Need help on converting 0-5v signal to 0-10v signal

[Mahonroy]

Hi guys,
I am currently using a parallax propeller micro controller to generate a signal.
I am using a ADC to create a 0-5v signal, and an op-amp to convert this into a 0-10v signal. I am experiencing a problem with it. Everything works fine until I go below 1.9v on the input of the op amp... at this point, the output of the op-amp pegs at 3.93v. When I supply 0 volts to the input, it is also stuck at 3.93v for the output of the op amp. Above 1.9v, and the op-amp works perfectly fine and has a gain of 2x which is what I want.

I'm thinking it has something to do with the op-amp I'm using, and the offset gain associated with this op-amp, but honestly I don't really understand it (I'm still somewhat of a newb with electronics). If I could provide 0 volts to the input of the op-amp, and have it just be closer to 0 (no higher than 0.5 volts) then this would be satisfactory. I attached a schematic which has the parts I'm using as well.

Thanks and any help is greatly appreciated!

 

[Harald Kapp]

The 741 opamp's output does not go all the way to ground when used with a single supply voltage. The datasheet says output voltage = +-16V at Vsupply=+-20V and Rl>=10kΩ. The output is therefore 4V away from either the positive or negative supply. This means that when used with a single supply, the output does not go below 4V (close to your measurement.

You have 2 options:
1) Use a dual supply, e.g. +-15V or +15V / -5V
2) Use a so called rail-to-rail opamp (e.g. OPA341, to name just one) which wil go to 0V even on a single supply.

 

[BobK]

An LM324 would be able to output 0-10V if you have a single supply of 12V.

Bob

 

[Harald Kapp]

Right, a good choice in this application.

 

[Mahonroy]

Thanks for the responses! So its recommended I go with the OPA341, or an LM324? I also had someone recommend using a LMC6081.
How do I decide which one to go with? They all look like good options.... though the LM324 has 4 chanels on it, which I only need 1.

 

[BobK]

I think they can be has as singles as the LM321. Any op amp that can output close to the 0 rail used with a power supply that allows it to output 10V will work.

Bob

 

[zazhaf]

You can use LM714 which contain only 1 op-amp.

And based on the theory that "maximum" output saturation voltage (VSat) of an op-amp is about : (+VSupply) - 2v and the "minimum" output saturation voltage is : (-VSupply) + 2v, so you'll need to use dual supply about +-12V.

Here some alternative circuit you can use.

 

[Rleo6965]

Look for opamp that an open collector output. You just need to place pullup resistor between output pin and +12V supply.

 

[Harald Kapp]

An open collector output is no guarantee that the output is able to be driven all the way down to 0V. The internal circuitry of the opamp needs to match this requirement, too. That's why dedicated rail--to-rail opamps exist.

 

[BobK]

I have never heard of an opamp with open collector output.

Bob

 

[KrisBlueNZ]

I've never heard of an op-amp with an open collector output either. Maybe he's thinking of comparators.

You need an op-amp with rail-to-rail input and output. Most of these are designed for low-voltage operation only; 12V is too much for them. Here is a filtered selection guide at Digi-Key that contains single devices with suitable power supply specifications, and rail-to-rail I/O: http://www.digikey.com/product-search/en?pv659=257&pv659=736&pv659=764&pv659=330&pv659=442&pv659=425&pv659=855&pv659=428&pv659=467&pv659=856&pv659=826&pv659=421&pv659=422&pv659=32&pv659=419&pv659=297&pv659=653&pv659=543&pv659=595&pv659=755&pv659=647&pv659=762&pv659=578&pv659=35&pv659=48&pv659=577&pv659=696&pv659=499&pv659=120&pv659=1025&pv659=417&pv659=429&pv659=777&pv659=4&pv659=108&pv659=949&pv659=927&pv659=1083&pv659=778&pv659=1096&pv659=922&pv659=1099&pv659=23&pv659=230&pv659=747&pv659=312&pv659=174&pv659=444&pv659=847&pv659=381&pv659=91&pv659=711&pv659=614&pv659=432&pv659=434&pv659=824&pv659=1097&pv659=1008&pv659=92&pv659=341&pv659=64&pv659=292&pv659=478&pv659=495&pv659=236&pv659=823&FV=fff40027,fff800dd,a40070,a40087,a40290,a40292,1000001&stock=1&quantity=1&pageSize=250

There are over 700 devices on that list; you can narrow them down quite a lot by selecting ranges for package type (SMT or THT - but eliminating SMT will reduce your options dramatically), input offset voltage, frequency response (slew rate, gain-bandwidth product, and -3dB bandwidth). Once you've narrowed down your options pretty closely, download the data sheets and make a final decision.

If you tell us more about the application - for example, the frequency range needed, and the offset voltage accuracy needed, I can post a narrower Digi-Key product filter.

Edit: You don't strictly need rail-to-rail I/O. You just need inputs that go down to 0V, and an output that goes down to 0V as well, and to within 2V of the positive supply. Digi-Key doesn't have that level of detail in the selection table; that's why I went with RRIO.

 

[Mahonroy]

Well, I decided to go with the LMC6081, it generates a nice 0-10 volt signal...... but sure enough it cooks quickly when its operating on the circuit board. It fails either by making a short from the power to the output, or it fails bu only outputing a small amount.

So it looks like I need to choose a different one to go with.

I have a digital to analog IC that is creating a 0-5 volt signal. I am amplifying this to a 0-10 volt signal. I am providing 15 volts of power to the op amp. It doesn't really matter if the op-amp is super fast, the device will still operate just fine. It does not have to be super accurate either (within 0.1 volts is good enough).

 

[Harald Kapp]

but sure enough it cooks quickly when its operating on the circuit board

This is not a normal behavior. Show us your circuit diagram, check all connections. What load (aka output current) are you driving?

 

[Mahonroy]

This is not a normal behavior. Show us your circuit diagram, check all connections. What load (aka output current) are you driving?

My circuit is in my original first post. As far as the output current... unfortunately that is part of the problem... I am using this circuit to control an external circuit (replaces a manual control) and I am not sure what the current draw is.... though I might be able to measure it. It should be less than 25 mA though since the old op-amp does not fail.
Here is the original op-amp I was using:
http://www.mouser.com/ProductDetail/STMicroelectronics/UA741IDT/?qs=/ha2pyFaduiUbkYP7qvnjoyGiwp9Sm/DHia9J%2bDLEc8uV8wj4/636Q==
And here is the new one I tried using, but is currently having failures:
http://www.mouser.com/ProductDetail/Texas-Instruments/LMC6081IMX-NOPB/?qs=/ha2pyFadui9x9U6%2bFkd/tuq/VxHrLk21o2RNPNmcmVKb2kxIqUcyEmdJHMja3GL

Something to note though... my circuit is actually giving +15 volts to the op-amp (not +12 like is indicated on my drawing). I also get a LOT of external EMI on the power line of this circuit from external machines in the room. I am wondering if this is too close to the op-amps max of +15 volt limitation (and the EMI is potentially giving it spikes higher than +15v)?

 

[Harald Kapp]

Something to note though... my circuit is actually giving +15 volts to the op-amp (not +12 like is indicated on my drawing). I also get a LOT of external EMI on the power line of this circuit from external machines in the room. I am wondering if this is too close to the op-amps max of +15 volt limitation (and the EMI is potentially giving it spikes higher than +15v)?

That's entirely possible. You can use an inexpensive voltage regulator (e.g. 7812) plus a few capacitors to provide a stable 12V to the opamp. An inductor in series with the 15V power supply (before the regulator) will help to eliminate noise on the power line.

Before you build a regulated supply, try the circuit with a 9V battery and an output of e.g. 3V of the DAC. The opamp should now deliver 6V. Is the opamp still getting hot? What if you remove the load? What is the load current?

 

[Mahonroy]

I will try that out.

Quick question though... can I toss in a 14 volt or 15 volt zener diode to protect the power input at least?

 

[KrisBlueNZ]

Yes, if you put at least a resistor between the incoming power and the zener, and a smoothing capacitor across the zener (e.g. 470 µF 25V electrolytic), and power the op-amp at the zener.

Choose the resistor to drop about 1V at the expected current. If you're not sure, as in this case, assume 100 mA, so that would be around 10Ω. Use a fusible resistor if possible. If it gets warm or hot, you know there's some noise, spikes, or surges on the incoming power.

Harald and I would much prefer if you tried to answer his questions and figure out what the problem actually is, instead of sticking random bandages onto the circuit in the hope that they will help.

 

[Mahonroy]

Hey guys, first off I want to say thanks for all of the help so far! (This thread has been stale for a while).

So I ended up going with a rail-to-rail op amp (OPA171AIDR) and for the most part it has been working great. I provide it a 0-5 volt signal and it outputs 0-10 volts. I am supplying it with 15 volts, and it can take up to 36 volts. I am also using a 15 volt zener diode for protection on the supply, and a 0.1uF decoupling capacitor on the supply.

Most of the devices work great.... but occasionally the op-amp is having a failure and I am trying to figure out why and how to fix it. Once the op-amp has failed, I give it a varying 0-5 volt signal, and it outputs 1.2 volts no matter what (I have a gain of 2x, so it should be between 0 and 10 volts). This leads me to believe the op-amp is failing and is shorting to ground. The op-amp also begins to heat up.

Any ideas on what is causing this? Thanks again and any help is greatly appreciated!

 

[Mahonroy]

Here is an updated schematic by the way:

The resistor in front of the Zener diode is just a 10 ohm 1/6 watt resistor. The 15V zener can dissipate 1 watt, the part number for that is "ZM4744A-GS08". Not gonna lie, I had a bit of a hard time trying to figure out the proper combination of zener diode and the resistor to put in front of it, mainly because the +15v input voltage in reality is something between 14.5 and 15 volts, and the resistor zener calculator I was using varied quite a bit.

 

[Mahonroy]

Is it possible that the "analog output" is pulling too much current? This opa171 is rated at 25mA max output current... is this a failure that would be associated with that?