Zero drift amplifier input bias current - Page 1
Forum section: electronics / general

author: macaba
author_rank: Regular Contributor
timestamp: July 30, 2023, 12:21:57 pm
content: I recently started simulating designs that involve DC amplification of high resistance low level signals but there is a lack of data and discussion regarding the input bias current behavior of zero drift amplifiers. This is a classic case of where reality would not match simulation so I wanted to do some measurements.I had ADA4523 and OPA189 on my bench, both are not particularly ideal for the task (I probably want theothergroup of AZ amplifiers - higher voltage noise, lower current noise, potentially better behaved inputs) but thought it would be an interesting place to start. Both amplifiers have datasheet typical values in the 50-100pA range.Using a Keithley 617 and the built-in voltage source, I measured the input bias current over CM voltage, with various values of input capacitance (SMT C0G). See attached images.OPA189 is horrifying (with any value of input capacitance) and ADA4523 is better behaved (1nF and above, though 100nF & higher is impractical due to dielectric absorption). The OPA189 datasheet has a chart (Figure 8-5) that suggests it could work for high resistance inputs but I suspect this is pure theory.My next step is probably to order some more appropriate AZ amplifier parts or design a DMM-style discrete JFET auto-zero frontend, but I'd like to get some ideas/feedback at this early stage.

author: MasterT
author_rank: Frequent Contributor
timestamp: July 30, 2023, 12:44:24 pm
content: I've seen similar misbehavior many times, AZ getting upset by caps on any inputs or output, ussualy it's exposed as abnormal offset voltage or low freq. noise. Cure is isolation by resistors, try to insert R in between cap & input, and inverting input is no exception

author: Kleinstein
author_rank: Super Contributor
timestamp: July 30, 2023, 03:08:24 pm
content: The voltage dependence of the input bias looks really bad.The microchip appl. notes suggest that one shoud aim for a similar capacitance for the inverting and non inverting inputs. I would extend this to a similar impedance in the 1-100 MHz range. This only makes sense if one also really takes care about the decoupling as the inverting input often gets connected to the output and not directly to ground. A series resistor can be a good idea, but for the very low noise types like the ADA4522 or OPA189 this may also add significant noise. It would however be less of an issue with higher voltage noise / lower current noise types that are more suitable for a high impedance source. The high impedance AZ amplifiers tend to be low supply voltage (e.g. 6 V max) only. So in this case the setup would be with a bootstrapped supply for the AZ amplifer. So at least the voltage dependence is not a problem. The capacitance dependence gets a low less with series resistance. I have rel. good experiance with AD8628 and MCP6V66 / MCP6V76.The Keithley 617 may also have additional input capacitance and may react to higher requency pulses.

author: iMo
author_rank: Super Contributor
timestamp: July 30, 2023, 06:07:24 pm
content: Now, many are using the 4522/23 and OPA189 in classic wiring with 7V->10V amps, with capacitance at the input against ground.How does this issue affects the performance actually? How to improve it?Below the typical wiring I've been talking about..

author: David Hess
author_rank: Super Contributor
timestamp: July 31, 2023, 02:34:13 am
content: Input current noise versus frequency tends to be poorly documented in chopper stabilized amplifiers, perhaps because it makes them all look bad or because they were never intended for high impedance applications. I am not sure about newer lower noise devices, but in the past the rules for getting good results included equalizing the DC and AC impedance at each input, which applies to non-chopper parts where the input current noise is correlated between the inputs, and operating with low AC impedances at the inputs.That last rule can be found in low noise two path amplifier designs where the chopper amplifier inputs have a 1 megohm DC input resistance, but say 0.1 or 1 microfarads of capacitance to ground for the non-inverting input and same same between the inverting input and output.There are newer parts which claim to have solved all of these problems, but I have not tested them yet to see how they perform with high impedance sources. I am working toward a test setup for characterizing low frequency noise. There are now some outstanding precision JFET parts from TI, so they would have to perform very well indeed to be worth using.

author: iMo
author_rank: Super Contributor
timestamp: July 31, 2023, 05:44:04 am
content: Well, based on the above to cure the issue with for example OPA189 we have to wire it as follows:the 3k3=10k||5k and with the C6/C7 1n caps it equalizes the impedancies both inputs see.Is that correct?

author: Kleinstein
author_rank: Super Contributor
timestamp: July 31, 2023, 06:43:56 am
content: With AZ amplifiers there is no need to balance the DC impedance. The DC bias is anti correlated and thus no compensation and a lower resistance is preferred. A balance would only help for high temperature (e.g. > 60-100 C) with more conventional correlated leakage current.With conventional OP-amps it also depends if an added series resistor to get balance is worth it. They still add noise and with internal compensation of the bias the correlation is no longer that good.This was mainly a point with old BJT based ones (e.g. NE5534, 741).For the higher frequency part I see the slight problem that the output sides usually has some series impedance. So it is not just balancing the capacitor at the non-inverting input and the feedback.It may help to additionally load the ouput with an RC snubber (e.g. 50-100 ohm and 1-10 nF). To some degree one may be able to use the capacitance at the input to trim the input current. So more like individual try and error.There are a few examples where they show the frequency dependence of the current noise: What I found so far they showed a flat curve towards low frequencies and the expected extra near the chopper frequency range.The current noise specs in general are poor and quite some numbers look too good to be true (e.g. a lower bound from the input bias current).

author: dietert1
author_rank: Super Contributor
timestamp: July 31, 2023, 07:23:04 am
content: Some years ago somebody explained about the spikes from the opamp inputs generated by the chopper. Since then i started to place a capacitor to Gnd also on the inverting input. Of course one can only put 500 pF or 1 nF there without instability, but that seems to work well. For similar reasons i think these capacitors are important in a circuit having digital signals. I never had the time to quantify the advantage of the measure, though.In the 7V => 10 V schematic of imo the OPA189 is likely stable with an additional 1 nF capacitor from the inverting input to Gnd ("C6 prime").Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: July 31, 2023, 07:28:55 am
content: A capacitor to ground for the inverting input can work as part of a compensated divider. For the 7 to 10 V step there is not much gain and thus quite some capacitive loading. So 1 nF is likely too much - maybe 50-100 pF could work. With a higher gain the capacitance should be OK.

author: dietert1
author_rank: Super Contributor
timestamp: July 31, 2023, 09:03:03 am
content: If one puts a 1 nF cap C6 prime onto the inverting input, it forms a capacitive divider 0.5 with C7, while the resistors make a 0.7 divider. Not exactly compensated, but roughly. Capacitive output load will be 500 pF and the OPA189 does that.The point was the balance of these caps should be more important than the 3.3 K resistor R10. C6 could increase to 2 nF.Regards, Dieter

author: macaba
author_rank: Regular Contributor
timestamp: July 31, 2023, 10:51:50 am
content: Quote from: MasterT on July 30, 2023, 12:44:24 pmThanks, this was a good clue.Quote from: Kleinstein on July 30, 2023, 03:08:24 pmI agree, when I saw this, I also thought the same as you - there is still value in bootstrapping an AZ amplifier to make the input bias compensation (current injection through high value resistor) easier. Do you have links to the microchip application notes?Quote from: iMo on July 30, 2023, 06:07:24 pmYou have to build the circuit and measure it. No amount of simulation or discussion will help here. (this is coming from someone who really likes simulation!)Quote from: David Hess on July 31, 2023, 02:34:13 amI eagerly await your results. Do you mean OPA140 and friends? (or JFE2140?)Quote from: dietert1 on July 31, 2023, 07:23:04 amThis is the same thing I had in mind, and I've seen it on datasheets too. However, as the following will show... this does *not* apply to OPA189.Continuation of experimentationWith all those comments in mind, in an attempt to tame the OPA189, I explored various topologies involving R and C and C on the inverting input too. All of them failed except one - simple resistive-only isolation of both OPA189 inputs. It hates any kind of additional capacitance on the inputs (which defies the 'taming charge injection spikes' hint we've all seen before).See attached image.I think the only reliable rules so far are:- You have to measure reality. Discussion or simulation no good here.- Once you've picked & measured an AZ amplifier for your design, you cannot substitute it for another part unless you perform all the same testing again.

author: dietert1
author_rank: Super Contributor
timestamp: July 31, 2023, 01:11:58 pm
content: Thanks for the reminder to check things. I remember doing something similar with the Advantest R6581T input. The picoamperemeter was a HP 3478A with a 1 MOhm "shunt".Regards, Dieter

author: tszaboo
author_rank: Super Contributor
timestamp: July 31, 2023, 04:39:10 pm
content: Quote from: David Hess on July 31, 2023, 02:34:13 amWhat I've seen, the bias current was correct, but it was an average bias current, with huge swings from it when the AZ was happening. So if you had a high impedance source it was super noisy. I had to order a bunch of different Zero drift amplifiers and test them. Probably not surprising, but the Linear Technology part was like 100 times better than the equivalent TI and AD part, and you couldn't see this at all from the datasheet.Yes, and forget simulations and theoretical calculations.BTW I think I used the OPA189 in a recent design, maybe I'll take a look at it's noise, since i haven't investigated that.

author: David Hess
author_rank: Super Contributor
timestamp: July 31, 2023, 04:43:21 pm
content: Quote from: macaba on July 31, 2023, 10:51:50 amThat is right. The OPA140 is:Input Offset: 120 microvolts maximumInput Offset Drift: 0.35 uV/C typ 1 uV/C max0.1 to 10 Hz Noise: 42 nVrmsInput Bias Current: 0.5 pA typ 10 pA maxCommon Mode Rejection Ratio: 120 dB min 140 dB typBipolar parts can achieve an input offset drift about 10 times better, and chopper parts 2 times better again, but 1 uV/C is still as good or better than the best matched JFETs. That low frequency noise level is about the same as an OP-07 or LT1001, and about twice as good as a low input current super beta bipolar part which the OPA140 would replace for even lower input bias current at room temperature.I am in the process of moving to New Hampshire and still setting up my home lab. I am looking for the best way to measure noise down to 0.01 Hz. Spot noise measurements are straightforward, but maybe I will end up with something more sophisticated. A low frequency low noise digitizer might be ideal, so I am investigating whether a high resolution multimeter could be used that way, which is how I have made accurate spot frequency measurements in the past down to 0.1 Hz.

author: David Hess
author_rank: Super Contributor
timestamp: July 31, 2023, 04:47:33 pm
content: Quote from: tszaboo on July 31, 2023, 04:39:10 pmI do not remember who published an article about it, but it might have come from Bob Pease in a discussion about how datasheets lie, or a discussion about operational amplifier noise. Old datasheets also often confuse common mode rejection versus frequency with power supply rejection versus frequency because they used the wrong test for one of them.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 31, 2023, 06:29:24 pm
content: The JFET input noise may have a different spectral composition than the precision BJT parts. Often one is indeed interrested in the frequencies lower than 0.1 Hz. The 0.1 - 10 Hz range is more like choosen to be easy to measure (AC coupling still works and 2 decades is large enough so that the exact filter shape is not that critical (still a factor to cause condusion). For the lower frequency noise a relatively fat DMM (to work well in 1 PLC mode) should be a reasonable choice. If not with a low of DC offset like a voltage reference the amplification can be quite large in a DC coupled configuration.A point to whatch for is thermal stability: temperature fluctuation combined with a TC may look a lot like 1/f noise.The current noise specs are not just a problem for the AZ OP-amps, but also for many BJT based OP-amps. To get similar good looking numbers the wrong way (splitting the resistor to both inputs), e.g. used with the OP27 is also used with many other OP-amps. This should give a values for the current noise that is 30% to maybe 50% too low if the current noise is not fully in phase correlated. Already the definition is a bit tricky as there are 2 (or even 3 if the inputs are different as in CF parts) parts to the current noise as an independent and correlated part.

author: iMo
author_rank: Super Contributor
timestamp: July 31, 2023, 07:05:18 pm
content: For example MCHP on the MCP6V76 input glitches:QuoteTI on OPA189QuoteADI on ADA4522 input glitches or charge injection - none.

author: dietert1
author_rank: Super Contributor
timestamp: July 31, 2023, 07:16:24 pm
content: The interpretation of macabas plots depends on the input capacitance of the current meter. One could also try to look at the input current with a transimpedance amplifier and a scope. Certainly the image will also depend on the common mode voltage.As far as i understand missing capacitor(s) on a chopper stabilized opamp like the OPA189 won't generate offset current but an unstable input offset voltage (extra low frequency noise). Similar to the errors a SAR ADC produces without some buffer cap on its input.I have been using the OPA189 and ADA4522 for filtering references and in reference gain stages with good results. But of course, if there are quantitative comparisons to other parts, i am interested to see them.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: July 31, 2023, 08:44:20 pm
content: Ti has some article / set of slides on the input current using a TIA to measure the actual current.The charge injection of CMOS switch chips is known to slightly depend on the capacitance, especially in the range of small capacitors. It helps if both sides see a similar capacitance. So to me it is no surprise that the input current is somewhat effected by the capacitance.

author: macaba
author_rank: Regular Contributor
timestamp: July 31, 2023, 09:32:24 pm
content: The Keithley 617 is a TIA. I also have a Keithley 428 (a high bandwidth TIA) so there’s the possibility of seeing dynamic behaviour but I am not optimistic - AZ switching frequency is usually above 100kHz, and the Keithley 428 bandwidth depends on the current range (with most sensitive range having the least bandwidth). Worth a quick attempt though.

author: iMo
author_rank: Super Contributor
timestamp: July 31, 2023, 09:45:16 pm
content: Interesting paper on on-chip glitches and ripples reduction technology used in AZ/choppers opamps written by ADI designer Mr. Kusuda.ADA4522 is usingthe technique depicted in Fig.13.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 01, 2023, 07:43:05 am
content: The dynamic is very fast. Ti in there paper on this used a really fast OP-amp (OPA657) in the TIA to get high speed. The current spike is on the order of 20-100 ns wide.Attached is a copy of the Ti appl. note, that for some reason is hard to find on the website.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: August 01, 2023, 03:46:44 pm
content: Quote from: tszaboo on July 31, 2023, 04:39:10 pmCan you tell what amp it was?

author: dietert1
author_rank: Super Contributor
timestamp: August 01, 2023, 05:24:09 pm
content: Quote from: iMo on July 31, 2023, 09:45:16 pmHow do you know? How to find out which ones are  and  (charge injection balancing)?Regards, Dieter

author: iMo
author_rank: Super Contributor
timestamp: August 01, 2023, 05:37:42 pm
content: Do click on the link and you will see the reference.

author: David Hess
author_rank: Super Contributor
timestamp: August 01, 2023, 10:28:46 pm
content: Quote from: Kleinstein on July 31, 2023, 06:29:24 pmI discovered the OPA140 when researching suitable improved replacements for the old AD542, which at one time was about the best precision JFET part. The LT1012 is the bipolar alternative that I would have used by default. My primary interests were high common mode rejection, low 0.1 to 10 Hz noise, low input bias current, and low input offset voltage drift.The difference in noise is apparent. The JFET in this case has lower high frequency noise, but its higher corner frequency means comparable low frequency noise.OPA140 140 dB cmrr 1 uV/C max 0.5 pA 0.1 to 10 Hz 230 nVpp 8 Hz 5.1 nV/SqrtHzLT1012 132 dB cmrr 1 uV/C max 30 pA 0.1 to 10 Hz 500 nVpp 2.5 Hz 14 nV/SqrtHzLT1001 126 dB cmrr 1 uV/C max 500 pA 0.1 to 10 Hz 300 nVpp 4 Hz 9.8 nV/SqrtHzAs you point out, it is straightforward to make a spot noise measurement over the 0.1 to 10 Hz range. Since the corner frequency usually lies within this range, it makes for a good proxy measurement of low frequency noise. However older chopper stabilized parts have *higher* noise within this range, so are more suitable for even lower frequency applications, where their 1/f noise is flat, and a 0.01 to 1 Hz test is required for proper evaluation which is not so easy.I have used a high resolution DMM to make low frequency spot noise measurements before and got excellent results which agreed with the datasheet values. I am looking to improve on this and then run some tests on modern chopper parts.QuoteYea, at low frequencies it is difficult to distinguish 1/f noise and thermally caused drift.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 02, 2023, 07:10:27 am
content: The thermal part is not just drift. In some cases one even gets some thermal oscillations at low frequencies. I got an example with some 0.1 Hz (not very stable) AFAIR.The AZ amplifiers usually have a rather flat noise in the LF range, with more thermal effects limiting to the really low frequencies below 0.1 Hz. The chopper artifacts are usually above some 5 kHz in modern parts often higher, like 50-500 kHz.

author: tszaboo
author_rank: Super Contributor
timestamp: August 02, 2023, 05:07:58 pm
content: Quote from: MegaVolt on August 01, 2023, 03:46:44 pmIts been a while but I believe it was the LTC2057."The current noise spectrum of the LTC2057 is shown inFigure 2. The characteristic curve shows no 1/f behavior.As with all zero-drift amplifiers, there is a significant current noise component at the offset-nulling frequency. Thisphenomenon is discussed in the Input Bias Current section."

author: iMo
author_rank: Super Contributor
timestamp: August 19, 2023, 08:59:07 am
content: Based on the discussion here I added 1k resistors (R2 and R3) into both inputs of my OPA189 10V buffer.On the first glance the output voltage dropped by some 7-10uV.Now - what about the feedback capacitor C2 - I have not any capacitor there yet.How the C2 affects the story?

author: dietert1
author_rank: Super Contributor
timestamp: August 19, 2023, 09:34:51 am
content: C2 is meant for opamp stability, to provide fast feedback while driving a capacitor. You could try and insert a 1 KOhm resistor between negative input and C2. I think this was the proposal of macaba. One would then expect a shift of similar size but opposite sign.I ordered some OPA847s to try and reproduce the TI study.Regards, Dieter

author: iMo
author_rank: Super Contributor
timestamp: August 19, 2023, 09:37:45 am
content: Adding the C2=4n7 foil as in the above schematics increased the output voltage by ~30uV.PS:Adding a series 1k resistor with the C2=6n8 foil dropped the output voltage by ~20uV, thus I am now at the same level I was before all those above changes..Not sure the fine-tuning of the output voltage should work this way

author: dietert1
author_rank: Super Contributor
timestamp: August 19, 2023, 02:53:25 pm
content: Yes, in order to decide one needs some criterion. Candidates are low input bias current, low input offset voltage and low frequency output noise ("stability").From macabas tests we already saw that the common mode input voltage is a parameter. Using the 10 V output voltage as supply for the chopper amp may not be the best idea.Regards, Dieter

author: iMo
author_rank: Super Contributor
timestamp: August 19, 2023, 03:59:13 pm
content: I do not use 10V as the supply for the chopper.. See my V3 in the ADR1001 thread..

author: miro123
author_rank: Frequent Contributor
timestamp: August 20, 2023, 11:55:13 am
content: According to opa 189 datasheet page 228.3.3 Input Bias Current Clock FeedthroughZero-drift amplifiers such as the OPAx189 use switching on the inputs to correct for the intrinsic offset and driftof the amplifier. Charge injection from the integrated switches on the inputs can introduce short transients inthe input bias current of the amplifier. The extremely short duration of these pulses prevents the pulses fromamplifying, however the pulses may be coupled to the output of the amplifier through the feedback network.The most effective method to prevent transients in the input bias current from producing additional noise at theamplifier output is to use a low-pass filter such as an RC networkSHared link by Kleinstein shows that glitches are in GHz range. poor capacitor grounding and attching of Dc measurement equipment with long wires can create wrong conclusionsInput as well as feed-trough insulation combined with RC output filters seems the practical way to go as hobbyist /limited T&M equipment and lab facility/https://www.eevblog.com/forum/metrology/zero-drift-amplifier-input-bias-current/?action=dlattach;attach=1842343

author: iMo
author_rank: Super Contributor
timestamp: August 20, 2023, 02:19:11 pm
content: Interestingly, when looking at the schematics and designs with choppers here, even the high-end ones, such special precautions has not been provided, afaik.Perhaps those 10-12ns/2uA needle like pulses get absorbed by always existing parasitic capacitancies in those designs.The initial experiments here shows the resistors tame the effects, most probably the R with a parasitic C does the job. The large foil capacitors may not help much, low value ceramics would be a better choice, imho.A nice area for experiments, as many here are using choppers in their high-end references..

author: David Hess
author_rank: Super Contributor
timestamp: August 20, 2023, 11:59:29 pm
content: The designs I am familiar with which use older chopper stabilized amplifiers successfully all provided low AC impedances at both inputs, which means a large feedback capacitance and a large capacitance from the non-inverting input to ground. They were also limited to low bandwidth extending below 1 Hz.The designs which failed presented a high impedance to the inputs, which led to excessive low frequency noise.I should have the chance in the near future to run some tests comparing old and new parts. I am currently working toward having better ways to characterize low frequency noise.