NI USB-6009 using op-amp to amplify analog output

[eichlerjw]

Hi.

I'm using a NI USB-6009 DAQ to send a signal to a proportional pressure controller which varies the supply pressure to a pneumatic system. Unfortunately, the pressure controller requires a 0-10V analogue control signal and the 6009 can only do 0-5V.

So I thought I would build a little circuit with a gain of 2 to amplify the signal.See attached pics for a schematic and photograph of the setup. The 741 op-amp is powered by a 24V DC supply. 24V is a little on the high side for the op-amp but I have this supply already built in and (using a lab bench supply) the circuit works exactly the same at 12 and 18V.

Bascially, with input voltages from 5 down to 2 volts, the circuit behaves perfectly. The output is just the input multiplied by two.

However, the output never goes below 3.9 (ish) volts, no matter what the input voltage is.

Can anyone suggest a solution (or know where else to ask)? I really need to be able to control my output within the range of 1-10V.

Many thanks. John

[ChrisR.]

The 741 doesn't have an input common-mode voltage range that extends to the negative rail.  You need either to power the 741 from a dual supply (e.g. ±15V) or to replace the 741 with a "single-supply" opamp.  Those are opamps intended to measure voltages near ground when there is not a negative rail.  There are hundreds out there to choose from.

Your schematic shows the LED directly across the 24V rail.  I assume there's a resistor in series with it.

Chris

[eichlerjw]

OK, thanks for the reply

I'll go for replacing the op-amp with a suitable type, since I don't have any other power supplies.

Do you think the LM358 would be suitable? (I can get these from my local store tomorrow). Please excuse my ignorance - I'm new to electronics.

I am thinking of wiring the op-amp in the same way as my previous attempt. Like the second half of this cricuit.

schematic

Do you think this would work?

I can wire up a simple voltage divider to reduce my 24V supply if needs be.

Thanks again.

J

[ChrisR. · Accepted solution]

Yes, an LM358 should work.  You won't be able to get all the way down to 0V, but it should get you down to 0.5V or so.  The LM358 is a dual opamp, so you won't be able to drop it into the same socket as your 741.  You can probably get away with leaving the unused half disconnected.  (That's not true of all dual opamps.)

Chris

[eichlerjw]

OK, great.

Can you suggest an Op-amp that would just drop into my socket. If not, the board didn't take too long to build.

cheers

J

[ChrisR.]

I don't know what's at your local store.  Here are some to choose from:

OPA137

AD820

OP113

If you can't find anything, it won't be hard to change your circuit to use a dual.  Pins 2,3, and 4 can stay.  Connect power to pin 8 instead of pin 7, and the output is pin 1 instead of pin 6.

Chris

[eichlerjw]

Many thanks!

I modded the circuit board this morning and replaced the Op-amp with an LM358N. Everything works perfectly.

If anyone else is interested in this, I have attached an updated schematic.

Thanks for your help Chris!

J

[dierks]

Does anyone have a recommendation for a product that I can just buy to do this?

I'd rather not spend the time to build an amp, and buy one so i can focus on other tasks.

Lookig for up to 5 amps, 32V and capable of DC-10kHz. It will operate in the range of 1A 18V most of the time, around 2kHz.

Thanks so much in advance!

[AaronBaldwin]

I'd rather not spend the time to build an amp, and buy one so i can focus on other tasks.

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[Henrik_Volkers]

We use BEAK amplifier for shaker exitation.

http://www.beak-electronic.de/en/products/linear-power-amplifier/120-va/-baa-120-beak

They are OEM for a lot of big names 🙂 

The student version would be to mod a 'hifi' amp to DC, but I found a good working current limit can save your setup 😉