# How can I produce an analog control signal from -2 to 12V?

I have a sensitive micro/nano positioning controller that takes -2V to +12V signals as input. This controller then produces high voltage -20V to +120V as it's own output to actuate a piezo crystal, which contracts and expands microscopically depending on the potential it experiences.

I am aware of how to produce a range of signals from 0-12V DC using a potentiometer and a 12V battery, however, I am not aware of a way to produce -2V to 12V DC signals.

At first I was a bit confused since the potential difference between -2 and 12V is going to be 14V (i.e. "0" to 14V), but then I realized that the controller device itself has it's own "0V" ground, so it's indeed possible to have -2V and +12V relative to that.

I am open to anything since it's critical to get this right, but I don't have too much experience in building electronic circuits using op-amps. Since I am controlling very small movements using this piezo crystal, I would ideally like to produce these -2 to 12V signals with a digital controller - maybe something that goes from 12-bit or 16-bit PWM input to a -2V to 12V output. If there is a way to use a potentiometer I could also use a stepper motor to turn it, but I am still confused as to how to get the -2 to 12V.

• Use a DAC with bipolar power supplies. You would somehow have to generate the negative supply, but it would be pretty low current. If you want to use PWM then you need a PWM to DC converter circuit which is simply a low pass filter and possibly op amps to get the -2V to +12V range. Again you need to generate a negative supply voltage. Commented Mar 12, 2018 at 21:19
• Do you have a floating 14 or 15V supply? If so then a pot, cap and Op AMP unity gain buffer can create a virtual ground at 2V. Stepper motors with 2 coils is not related is it? That takes a stepper bipolar quadrature driver circuit not a DC offset circuit with 200 cycles per rotation typ. Maybe you want a servo positioner Commented Mar 12, 2018 at 21:27
• Thanks. I will go with an evaluation board for a bipolar DAC for now. I am looking at EVAL-AD5754R. analog.com/media/en/technical-documentation/user-guides/… At \$24 it's not a cheap IC but hopefully it will pay off in the future once I spend some time learning (first time using a DAQ IC). Commented Mar 12, 2018 at 22:41
• Looks like someone has done some work interfacing the AD5754 with arduino and has even made a shield (although I will go with the eval board first). laserlance.com/projects/arduino-dac-library-and-shield Commented Mar 12, 2018 at 22:46
• I also found this interesting article "Challenge the Conventional - Make Unipolar DACs Bipolar" maximintegrated.com/en/app-notes/index.mvp/id/5581 but it will have to wait for future study. Commented Mar 12, 2018 at 22:49

Since your immediate question seems to be how to create a "signal" that is adjustable between -2 & 12 volts. I will prescribe a simple experiment which you can perform in the comfort of your own living room, and which will illustrate the basic principle. The principle is extendable to many implementations, including DACs and the like.

Hook up the pot and 12 volt supply just as you describe to get a "signal" ( actually a DC voltage) which can be varied from 0 to 12 volts by turning the pot thru its mechanical range.

Next hook up another power supply as follows. Ideally, this would be a 2 volt supply, but it can be 5 volts or similar. An adjustable bench supply adjusted to 2 volts will work fine. Both of the power supplies must be isolated from earth ground (most bench supplies are, or allow you to do so via the output terminals).

Connect the positive output terminal (usually red) of the 2 volt supply to the negative terminal (usually black) of the 12 volt supply. This connection becomes your "ground" or "DC Common". Next, re-connect one end of the potentiometer between the positive terminal of the 12 volts supply and the other end to the negative terminal (black) of the 2 volts supply.

Connect your DVM negative lead to the "DC Common" (either the red terminal of the 2 volt supply or the black terminal of the 12 volts supply) and the DVM's positive input to the pot's wiper.

Energize the power supplies. Rotate pot shaft and note how voltage varies smoothly and continuously from minus 2 volts to +12 volts. See?

The principle is that you can stack voltage sources, one on top of the other, connected positive to negative, and add up their voltages. As long as the voltage sources are isolated, you can choose any inter-source connection as your ground or DC common. Voltage is relative - always a good principle to remember.