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  • Author Author: jc2048
  • Date Created: 27 May 2020 1:42 PM Date Created
  • Views 6126 views
  • Likes 16 likes
  • Comments 16 comments
  • op177
  • experimenting
  • op amp
  • op27
  • offset_voltage
  • tl081
  • jc2048
  • icl7611
Related
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Experimenting with Op Amps: Input Offset Voltage

jc2048
jc2048
27 May 2020

When we are taught about op amps, the usual starting point is a perfect op amp. That amplifier

has infinite gain, infinite input resistance on the two inputs, zero output resistance, and so

forth. That's done so that the focus can be on understanding the basic operation and how the

feedback operates with the various common circuit configurations, without all the messy real-

world details that apply to an actual physical circuit. Although real op amps can be very,

very good (exceptionally so compared to what most of us would end up with if we tried to

design one with discrete parts), inevitably they fall short of perfection, so we need to

understand those imperfections and how much real effect they have in order to design with

them.

 

One such imperfection is input offset voltage. An ideal op amp would have zero offset voltage.

That is, the difference in voltage between the two inputs would be zero to get zero at the

output. In practice the manufacturer can't achieve that. That might be because of small

differences between the characteristics of the input transistors, or it might be because the

internal circuit isn't perfectly balanced leading to a slight offset by design. The net result

of that offset, from our point of view as designers is that it's as though the perfect op amp

has a small voltage generator in series with one input. That then upsets the operation of our

circuits a little bit. For precision work, we may want to pay more for a precision op amp that

has much tighter specs for input offset voltage.

 

So how to look at it and measure it? This was going to be easy, I thought. Dredged up from the

rank depths of my memory was a circuit that would show me not only the input voltage offset

but also the open-loop gain and how close to the rails the outputs could get. I don't know

where I might have seen it and can't find it now. It might have been a book, an application

note, or maybe even online. Or maybe I imagined it.

 

The circuit was something like this

 

image

 

and the result would be along these lines:

 

image

 

The input is a triangle wave and is simply attenuated by a factor of approximately 1000 by the

resistors, giving better control of the mV-level signal at the input.

 

I should have thought a bit harder about what the result might be but, in a fit of enthusiasm,

I quickly repurposed one of my old test pcbs

 

image

 

and tried it on some of the odd selection of op amps I've accumulated over the last couple of

years.

 

Here are the results

 

TL081

image

 

OP177

image

 

ICL7611 [powered from +8v, -8V rails]

image

 

OP27

image

 

They look plausible but there's a snag. I can't show you because I didn't save a trace of it,

but the output always lagged the input zero-crossing point, irrespective of whether the input

was on the ramp up or the ramp down. If it represented the offset, it would have lead one way

and lagged the other. So what am I looking at? A further clue is the rounded corner where the

output starts to move. I'm looking at how the op amp output behaves when it comes out of

limiting at the rail, and it doesn't hurry, does it? The plots do show me how close the

outputs can get to the rail, so it wasn't a total fail as an experiment. The results are fine

for showing how the parts behave as comparators [not too good, if you like fast and snappy],

but the offset is quite overshadowed by the slow recovery.

 

At that point I decided to change course and rewired the board like this

 

image

 

Now the op amp is simply amplifying the offset voltage and I can read it on a bench meter. Here

are the results for the same four op amps

 

TL081 FET-input bipolar. Typ: 3mV Max: 15mV Reads: 576uV

image

 

 

OP177 Precision bipolar. Typ: 20uV Max:60uV Reads 22uV

image

 

ICL7611 CMOS Typ: Not Given Max: 15mV Reads: 138uV (Iq was set to 1mA)

image

 

OP27 Precision bipolar. Typ: 30uV Max: 100uV Reads: 25uV

image

 

The reading on the meter is 1000 times the offset.

 

(More or less: strictly the gain is 1001, the resistors are only 5%, and there must be a small

contribution to drive the output to the measured voltage, though it won't be very much. So all in all

a little approximate, but good enough for a quick blog.)

 

The precision parts [OP27 and OP177] lived up to their billing, being close to their typical

figures. The FET-input TL081 was worst, but well within spec and better than might have been

expected [it has offset pins, so could be trimmed closer, though, with a lot of the

applications it gets used for, it wouldn't be an issue anyway]. The CMOS part was a lot better

than the datasheet allows for - so it looks like I got lucky with that one too.

 

What would be interesting would be to look at the spread on a group of similar parts, but

unfortunately I don't have a lot of op amps so that will have to wait for another day.

 

If you found this interesting and would like to see more blogs I've written, a list can be found here:

jc2048 Blog Index

 

References

 

[1] User's Guide to Applying and Measuring Operational Amplifier Specifications

Ray Stata, Analog Devices Application Note AN-356

 

Note that that derives from something published in1967, so the methods that manufacturers use now may be much more spohisticated.

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Top Comments

  • jw0752
    jw0752 over 5 years ago +5
    Hi Jon, Thanks for another interesting area of exploration. I will have to set up and run some tests. Since I salvage lots of components from boards I have several types where there are quantities. Please…
  • jw0752
    jw0752 over 5 years ago +5
    Hi Jon, Tonight I ran your offset experiment with various stock numbers and a quantity of each. I had quite a time as I initially set the experiment up on a single supply. Things didn't go well and I was…
  • ralphjy
    ralphjy over 5 years ago +5
    Wow, brings back a lot of memories. I just realized that the 741 passed its 50th anniversary. I think I first used one back in 1971. I remember the TI versions had better offset than the Fairchild ones…
  • fmilburn
    fmilburn over 5 years ago +5
    Hi Jon, Great post. By coincidence I have been playing with an op-amp I built from discrete parts a week or two back and experimenting with it. It was to be a 1day experiment but due to unexpected complexity…
  • DAB
    DAB over 5 years ago +4
    Yes, there are many aspects about opamps that every engineer should know and explore, but usually do not. Thanks for taking a look into one of them. DAB
  • genebren
    genebren over 5 years ago +3
    Very interesting blog. You have a knack for taking on some interesting topics and bringing a lot of interesting lessons to the readers. Thanks!
  • Andrew J
    Andrew J over 5 years ago +3
    Very interesting Jon. I’m creating a prototype PCB at the moment with an Opx192. Actually, two of them and they both have a spare unit. What I could do is wire these up to run a similar test, in two different…
  • jc2048
    jc2048 over 5 years ago in reply to Andrew J +3
    Hello Andrew. It might be interesting to see how it compares with precision bipolar parts [they get a CMOS part that good and consistent by trimming it at the test stage]. But I think you're going to have…
  • jc2048
    jc2048 over 5 years ago in reply to fmilburn +3
    That sounds interesting. Look forward to reading more.
  • Jan Cumps
    Jan Cumps over 5 years ago in reply to jw0752 +3
    I found an ST LM324N quad opamp. 100R is 115R 100K is 95K 47 nF = 56 nF Vcc = +- 15V Tested on a breadboard, but I tried to mitigate noise: zeroed the measurements at breadboard before starting the measurements…
  • kkazem
    kkazem over 3 years ago

    Excellent op-amp blog. 

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  • fmilburn
    fmilburn over 5 years ago in reply to jc2048

    In case you missed it: https://www.element14.com/community/people/fmilburn/blog/2020/06/03/op-amp-made-from-discrete-components

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  • jw0752
    jw0752 over 5 years ago in reply to Jan Cumps

    Hi Jan,

    Your results seem very consistent with what I observed. Even in the same package there is a variance in the offset which makes sense. The LM 324 is a very popular chip and I salvage a lot of them from industrial and dental medical circuit boards.

    John

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  • Jan Cumps
    Jan Cumps over 5 years ago in reply to jc2048

    The photo doesn't reflect what I did in the end. I put the players close to the pins image.

     

    I'll redo one measurement with decent decoupling; I was expecting that it wasn't needed because we're playing in the DC field so no buffer caps required ...

    image

     

    edit: buffer caps didn't impact the measurement. I guess there's too little AC going on in this design that the caps matter. No swing.

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  • jc2048
    jc2048 over 5 years ago in reply to Jan Cumps

    but I tried to mitigate noise

    By not having any decoupling capacitors and placing the resistors kilometres away from the chip?

     

    On my board, I had a 10uF electrolytic on each rail and a 100nF under the socket directly across the supply pins. I probably should have put that in the blog. I'm so used to doing it now, I don't always think to draw it on the circuit.

     

    Thanks for trying it with the LM324.

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