# Create a differential output signal

I need to supply the galvo driver with a signal "Voltage Range ±3.0V p-p or ±10.0V p-p (Differential Input)" - that's what it says in the datasheet. I have a 0-5V single ended DAC and differential opAmp AD8132. So, I am planning to create the differential signal by the following schematic:

The output of the DAC will give me a signal ranging from 0 to 5V and I can scale it down using R26. The other pot (R25) will generate the proper voltage to offset DAC's signal to 0V DC. This way, a 0-5V wave on one input and +2.5 on the other input will give me ±2.5V between the outputs of the dual buffer U4. These signals then fed into differential opAmp U5 with G=1.

Question 1: is this the proper way to generate a differential signal? I believe that I could place a 0.1uF cap between R26 and +in of U4 and it would automatically offset it to 0V, but since a part of my signal is DC, it would compromise the signal, right?

Question 2: What should I measure on each of the differential outputs of U5 relative to ground?

• Your circuit is not going to work without connecting U5 and U6 to a power source. Jan 27 '15 at 4:45

• Gain:

I believe the gain in this design is too low to get a +/-10V output signal.

$$V_{out diff} = \frac{390Ω}{390Ω}.(V_{i+} -V_{i-})$$ $$V_{out diff} = 1.(5 - 2.5) = 2.5V_{diff-mode}$$
or $$+/-1.25V_{comm-mode}$$

Alternatively you could make use of the Vocm, instead of the V- input to offset the ADC value.

• Question 1:

Correct, this would destroy the DC component and is only possible if you could guarantee that the ADC generates a continuous AC signal, which I presume is impossible in your application. Even if that would be the case, you still have to take the lower cut-off frequency into account. Signals below 1Hz would require considerable capacitor values.

• Question 2:

You would measure an in-phase signal with half the amplitude of the differential mode signal on the +Vout pin and an opposite-phase signal on -Vout.

You may want to read this application note as well.

PS: You may want to consider a fixed voltage divider for R25/R26 in series with a smaller value potentiometer if calibration is needed.Like this for instance.

This allows for more accurate calibration. The current single potentiometer design is very course.

• Thanks for the app note - good one. The current schematic has problems because if I change DAC's output range using R26 (say 0-3V), then I have to adjust R25 to make 1.5V offset. Are you saying that I should connect DAC's output to the +in, 0V to the -in, and 0V to the Vocm and I do not need to generate the offset voltage on the right side of the schematic? Jan 27 '15 at 15:26
• Correct. Wire the AD8132 as a single-ended input and use Vocm as offset, like fig 4. in the AN. Jan 27 '15 at 22:50
• Not as simple. I want my signal to be offset to 0V, so I connect Vocm to ground. However, because my input signal is DC offset, I get this same offset on the output. Jan 28 '15 at 13:41