[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

[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.

[wildwestpat]

Hi Al and Mike

I don't think this is getting us any where.

The doubling and halving of drive signals is being brought up because of the desire to connect a single drive line to an input that is expecting to see a balanced line transmission and that is easy to sort out in amplitude terms.

On noise reduction the only thing that matters is if the machine will work and not suffer from spurious actions due to lack of common mode rejection.

Kind regards all round - Pat

[mike_Kilroy]

agreed; the bottom line is what matters (and my lazy streak says don't design/build/add stuff if not needed) so why not try it and see it works as is before going off building more stuff.

Glad we can continue to agree and disagree in such a friendly, fun, & hopefully entertaining/educating manner!