EEVblog #479 - Opamp Input Bias Current

[EEVblog]

Fundamentals Friday
All about opamp input bias currents.
Dave goes through the theory, and then does some practical measurements and tweaking.

[AlfBaz]

Great vid Dave, I think what you have highlighted here for me is the difficulty in knowing which of these parameters causes unexpected output errors. Not to mention the many other things that can go wrong especially when working in the device extremes, such as precision, HF, small signal etc

[EEVblog]

Great vid Dave, I think what you have highlighted hear for me is the difficulty in knowing which of these parameters causes unexpected output errors.

Yes, that's one of the major things I was trying to convey.

[ChadSeibert]

Great vid Dave, I think what you have highlighted hear for me is the difficulty in knowing which of these parameters causes unexpected output errors.

Yes, that's one of the major things I was trying to convey.

AD's opamp handbook has been demonstrating that quite handily as well . More info about bias current compensation can be found in chapter 1, page 38, for those interested.

[Rufus]

We said in the previous thread the bias currents are not significant enough for the output error you were getting.

You can consider the bias current to flow in 1k//100k and multiply that by the circuit gain. You can roughly consider the bias current to flow through the 100k to get the output offset but you can't say it flows through the 100k and then multiply by the circuit gain.

[kkp]

I'm with Rufus (and Kirchhoff ) on this one. But:

What flux are you using? If you used the water washable kind, either as pen or in the solder, or have been using the water washable kind a lot with the sponge on the soldering station, it could explain the 3nA flowing in the 100k. It is quite conductive.
(hey, that's a good opportunity to revisit guard traces.)

[colotron]

Nice video. One more test to do:
Put a mid-rail voltage on both inputs, before the 1K resistors. With mid-rail a refer to (positive supply - negative supply) / 2. This way we will have a 0V differential input with mid-rail common mode. Does it improve the input bias current? (I don't know, but I will like to see it!!!).
Being a rail to rail opamp this mid-rail voltage is used when we have to measure negative voltages.
If the last works, the input bias problem will still be there for close-to-rail voltages on non-inverting input.

[ftransform]

Are these hardware based solutions to op-amp offsets actually recommended though? Is it not better to eliminate the offsets through software, i.e. hook up a voltage reference to the input, measure the output in steps, then form a polynomial equation to deal with it? I swear I have read something along those lines in an app note but I can't find it.

I believe the app note pretty much said that on the longer term scale trimmed op-amp parameters would end up drifting more then software, like the strain/tempco in the resistors will lead to a summed error with the op-amps drift vs just the op amps drift, and that it is just a messier and more expensive solution over all.

[EEVblog]

Nice video. One more test to do:
Put a mid-rail voltage on both inputs, before the 1K resistors. With mid-rail a refer to (positive supply - negative supply) / 2. This way we will have a 0V differential input with mid-rail common mode.

I was doing that using a split supply.

[EEVblog]

Are these hardware based solutions to op-amp offsets actually recommended though?

Yes, but depends entirely on your system requirements. Either solution can work (or both at once).

[ftransform]

Are these hardware based solutions to op-amp offsets actually recommended though?

Yes, but depends entirely on your system requirements. Either solution can work (or both at once).

What is the advantage of using both? I figure it would be worse because there are more parts that can drift, so its better to use one or the other.

[EEVblog]

What is the advantage of using both? I figure it would be worse because there are more parts that can drift, so its better to use one or the other.

It won't drift if you do it right. Low ppm resistors are cheap, and either way you have to use low ppm resistors anyway to prevent any drift.
Doing both might for example allow "good enough" performance from the factory using default cal values, that could potentially be software tweaked better if really required.

[free_electron]

One thing to stress is that there are opamps that actually DELIVER current.
Dave mentioned that in the first minute but I find this important enough to re-state this here.

So , if you connect a large resistor, say 10 megaohm to such an opamp to ground... you can see a few 100 mV dc across it . have any kind of gain in such a system and it goes banana's...  this is a frequent problem when making systems that need to interface with high impedance sensors. the sensor resistance itself will muck things up.

JFet opamps are especially dangerous. The gate of the JFET is essentially a diode in reverse. pull that pin low and you will pull current out of the gate as the gate diode now conducts !

That is why JFET opamps like TL071 can be 'annoying' to work with. The current source sits on top and the gates of the jfets come out. you can actually pull the entire current bias current out of those pins... if you do that the opamp starts working. As you deviate the current you effectively limit the input swing rate of the opamp.

[free_electron]

in the AD8628 circuit the typical trick is to use a trimpot with its wiper to ground.

you would connect a 2k trimpot with its outer ends connected to _ input and + input and the wiper to ground. this let's you balance the input currents.

Code: [Select]


           ----------|+\
          |          |  >---+-----
          |        --|-/    |
          |       |         |
          |       +-[Rfb]---    
          |       |
           -[2 k]-
             /|\    
              |
            _|_
             GND

         

this lets you trim the effect of the input bias away ( on classical opamps )

if you don't want to use a multiturn trimpot you can take a 200 ohm trimpot and put a 900 ohm resistor left and right.

[Galaxyrise]

Excellent! I believe this video answers my first question in https://www.eevblog.com/forum/projects/keithley-2000-gain-repair/

Thank you

[Rufus]

I'm with Rufus (and Kirchhoff ) on this one. But:

What flux are you using? If you used the water washable kind, either as pen or in the solder, or have been using the water washable kind a lot with the sponge on the soldering station, it could explain the 3nA flowing in the 100k. It is quite conductive.

In the previous video the output offset changed with a change in the decoupling arrangement of the chip power supply. That indicates at least some of what is going on is not related to dc currents and voltages.

[gerrysweeney]

Great explanation Dave, its funny I ran into exactly this problem in the PSU I am designing just a few weeks ago and posted in my vblog. I did not manage to articulate the problem as well as you have in your video but the exact same problem I had to resolve. It was ultimately resolvable for me by putting in that +input resistor to balance input offset current and ultimately better op-amp selection.  My less articulate explanation starts at about 14 mins into the video.

http://gerrysweeney.com/fully-programmable-modular-bench-power-supply-part-13/

Gerry

[kg4arn]

We said in the previous thread the bias currents are not significant enough for the output error you were getting.

You can consider the bias current to flow in 1k//100k and multiply that by the circuit gain. You can roughly consider the bias current to flow through the 100k to get the output offset but you can't say it flows through the 100k and then multiply by the circuit gain.

I agree with Rufus.  The bias current flows through both resistors in parallel, not mainly the higher valued feedback resistor.  This is why the compensating resistor value is Rin || Rf.  But don't take my word for it:  see The Art of Electronics page 194.  Something else is at play here.

[Dave]

I don't think your current measurement with the Keithley 480 was accurate. The input burden voltage of that meter is supposed to be below 200uV. While this figure is very low, it's still way more than those microvolts of error that the bias current caused in the circuit.

[P_Doped]

I think the current measurement was fine.  There was no indication that the ammeter was in the circuit when the output referred voltage measurements were being taken.  The input bias current would be independent of how balanced the inputs were, so the impedance imbalance introduced by the ammeter should not have affected the measurement of the current.

[The Chump]

Another agreement with rufus here. You've applied a rule of thumb and forgotten how to use it properly

Why not just calculate the current in the 100k feedback resistor due to 300uV of output voltage.
You will find that 3nA is far in excess of either input bias current or input offset current specs.

Time to be really really sure that you've built what you think you've built:
have you checked that the resistors are the correct value? (both by inspection and measurement)
have you checked the gain eg with a 100Hz sine input?
have you checked that the split rails are symmetrical? (you are only metering the total rail)
have you checked that the circuit is not oscillating?
have you checked all the other things I've not thought of?

If the error remains after you've eliminated all these other things, i'ts time to ask AD why the parts appear to be way out of spec.

Cheers

Alex

[mrflibble]

JFet opamps are especially dangerous. The gate of the JFET is essentially a diode in reverse. pull that pin low and you will pull current out of the gate as the gate diode now conducts !

Thanks! Do you happen to have more information or reading material on this? Assuming that the exact same issues can occur on TL072's...

[dtweed]

You can consider the bias current to flow in 1k//100k and multiply that by the circuit gain. You can roughly consider the bias current to flow through the 100k to get the output offset but you can't say it flows through the 100k and then multiply by the circuit gain.

Someone said this in the YouTube comments too, but so far, Dave has ignored this point.

You are, of course, quite correct.

[free_electron]

You can consider the bias current to flow in 1k//100k and multiply that by the circuit gain. You can roughly consider the bias current to flow through the 100k to get the output offset but you can't say it flows through the 100k and then multiply by the circuit gain.

Someone said this in the YouTube comments too, but so far, Dave has ignored this point.

You are, of course, quite correct.

but most of it flows through the lowest value ... so that portion does get amplified.

[dtweed]

but most of it flows through the lowest value ... so that portion does get amplified.

No, the "lowest value" has no voltage across it. All of the voltage is across the feedback resistor, and the input bias current flows through it.

[Rufus]

Someone said this in the YouTube comments too, but so far, Dave has ignored this point.

You are, of course, quite correct.

Dave makes mistakes like the rest of us (on live video probably less than the rest of us would).

We should point them out but he doesn't have to fess up or apologize or anything. He isn't arguing about it which I just take as acceptance.

[kg4arn]

but most of it flows through the lowest value ... so that portion does get amplified.

No, the "lowest value" has no voltage across it. All of the voltage is across the feedback resistor, and the input bias current flows through it.

@dtweed
Are you serious or are you mocking the video?

[dtweed]

Someone said this in the YouTube comments too, but so far, Dave has ignored this point.

You are, of course, quite correct.

Dave makes mistakes like the rest of us (on live video probably less than the rest of us would).

We should point them out but he doesn't have to fess up or apologize or anything. He isn't arguing about it which I just take as acceptance.

Ordinarily, I'd agree with you. But in this case, Dave makes the error at about 20:00 in the video, and much of what follows is based on this erroneous calculation.

Also, this isn't just some hobbiest putting out random videos, this is now Dave's livelihood, and I would think he'd have a vested interested in getting the fundamentals right.

[dtweed]

but most of it flows through the lowest value ... so that portion does get amplified.

No, the "lowest value" has no voltage across it. All of the voltage is across the feedback resistor, and the input bias current flows through it.

@dtweed
Are you serious or are you mocking the video?

Quite serious. As @Rufus said, you can etiher think of the current as going though the parallel resistors, and apply the gain to the resulting voltage, or you can think of it as going only though the feedback resistor -- but you can't do both.

[EEVblog]

Dave makes mistakes like the rest of us (on live video probably less than the rest of us would).
We should point them out but he doesn't have to fess up or apologize or anything. He isn't arguing about it which I just take as acceptance.

Yes, I goofed that up.
But it's not like I can pull the video. The best I can do is post-annotate.

[EEVblog]

Ordinarily, I'd agree with you. But in this case, Dave makes the error at about 20:00 in the video, and much of what follows is based on this erroneous calculation.
Also, this isn't just some hobbiest putting out random videos, this is now Dave's livelihood, and I would think he'd have a vested interested in getting the fundamentals right.

It is my livelihood, yes, but the videos are still "off the cuff".
I am not teaching university here, nor producing paid-for content or some course.
I shoot the video and don't review it, so unless I catch myself at the time I'll miss any errors. I try to sanity check as I edit it, and if there are any errors that's were I'll catch it. But proof checking your own stuff is useless as most will know. Then I upload and it's too late at that point.
No matter how hard I try to check for errors, they are always going to slip through. Heck, even textbooks have errors.

The fact that the errors dropped as expected during the practical measurements when dropping the resistor values probably reinforced in my mind that I made no mistakes in the theory part.

[xrunner]

Also, this isn't just some hobbiest putting out random videos, this is now Dave's livelihood, and I would think he'd have a vested interested in getting the fundamentals right.

Warning - Analogy coming at full speed ...

dt - have you ever watched reality TV shows? My favs are Man vs. Wild and Survival Man. Basically they are survival experts thrown into a new environment on each show. Being experts, they can manage quite well yet, even so, they can't always be perfect in a survival setting. That's the way I look at these videos and enjoy them. It's a survival expert trying to tackle a new island or a new situation where you are running out of water. You do what you can at the moment, and later in the studio you can see how a perfect show could have been made.

But, that's not what people want to watch. 

[EEVblog]

Perhaps I've painted myself into a corner with calling it Fundamentals Friday and people expect it to be somehow different/better fact checked to my regular content?
The analogy to a textbook highlights the difference I think. You can spend a couple of years writing an electronics textbook, check it a dozen times, have a third party technical review, and still have a dozen technical errors creep into the final print.
I conceive, shoot, build, and edit a video in less than a day with a streamlined process that has minimal checking. The odds of technical errors creeping in will always be high I'm afraid, that's just the way it is.
In the end, only a few people actually picked up on the error, out of how many, 15,000+
In this case, if the practical measurements didn't show anything to support the theory, then I would have noticed something and went back to review my theory material and likely found the error.

[EEVblog]

I don't think your current measurement with the Keithley 480 was accurate. The input burden voltage of that meter is supposed to be below 200uV. While this figure is very low, it's still way more than those microvolts of error that the bias current caused in the circuit.

Sure, but it at least still showed the current to within an order, and showed it change value and go negative, which was the intent.

[free_electron]

but most of it flows through the lowest value ... so that portion does get amplified.

No, the "lowest value" has no voltage across it. All of the voltage is across the feedback resistor, and the input bias current flows through it.

Theoretically yes... Measure it and you will see there is a voltage.

[EEVblog]

Yes, I certainly formed the view that that was your intention.

Hmm, unfortunately it wasn't.
For any of my videos to take extra time to fact check, produce, refine, spin and release, that is time I simply do not have available.
No doubt some will say "but it doesn't take much effort", but unfortunately it does. And errors will still creep through.
I doubt there is a single technical video I have done that doesn't has some form of error or slip of the tongue/brain fart etc in it.

Perhaps you can make FF videos available to a selected group of volunteer reviewers (like they do with textbooks) prior to making them available to everyone on YT.

Good idea in theory, in practice probably not workable. It would lead me to having to do the usual edit/process/upload cycle, and then have to refine any errors and repeat the cycle. Ok, if they are gross errors, fine, that needs to be done. But that's also a possible slippery slope toward "perfecting" videos.
That could also involve having to go back and re-shoot scenes, keep continuity etc, days after the fact too.
And then, take the current video for example, only a few people out of 15,000+ were able to spot the error. And no one person or small select group would be able to be knowledgeable and fact check everything right on every subject. So errors would likely still slip through.
It would also lead to the inevitable ticked-off reviewer because they might come back with a bunch of suggestions on a better way to explain something etc, and then for the sake of the not "perfecting" things above, I'd have to politely decline to do/fix that. And they would get peeved and won't bother any more etc.
I can see it being a very messy business...
And with having to try and push out maybe 3 videos a week on average, it's a tough enough ask already.

[dtweed]

Perhaps I've painted myself into a corner with calling it Fundamentals Friday and people expect it to be somehow different/better fact checked to my regular content?

No, not really. But I think this particualr error deserves at least one of your "post-annotations", since it keeps getting mentioned through the rest of the video.

I never really expected that your video production process would be any different for Fundamental Fridays, but I do have some expectation that you do a little more preparation before starting to shoot, including making a list of the points you intend to cover, and thinking about the sequencing of the presentation.

Overall, I think you're doing a great job!

[EEVblog]

I never really expected that your video production process would be any different for Fundamental Fridays, but I do have some expectation that you do a little more preparation before starting to shoot, including making a list of the points you intend to cover, and thinking about the sequencing of the presentation.

I put a little bit of thought into it, but it's still essentially press record then figure out what to say.
e.g. I don't recall exactly, but for this one I knew I wanted to structure the sequence pretty much as you see it.
eg. what is IBC, simplistic diff input circuit, a couple of problem scenarios, showing that Ib will flow through Rf in an ideal situation (not often known), and work from there to explain how complex it can get to figure it out, or that you often can't really figure it out because the specs are so wide and interacting etc.
It's often much better in my head than what eventually comes out of my mouth 

[dtweed]

but most of it flows through the lowest value ... so that portion does get amplified.

No, the "lowest value" has no voltage across it. All of the voltage is across the feedback resistor, and the input bias current flows through it.

Theoretically yes... Measure it and you will see there is a voltage.

Any voltage you measure at the inputs is basically the result of the amplifier correcting for V_OS, which is a completely separate issue (as Dave described). The effects of I_OS, and I_B appear only at the output, and the voltage shift at the input as a result of this would be the output shift divided by the open-loop gain of the opamp.

[wilheldp]

I saw Dave's tweet about the Owon scope video getting twice as many YouTube views as his "Fundamental Fridays" video on opamp bias current.  I'm admittedly not as up-to-date as many members of this Forum on electronics design, so I don't find in-depth design discussion or white-board diagramming very interesting.  I tend to find the "theory" videos like this one kind of dry, and they don't hold my attention as well as reviews or teardowns.  I will still watch any video that Dave produces because I like his style and find the vast majority of the videos interesting and engaging.  I'm just not a huge fan of the Fundamental Fridays series.

[EEVblog]

I'm just not a huge fan of the Fundamental Fridays series.

Cool, thanks for the feedback.
I guess the FF Friday videos simply aren't of much interest to those who know about the topic, or as you say, watch more for the entertainment/infotainment value (which I know is a lot of people).
The FF videos really don't have much if any of the "entertainment" aspect.
As always, it comes down the fact that not everyone is going to like every video style.
But it's not like they are not getting any views, so still very worthwhile doing, and feedback is very good.

[wilheldp]

I'll continue to watch them, Dave, and they are valuable.  But as you said, the entertainment value is fairly low.  Electronics aren't my business...just my hobby.  You will inevitably hit on a topic that will be applicable to one of my projects, so in that respect, I don't want to discourage more of the FF videos. 

Keep up the good work.

[3roomlab]

i have a noob question

when a opamp datasheet says bias is -10nA

is it trying to say current is going in or out? or it doesnt matter and we can take the value as +/-10nA?

[EEVblog]

i have a noob question
when a opamp datasheet says bias is -10nA
is it trying to say current is going in or out? or it doesnt matter and we can take the value as +/-10nA?

It means it will source (output) 10nA.
So if you put a 10M resistor across the input to ground you'll measure about 100mV at the input pin.

[David Hess]

i have a noob question
when a opamp datasheet says bias is -10nA
is it trying to say current is going in or out? or it doesnt matter and we can take the value as +/-10nA?

It means it will source (output) 10nA.
So if you put a 10M resistor across the input to ground you'll measure about 100mV at the input pin.

Unfortunately you cannot depend on the polarity specified in the datasheet meaning anything; you have to be very careful about the polarity of the input bias current specification. (1)

Datasheets for the 324 list a 50nA input bias current but datasheets for the 741 list an 80nA input bias current.  They cannot both be positive or negative because the 324 uses a PNP input stage and the 741 uses an NPN input stage.  If you want to know the direction in these cases, you have to see a schematic of the input stage.  A 301 operational amplifier is kind of like a single supply *positive* rail version of a 324 so it uses NPN input transistors but its input bias current specification is the same, just 70nA without indicating direction.

Does this ever matter?  The 741 has an NPN input stage so the input bias current goes to the negative supply.  But what about the old 4136 which was advertised as a quad "741 performance" operational amplifier?  It has a PNP input stage so the bias current goes to the positive supply.  Both datasheets just list 40nA without anything to distinguish the direction so you have to check the device schematics. (2)

There are more sophisticated ways than simple resistors to cancel input bias current.  A single transistor can be used or another of the same type of operational amplifier.  When this is done, the direction of the input bias current matters or you will end up doubling it.

(1) None of the old or new datasheets I reviewed for this post indicated the direction of the input bias current *except* when it could go either way (usually but not always do to input bias current cancellation) in which case "+/-" was included.

(2) The details are lost in the fog of time but I know some designers got caught by this when they tried to use the 4136 in an application where the direction of the input bias current mattered.  I suspect this involved external input bias current cancellation.

[3roomlab]

i have a nearly unrelated question about input impedance

in extremely high input impedance opamps, many have opted for "air" deadbug build as air is very high resistance (40TOhm/cm?) compared to PCB. from one of dave's vids which spoke about ohm/square resistance of copper, since we know the resistivity of FR4 (about 30G/cm?) and the thickness of FR4 (1.6mm?), that would make FR4 sit at 187GOhm/square. is my deduction of this resistance correct? (http://www.edn.com/electronics-blogs/all-aboard-/4431390/Sheet-resistance-of-copper-foil--Rule-of-Thumb--13)
(PTFE @ 1e+15Ohm/cm, would result in 6250T Ohm/square !?)

if then assuming @ 187GOhm/square, and we are mapping FR4 with blue squares (see pic). will it be right to assume that pads with long-ish footfalls adjacent to other similar pads suffer a higher leakage as more squares get to fit into the "spaces" ?
ie : 3 squares fitting between 2 pads = 187/3 = 62GOhm (what do we call this? actual creep resistance?)

so with that in mind, would it make sense to use/edit pads that will connect lesser "leaking" squares? i wonder do any PCB designer use this ohm/square method to design high resistance designs?

but strangely, does using a thinner FR4 PCB = higher resistance? ie : 0.8mm board = 375GOhm/square? (i suspect something is wrong with my fundamentals again ha !)

**update
i had a go editting the SOIC8, and was able to get 2 - 2.5 squares in series between pads (reduced pad size and staggering in and out alternate). if only i could measure how much that is ... ha! 375GOhms? 560G ? anyone want to try and measure actual creepage?

[David Hess]

...

if then assuming @ 187GOhm/square, and we are mapping FR4 with blue squares (see pic). will it be right to assume that pads with long-ish footfalls adjacent to other similar pads suffer a higher leakage as more squares get to fit into the "spaces" ?
ie : 3 squares fitting between 2 pads = 187/3 = 62GOhm (what do we call this? actual creep resistance?)

I think you did it right however I have never seen any designer bother with this because ...

so with that in mind, would it make sense to use/edit pads that will connect lesser "leaking" squares? i wonder do any PCB designer use this ohm/square method to design high resistance designs?

but strangely, does using a thinner FR4 PCB = higher resistance? ie : 0.8mm board = 375GOhm/square? (i suspect something is wrong with my fundamentals again ha !)

... the larger problem is surface resistance do to contamination.  There is also a question about how consistent the printed circuit board material is.  Manufacturers seem to be better about the material used for parts packaging than FR4 board material.

The one exception I have seen is when guard traces are used.  Then adding vias from top to bottom through the guard traces will intercept leakage through the volume of the board material but you have to be doing something pretty extreme for this to matter and it may have more importance preventing coupling capacitance to other nodes.  The RF guys use via like this for completely different reasons.

The surface contamination problem is a real killer and especially so with surface mount parts where air wiring is even more difficult and spacings are closer.  The closest I have seen this done with surface mount parts is cutting slots through the board and not for high voltage isolation but how do you do this with such small pin spacings?  Unfortunately the newest ultra low bias current parts are not available in through hole DIP packages somewhat limiting their ultimate performance.