converting +/- 10 V single ended to differential

[intern121]

Hi!

I am planning on using an NI DAQ card with ADCs and DACs on it for setting up a control loop. The DACs have an output range of +/- 10 V, single ended and I want to use 2 of these outputs for a fast steering mirror driver that accepts +/- 10 V analog differential inputs. It would also be nice to have a programmable/trimmable attenuation along with this converter, because I'm not yet sure the resolution of the driving signal will be good enough to use it on the whole +/- 10 V scale.

I have been looking at ADC drivers at Analog Devices for converting single ended to differential, but have not found parts that support my voltage range, I think. (At least, that's what I assumed from looking at the supply voltages that only went up to 5.5 V). Also, I don't want to design a circuit, but just use an evaluation board or something that's already in a box and you just need to connect cables to it.

Can anybody recommend something to me?

Thanks!

 

[crutschow]

To convert a single-ended voltage to a differential, just add an inverting op amp circuit with a gain of -1. The original signal and the inverted signal give you a differential signal. Of course the op amp voltages need to be ±15V (or ±10V if you use a rail-rail op amp).

 

[intern121]

Thanks! Won't the differential output signal be +/- 20 V then, though?

Of course, I could use one inverting amplifier and one non-inverting and set the gains to 0.5 for each. Finding the op-amps is not a problem. Finding evaluation boards for them, however, is. As I said, I'd like to avoid having to do the layout myself...

 

[ahsan_i_h]

You can use +/-10V input to +/-10V output signal isolator. Nokeval 641 is such an isolator.

Link: **broken link removed**

 

[FvM]

I presume that the +/- 10V differential input can be also driven by a single ended source. The advantage of the differential input to suppress common mode interferences will be almost preserved if the cable connection isn't very long and capacitive common-mode crosstalk may become a problem.

 

[crutschow]

Thanks! Won't the differential output signal be +/- 20 V then, though?

Of course, I could use one inverting amplifier and one non-inverting and set the gains to 0.5 for each. Finding the op-amps is not a problem. Finding evaluation boards for them, however, is. As I said, I'd like to avoid having to do the layout myself...

If your input signal is ±10V then you would use an inverting amplifier and a non-inverting amplifier, each with a gain of 1 to give a ±20V differential signal.

Such a circuit is quite simple requiring just two op amps (or one dual or quad amp) and two 10kΩ resistors (for the inverter). This can be easily done on a small perf board or solderless breadboard. Use sockets for the op amps (if using a perf board) and be sure and decouple each amp with a 0.1µF ceramic cap between the power pins and ground.

What is the highest control signal frequency?

 

[intern121]

The highest frequency is probably around 3 kHz. I am still confused about the convention of specifying differential voltage ranges. If a driver accepts a +/- 10 V differential signal, does it mean:

a. The difference between the two channels has a range of +/-10 V (that would mean the individual channels could have a range of +/-5 V, for example)
b. Each channel has a range of +/-10 V, so the difference has a range of +/-20 V

?

 

[FvM]

a) is right. But usually the voltage range of individual inputs isn't restricted to +/-5 V as this would leave no common mode range. More likely each input has at least an absolute voltage range of +/-10 V, may be more. So one pretty legal operation mode with single ended source is pseudo-differential, tying the -ve input to ground at the source side and the +ve to signal out, as suggested in post #5. The moderate frequency range promises that this is a suitable option for your application.

 

[intern121]

Ok, thanks!

I might actually go for the pseudo-differential operation.

Just for my understanding, what is the advantage of having a common-mode signal in the two channels of a differential signal, as opposed to having them symmetrical to 0?

 

[FvM]

The differential signal is better in terms of noise suppression, particularly with longer cables or higher frequencies where the cable capacitance respectively transmission line common mode impedance matters.

Pseudo differential safes the differential driver where it's not necessarily needed. You still keep the advantage of the differential input to suppress noise and DC errors intrdoduced by ground loops.