What is a precision op amp?

I've been sent an ad for an "ultra precision op-amp". It says "One of the highest precision performance of any op amp currently available.".

(It's an OP177GPZ).


OK, so what's so great about it being "precise"? I tried to Google it, but didn't get much other than ads for "precision op-amps".

I found this description from National:

"Precision op amps are defined by a combination of low offset, low noise, high open-loop gain, and specialized topologies." http://www.national.com/en/amplifiers/precision_op_amps.html

Doesn't really appear to be a functional definition. More of a marketing term.

Oooo, "specialized topologies" - I want one!

But what would I use it for?

Consider a boring old LM741 op-amp, with an "offset voltage" of 5mV. That's the voltage you'd have to apply between the + and - terminals to get an output of 0V (ideally, offset voltage is 0mV).

Now configure the op-amp with a gain of 100. The gain not only multiplies the input signal, it multiplies the offset voltage. So your output voltage is generally as expected, except it can be off by as much as +/-0.5V (100 * 5mV). Probably not what you wanted.

Now look at the OP177. Its offset voltage is 25uV, or 0.0025V. Same gain-of-100 circuit would produce an error of only 2.5mV in a signal that has been amplified by a factor of 100. Precision!

The noise behavior is another feature, which is important for dealing with very low level signals (e.g., ECG's).

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That is a good explaination. A low error op-amp.

@Nick Gammon

But what would I use it for?

Instrumentations and measurements.

Very low offset … YES better than the 741…BUT… Bandwith and slew rate is not better than the 741. ( I compare the datasheet )

If I was planing to use OP177, I use it for measurments purpose only. As for an amp and signal filtering, 741 is better. But the 741 slew rate is a bit high. And using the 741 as a comparator is out-of-question, the slew rate is bad. The 339 is way better.

In my opinion…

Bandwith and slew rate is not better

"precision" and "speed" tend to be opposites in analog circuity :-(

OK, so like many things to be good at one thing means there might be a trade-off that you are worse at something else.

So you might not get lots of ultra-precision op-amps if your requirement happens to be high speed.

Do note that the OP177 is a pretty old op-amp, and the 741 is positively ANCIENT. A quick search at digikey shows that there are about 500 parts with a bandwidth greater than 2MHz AND an offset voltage less than 100uV... It shouldn't be hard to find a modern "general purpose" op-amp that is "precision" compared to a 741, and "high speed" compared to an OP177...