Why does one op amp work and another one not?

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So I have a design with an input signal (steady) between 0 and 300mV. I wish to scale this signal up to 0-10VDC.

I bought an Op Amp called a MC33202

http://www.intusoft.com/onsemipd...

This did the trick and worked fine but was expensive.

On a second iteration of the design, I found a cheaper alternative to that op amp called a TL082CP

http://www.ti.com/lit/ds/symlink...

This was way way cheaper, but when hooked up to the same circuit, it outputs too high, it maxes out at the top rail voltage.

My Op amp is hooked up with one rail at 0 Volts, the other rail at 10 volts. Input goes into the +, the - is tied to the Vout and I have non inverting gain.

The only difference I see is that the non working op amp is a Jfet amp, whatever that is, the first op amp is a "rail to rail"

Why doesn't the cheaper op amp work???? I wish to learn what I did wrong.

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ANY opamp should work. Maybe if you post your diagram we can help.

With a 10V rail then you will be limited to a rail to rail opamps and even that is cutting it fine, maybe a 12V or even the good old fahioned 15V rail may be best.

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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There's a myriad of op-amps out there - all with subtle (and not so subtle) differences depending on the actual application. If you've only got a single power rail, then a rail-rail op amp is probably the best choice. non rail-rail devices will only go to about a volt or so of the rails. If you're running a negative rail, then the output will be able to get to 0V. Then there's the inputs - many op-amps don't like the inputs at the rail. So your choice of the mc33202 was probably a good one.

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smkipus wrote:

My Op amp is hooked up with one rail at 0 Volts, the other rail at 10 volts. Input goes into the +, the - is tied to the Vout and I have non inverting gain.

Surely there are a couple of resistors as well? Can you post a schematic?

 

In consideration of others, please RTFM!

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A TLO82 (or TL081 or TL084, or, for that matter, a 741, and quite a few others) WILL NOT work in this application with a single supply. Here is why. There are really three issues.

First is valid power supply range. TL08x family is designed to work with +/- 15V. That means Vcc+ is 15V above ground and Vcc- is 15V below ground. It is a little hard to tell what the minimum is but there are graphs for operation down to +/- 5V. So, unlike the LM358, for example (which is specifically designed for single supply operation), these are not.

Second issue is called "input common mode range". It is guaranteed to be at least +/-11V when used with a +/-15V supply. What does this mean. First of all, when used as a normal op-amp, the inputs are the essentially the same voltage (within millivolts). The common-mode voltage is the median between the input voltages, so it is the same as either input. A spec sheet note in the TI spec sheet which is at

http://www.ti.com/lit/ds/symlink...

says that the common mode limits are with respect to a voltage which is half way between Vcc+ and Vcc-. With a 30V worth of total supply, the common mode cannot get closer than 4V from either supply rail. This will kill you in low voltage situations. There is an 8V range that is excluded and, at 5V, you do not even end up with ANY valid common mode range. So sorry!

Third issue is output swing. Again, with +/-15V supplies, the MAXIMUM you can get out is +/-12V. So, the output cannot get any closer than 3V to either supply rail. That is a 6V excluded zone and at 5V, you have NO available output swing. So sorry.

An inexpensive amplifier that might work for you is the LM358. When operated from a single 5V supply, (within spec'd operating limits) the input common mode range runs from 0V to Vcc-2V (over full temp range). The output will swing down to within 20mV of ground. This does not quite classify it as "rail-rail", but for voltages close to ground, it should work for you.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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The TL08x is not a rail-to-rail op-amp. If you look at the curves, the output voltage is not able to go up to positive and down to negative supply voltages, there is a margin of about 2-3 volts depending on what kind of load you have on op-amp output.

A rail-to-rail op-amp cannot go exactly to rails either, but within about tens of millivolts, so that is why you see them working better in your application.

Some op-amps have rail-to-rail inputs, some have rail-to-rail outputs, some have both. TL08x does not like if the input is near supply voltages either. Some op-amps that are said to work with single-supply operation might work with input voltages near negative or positive supply, or even exceed them to some extent.

TL08x would work, if you had -4V negative supply (because of *input* must work down to 0V) and +13V positive supply (because *output* must work up to +10V).

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It is probably because of the different type of op-amp you have used like the others mentioned! (TL082 is not rail-to-rail)

But is the MC33202 really that expensive? I can get it at just above $1 (in Europe) as single quantity, that's not too bad for a rail-to-rail op-amp I think! But you could possibly look for another rail-to-rail op-amp at a better price!

I have used the TLV272 previously, priced similarly (slightly cheaper here) it should perform just as well for your needs I think. But apart from that one I don't have any experience with rail-to-rail op-amps.

- Brian

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For those who want the whole circuit, here is the image, I've learned a lot from the replies so far, I just assumed all op amps were "rail to rail" I mean, what else would they be? There's no where else for voltage to go besides the rails. Thank you all to your responses, here's my circuit.

The DAC puts out between Ground and 260 mV... the output goes off to a terminal strip.

Thank you!

Attachment(s): 

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I agree that op-amps OUGHT to be rail-rail.

The sad fact is that there are trade-offs to getting there. It is harder to get good bandwidth (for a given quiescent current) in rail-rail devices. And, the input structure is compromised to allow operation close to both rails; one rail (as demonstrated by LM358) is relatively simple. I suspect that there may be other trade-offs, like input offset or input offset temperature coefficient.

In fact, every op-amp should have no input offset (at any temperature), no input current, infinite gain, infinite bandwidth, and more. Sadly, none of these are available.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net