Creating a bipolar supply from a DAC with a single 5V output

Other Hardware General Electronics
1

Hi,

I am attempting to build a laser harp. I am using a 20kpps laser galvo and a 200mW green laser. My laser galvo driver requires a input (of at least) +-5V to actuate the mirror the full available range. I am currently running my commands from my arduino uno to an Adafruit mcp4725 DAC and using the single +5V output to drive the galvo (I have the -V and G hooked to ground). I have it working but the spread of the beams is very limited. I think this is because I am not supplying a bipolar power supply to the driver. I also may need a larger power supply to control the galvo but I got it from china with no datasheet or info of any use.

Is there a chip or something that can make the 5V output from the DAC into a +-5V to feed into the galvo driver? Also is there a way to increase that voltage(assuming the galvo driver will handle something more than +-5V)?

I was looking at op amps but there is so much info out there that I am now more confused than before.

Thanks for any help in advance.

Bruce

2

Is there a chip or something that can make the 5V output from the DAC into a +-5V to feed into the galvo driver?

You can get a chip called a voltage mirror which is basically an oscillator driving a capacitor ladder that will do this. However the current is very limited to a few tens of mA.

The other way is to use an isolated DC/DC converter, you can get much more current but these are quite expensive. Most DC/DC converters are not isolated.

3

How much current does this galvo require? Part number or datasheet?

4

this is the link to amazon where the galvo was purchased. It came with no data sheet. I did get an email with a few pictures of how to connect the different parts but that is all(even that was very vague). It came with a +-15V power supply to power the galvo driver but I haven't seen anything about the voltage to communicate with the driver board. It may be the same, I'm not sure.

5

The input resistance of the control input to the galvo driver is stated to be 200K Ohms, so it will draw only 25 uA from the DAC when driven by 5V.

So, it appears that what you really need to do is map the 0-5V DAC output to -5 to +5V. There are op amp circuits to do that.

Or, you can buy a bipolar DAC module.

6

Hi,
You can try the TC7662 it will +/- volts with two capacitors. I do not know if the current is enough for you applications. It use 5 volts.

7

Grumpy_Mike:
You can get a chip called a voltage mirror which is basically an oscillator driving a capacitor ladder that will do this. However the current is very limited to a few tens of mA.

The other way is to use an isolated DC/DC converter, you can get much more current but these are quite expensive. Most DC/DC converters are not isolated.

Why not a power op-amp like the Apex PA-xx series or else a smaller op-amp with a complementary push-pull power booster on the output?

Of course, all this depends on how much drive current the OP needs....

8

Why not a power op-amp like the Apex PA-xx series or else a smaller op-amp with a complementary push-pull power booster on the output?

Because the OP only has 5V at the moment and that sort of thing will only give you +/- 2.5V.

9

Let me understand this. If I put in 5V into an op amp the resulting output will be +/- 2.5V?

And if an op amp is used is there then a way to take the +/- 2.5V and amplify it into a +/- 5V?

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The way I read the description, you have a 15V supply to drive the thing, and require a control signal of +/-5V. Without a datasheet, WE cannot know if the 2 supplies are isolated or share grounds. You can find out, and if it is isolated, can be helpful in getting the +/-5V supply. Minimizing components would be good. Keeping slew rates low would also be good.

11

Let me understand this. If I put in 5V into an op amp the resulting output will be +/- 2.5V?

No.
You need a negitave rail supplying the op amp to get a negitave voltage out of it. The +/- 2.5V will be referance to + 2.5V above the negitave power input of your op amp, and that is only if you bias it that way.

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Oops. A closer look shows the PSU for the galvo unit to be +/-15V. That might be helpful in biasing an opamp or transistor.

13

Grumpy_Mike:
No.
You need a negitave rail supplying the op amp to get a negitave voltage out of it. The +/- 2.5V will be referance to + 2.5V above the negitave power input of your op amp, and that is only if you bias it that way.

Hey Mike thanks for the response. I hate to be that guy but a lot of what you're saying is going way over my head. Are you saying that I need another supply of power to the op amp along with the 5V coming from the DAC? Also will the op amp have two output lines for both +5 and -5V to connect to the laser galvo driver? Does this kind of circuit have a specific name that I can use to search for example diagrams?

14

What he is saying is the signal is 5V peak-to-peak. The bottom peak is referenced to the negative rail (supply) of the opamp. If you have a single supply (you do), the negative rail is ground. That means the lowest * the signal can go is the negative rail, which in your case is gnd. Since you have a 5V signal, the center of that will be at +2.5V. Essentially, it is what you have now. In order for you to get a negative swing, you need a negative supply. A +/- 5V signal is 10Vpp signal.

*opamps don't always go to the rails(supply). It is dependent on the particular opamp

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I hate to be that guy but a lot of what you're saying is going way over my head.

No problem, glad you asked instead of just ignoring things. Understanding the words is just as important as understanding the electronics.

Are you saying that I need another supply of power to the op amp along with the 5V coming from the DAC?

Yes.

Also will the op amp have two output lines for both +5 and -5V to connect to the laser galvo driver?

No.

This is a concept that beginners find hard to grasp, but when you say a voltage is is not an absolute measure but is always relative to some other reference point. This reference point is called ground or more precisely signal ground.

Now look at this diagram is shows three batteries wired in series.

Ground.jpg
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On the left the ground is the -ve connection of the lower battery. The top battery can be considered to be at a voltage of 4.5V with respect to ground.

In the middle the ground is considered to be the junction of the lower two batteries. You have what we call a split supply which could be described as 3V 0 -1.5V. Note we have a negitave voltage rail of -1.5V, that is because we are taking a reference from a different point to the first diagram.

In the third arrangement we have 1.5V 0 -3V voltage rails.

You need to supply your OP-amp with a split supply like this. When you do this the the output of the op amp can go to any voltage between the two power rails.

Diversion - modern op-amps can go "Rail to Rail" but others can't get closer than about a volt or so to the rail voltage

If you want a D/A to output a negitave voltage you have to feed it with a split supply. So however you drive things you need to have a split supply. This means using a second power supply, or one that delivers a split voltage, or something that makes a negitave voltage from your positive one.

One that I mentioned was a switched capacitor circuit, see this link https://www.allaboutcircuits.com/projects/build-your-own-negative-voltage-generator/ and the "555 driven negative voltage generator" about half way down that page.

The disadvantage of this sort of circuit is that the drive current is limited but that might not be a problem if the driver has a high input impedance.

Anything you don't understand then please ask.

Ground.jpg
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ok, I think I'm starting to understand what you're saying. The op amp when fed with a single +5V supply and ground makes the output +/- 2.5V because of where the reference point is. However in this case I need the signal going from the arduino into the DAC to get to the galvo driver since sometimes it tells the galvo/mirror to move a little and sometimes a lot(all by sending varying voltages within the given range).

If I add a second power source going into an op amp to get the -5V output that I require(along with the +5V that I already have coming from the DAC) to give me full operational range of the galvo then how would I be able to control the negative voltage the way that I currently control the +5V through the arduino/DAC?

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then how would I be able to control the negative voltage the way that I currently control the +5V through the arduino/DAC

It depends on how you wire up the DAC.
If it is fed with just a single supply, then you connect the op amp so that there is an offset voltages injected into the op amp as well, so when you set the DAC half way, that is the most significant bit a one with all the others a zero the output of the op amp will be zero.
Then anything above this would be a positive voltage and anything below it a negitave voltage. You also arrange the op amp to have gain to make the limits of the DAC output correspond to the limits of the power rails.

18

I was trying to do this in my head, but it's Saturday, so I cleaned off a little breadboard and grabbed a few components.

I didn't want to get into a DAC, so I opted for a freq synthesizer putting out a 5Vpp sinewave @ 2.5V offset. I also just grabbed a LM358, since it was close. You would need an appropriate opamp, as the 358 dropped like a stone at 10KHz.
For power, I set it up with 2 supplies set to 15V and hooked them in series, with ground referenced at the middle, which is what you have supplying the module. The only thing the -15V rail goes to is the opamp Gnd pin. Everything else references to the center (Gnd) of the supply.

Using this pdf Opamp Gain and Offset I set it up. The Ref Voltage was the +15V rail. It got me close, and a little adjusting on R1 has me sitting with a 9.34Vpp signal sitting at 350mV center.

19

The Ref Voltage was the +15V rail. It got me close, and a little adjusting on R1 has me sitting with a 9.34Vpp signal sitting at 350mV center.

Yes, that is what I was talking about.

In figure 2 of that PDF if you replace R1 and R2 with a preset pot then you can trim the offset down to zero.