well.

In general differential input and output stages can be expected to withstand voltages that are a little less than their respective power supply rails. The classification of video - audio drivers is one of frequency and phase response. Video being more demanding is more expensive and need not concern us here and similarly the phase response is academic unless video is being transmitted or the edge comparison has to be accurate.

As I have tried to point out there is no need for custom ICs the humble LM741 is more than capable unless tristate logic out puts are required to multiplex multiple sensors onto a common input.

The RS232 and the LPT ports on computers use balanced line drivers and the associated receivers to protect the signals from interference. These chips are expensive as they include major signal handling functions which are not required for a simple sensor driven input.

Hope this clears up the mystery on the reduced gain in analogue balance input when fed from a single signal source.

The American Burr Brown Corporation publish several good texts on the application of operational amplifiers and a host of application notes if you want to study differential inputs in greater detail.


Regards - Pat

[Al_The_Man]

I would have to agree with Mike on this one, why the trouble?
For the past 30 years I have used single ended Galil motion cards that use ±10vdc analogue extensively with differential amplifiers with no issues at all.
Al.

[mike_Kilroy]

Hi Mike

I am sorry to have to say I do not agree with your views for the following reasons:-

1. The whole reason for using balanced drivers is to get the very large decrease in interference picked up on the input leads. The balance input receiver has two inputs one drives the output in the oposite direction to the other. This means that any signal interference is induced equally in both inputs. The result is that the interference on one input is cancelled by the input on the other input. In electronic engineering terms this is called 'common mode rejection'.

2. The wanted signal is sent as a two lines that are of equal magnitude BUT opposite sign. That is when one moves to +3 volts the other moves to - 3 volts.

3. Disconnecting one input of the differential amplifier immediately halves the gain. Consider my point 2 above the input seen by the receiver is the difference between its inputs which would not be 6 volts and removing one input causes the input to fall to 3 volts hence the gain is reduced. However many digital balanced line receivers contain extra stages that further clean up the signal using voltage comparators so that the output will be a square wave.

4. The conceptual problem arises when a single ended output is connected to the differential receiver. One input is supplied with the active signal and the other input is connected to the common return of the driving signal. As in 3. above the gain is halved. BUT More importantly the noise picked up on the input lead is amplified. Also the source earth being connected to the other input has the potential to add noise as well.

In general differential input and output stages can be expected to withstand voltages that are a little less than their respective power supply rails. The classification of video - audio drivers is one of frequency and phase response. Video being more demanding is more expensive and need not concern us here and similarly the phase response is academic unless video is being transmitted or the edge comparison has to be accurate.

As I have tried to point out there is no need for custom ICs the humble LM741 is more than capable unless tristate logic out puts are required to multiplex multiple sensors onto a common input.

The RS232 and the LPT ports on computers use balanced line drivers and the associated receivers to protect the signals from interference. These chips are expensive as they include major signal handling functions which are not required for a simple sensor driven input.

Hope this clears up the mystery on the reduced gain in analogue balance input when fed from a single signal source.

The American Burr Brown Corporation publish several good texts on the application of operational amplifiers and a host of application notes if you want to study differential inputs in greater detail.


Regards - Pat

Pat, I guess you and I will just have to agree to disagree

There is no halving of input voltages so no loss of input resolution as the drives in question he mentioned:

Allen Bradley Ultra3000
Chapter 2 page 3 Pins 25 and 26

Granite VSD XE
Page 13/34 Pins 15 and 16

specifically state in their specs sections a max input of 10 volts not 20. So if Nick has +/-10v he is not going to loose any resolution feeding that to his drive inputs; if he does use a couple 741 op amps and makes a true differential +/-10v sig with full 20v peak range he will likely blow the input diff op amps in both of these drives.

Specific to your numbers above I would say....

1) differential input op amp does not care whether the + and - inputs are equal and opposite in voltage or not as you know; they simply amplify the DIFFERENCE between them. so my single ended input when fed with 2 wires from the source rather than simply grounding the - input at the drive, will reject all the same noise as a true differential output - no difference at all in noise rejection.

2) irrelevant to a differential amplifier.

3) if Nick sends +/-10v sig to the input, it is the same as +/-5v true differential = both give full 10v swing. If Nick sends true +/-10v differential signal (20v) swing, he can send his drive in for repair directly afterwards.

4) not so if fed via 2 wires from the source; even if one is common; it will pick up the same noise as the signal wire along the way and a difference amplifier will not amplify it as it is common to both.