Op-amp input bias current is given for a certain Rin ? - Page 1
Forum section: electronics / beginners

author: MathWizard
author_rank: Super Contributor
timestamp: September 09, 2025, 01:24:18 am
content: I have some of these OPA4388 quad op-amps, and for their input bias current at room temp, the typical is +/-30pA, and the max is +/-500pA. But they also say that's with an Rin of 100k. So do they mean with an external network on either pin, and it's eqv resistance was 100k, so the differential input impedance ? Ignoring the Ios, if you multiply that out for the max IB, you get 50uV.For my circuit in an op-amp test jig, I would only have Rin of ~100R on either pin. So what would my IB be? Surely it won't be 50uV/100R=500nA right ?https://www.lcsc.com/datasheet/C22368271.pdf

author: Konkedout
author_rank: Frequent Contributor
timestamp: September 09, 2025, 02:50:30 am
content: I don't think that input bias current should vary with Rin. For some reason the manufacturers decided to specify the input resistance used when they measured it. I imagine that it might be difficult to measure the pA level input bias current using an instrument with low input resistance, but I cannot e certain about that.

author: MathWizard
author_rank: Super Contributor
timestamp: September 09, 2025, 01:18:03 pm
content: Well it's not the TI version tho, but yeah I looked at their datasheet too. And I know there isn't really a voltage source there.For the input current measurements, it uses a 'servo-loop' configuration with the DUT and a 2nd op-am. And at least in the calculations and in the sim, if you can accurately measure down to the mV, between different setup modes, then you should be able to measure down to the pA. But yeah IDK how much leakage, noise and drift there will be.But right, when measuring the In,p , they'd see closer to 10Meg, ok my mistake, it's another node that would still always look more like 100R.On paper I can zero out the error of the 2nd op-amp, so I'm trying to choose one.But measuring 10's, or 100's of pA means you measure 10's or 100's of mV between setup's, so that should work.So today I want to make just the low current parts of the circuit, and see if it actually works, and measure a legit TI TL082, w/ typical IB=30pA and Ios=5pA.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 09, 2025, 03:50:32 pm
content: The input current with chopper stabilized amplifiers can change with the impedance seen at the inputs. This however tends to be more the impedance in the high frequency / shorter time scale (like 100 MHz, 10 ns). So the parasitic capaciance or an added capacitance can matter. Usually they suggest to have the same imperance for both inputs of a chopper amplifier - not just for the current.So it makes some sense to give the condution they use to test the OP-amps.

author: ahsrabrifat
author_rank: Regular Contributor
timestamp: September 09, 2025, 03:54:45 pm
content: Quote from: MathWizard on September 09, 2025, 01:24:18 amyour IB doesn't magically become 500 nA because your source resistance is 100 Ω. The datasheet statement “with RIN = 100 k” is telling you the source/test resistance used when the manufacturer measured the bias-current numbers, not that the bias current scales as 50 µV / R.

author: MathWizard
author_rank: Super Contributor
timestamp: September 10, 2025, 11:42:05 pm
content: Ok so it shouldn't change very much for most op-amps. I haven't tried that OPA4xxx yet, but on a breadboard I made the op-amp test jig for the picoAmp ranges, and I seem to have measured some 33pA as Iinv from a off-brand TL084, (but Ip was way bigger for some reason) but I need to solder up some good SMD op-amps, with datasheets I trust, before I'll trust 10's pA on a breadboard. But it still matches the calc's and sim. I can try some really large voltage dividers too.Now I've been reading some of this Microchip 'Op Amp Precision Design: PCB Layout Techniques', but I'm not using real SMD PCB's, I'm just using proto-board or copper clad. So no way can I make circuits as compact as real PCB's (for now I rather just make my own).But how much does the size matter for such circuits, if there's no thermal worries, and it's more for DC, how much larger area can you spread these circuits out over ? Say one of these op-amp circuits below was postage stamp sized, is it ok for me to make it 2,3,4x the area ? I'm not too worried about the extra trace resistance for larger size in this case.If I want to go trying to add guarding on a proto-board or copper clad board, I'd really want to spread things out. My 5.5D Keithley DMM is pretty spread out.For this op-amp test-jig project (which I already made a version lacking ranges) I bought some cheap 12-position rotary switches. I sure hope them, and all the bundles of cheap wire going to them, don't leak much. I bought some rubber gloves to wear while making some of this too.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 11, 2025, 08:15:36 am
content: The Tl084 is a JFET type and should thus have a well behaved input bias, without dependence on the input resistor. It is only the AZ amplifiers where the input impedance (may be more the capacitance than the resistance if > 10 K) can slightly effect the input current.For low frequency the circuit around an OP-amp can be relatively large. More area still allows for more hum pick up (magnetic via loop area and capacitive).One should sill keep the capacitance at the inverting input small as this can reduce the phase reserve. This is especially with fast OP-amps, even if only used for low frequency. Similar the decoupling should be relatively close with high GBW OP-amps (e.g. > 10 MHz).The OPA4xxx can be quad OP-amps of many types. For the input currents it is more about the input types (CMOS, JFET, AZ , BJT based with compensation, BJT based without compensation).

author: David Hess
author_rank: Super Contributor
timestamp: September 11, 2025, 05:54:25 pm
content: Quote from: Kleinstein on September 09, 2025, 03:50:32 pmThat sounds like something I read once, but I do not remember where.My general rule has been if the DC impedance is not low at the inputs of a chopper stabilized amplifier, then the AC impedance should be made low by adding capacitance. An example is shown below.

author: MathWizard
author_rank: Super Contributor
timestamp: September 15, 2025, 09:45:41 am
content: Yeah I've made BJT op-amps before, so should be picturing changing the input resistance on that at least. But I did see in the LT1013 datasheet, they give a chart of Vos vs input resistance.Ok I forgot how easy it was for this DC-servo loop (for the op-amp test jig) to oscillate, and how it's just aswell to use a scope on it anyways, since the volatges can be nice and large. So now with it pretty stable, or really stable, I got some deeper readings.So far none of ebay TL07x/8x seem real, but even the ones I got from Digikey or LCSC don't seem to have low enough bias current, it was closer to nA. I have 50 or 100, I only checked all of a quad TL084. Another brand TL084 from LCSC did better though, I'll check some more.The Vos measurements below can be made more accurate once IB,IOS are measured, and those errors can be cancelled out, with other range resistors and other networks. But I'm just using 1-2 ranges on the breadboard. Smaller ranges for power op-amps, and discrete BJT's, etc, might also add too much leakage.A chopper COS2388 has typical Vos of +/-5uV, IB and IOS of +/-30pA, and I measured-10.5uV12.5fA-5.986pAw/ my SDS804X HD. And my SDM3065X got-6.201uVIB=19fA, that or similar is repeatableIOS=-5.039pAAlso a chopper, the UMW brand OPA4388 has a typical Vos of +/-2.25uV, IB of +/-30pA and max IOS of +/-1nA. Yesterday I got-241uV for some reason-2.760pA-5.920pAso ideally Iinv=200fAand today-238.5uV-4.490pA-9.120pAideally Iinv=70fAI saw some funny voltage spikes when testing them, so I guess that's the chopper action changing caps. (edit, IDK what those spikes are, now I'm seeing them on JFET op-amps too, and I guess they are way to big to be on purpose)A RS8621 has a typical Vos of +/-200uV, IB and IOS =+/-10pA, I get-249.2uV6.871pA-5.998pANow sometimes on the order of seconds some of these outputs go wandering all over the place. I'm not sure if thats leakage currents or caps, or improper compensation, etc. I need to redo all my equations with all the leakage currents like from caps. And do the AC analysis too, but most of the numbers above match the detailed DC eqns and LTSpice (not just the simplified version in Renesas pdf).Now I want to put the main op-amp and the range resistors, on a copper clad board. I've broken up little pieces of ringed protoboard, to glue down as islands over the copper clad GND plane.So with all the parts up over the GND plane by a millimeter or so, I guess capacitance won't be much between stuff.But I was thinking of making guard planes, like in LTSpice below. I'd cut a closed loop in the GND plane, from under the op-amp output, and around the feedback resistor islands near that node, and down the middle under the resistors. And then tie that guard area to the output.So I'm hoping that will make it even more low leakage from part, to island, to glue to GND. I'm leaving a space for a 100Meg, but I may not want to add all the other ranges on all the switches, etc, it may leak too much.I want to guard the other side of those feedback resistors too, but that will take ganged switches, or even lower bias current parts, or some other tricks I don't know about yet.Ok so guarding the output side is probably pointless in this case.