Zero volt on the output of OP AMP - Page 1
Forum section: electronics / beginners

author: Tommy1984
author_rank: Regular Contributor
timestamp: February 18, 2018, 10:29:39 am
content: As you all know, if the one shorts a decent multimeter props, it shows 0V if the one is measuring voltage. But, when I short the op amp inputs, I get few micro volts on the output (depends on the op amp and its input offset)! So my question is, how to get zero volt on the output of op amp without effecting the measures value ( I mean if I'm using an op amp to sense current)Regards,Tommy

author: tszaboo
author_rank: Super Contributor
timestamp: February 18, 2018, 10:42:55 am
content: You can use an opamp which has offset pins, like NE5534.Or calibrate it out in software.Or us one of the methods described here:http://www.analog.com/media/en/training-seminars/tutorials/MT-037.pdfOr use a different opamp with insignificant offset.

author: Tommy1984
author_rank: Regular Contributor
timestamp: February 18, 2018, 10:51:41 am
content: Quote from: NANDBlog on February 18, 2018, 10:42:55 amThank you for your reply!Actually, I tried an OPA277 which has an offset trimmer (and very low input offset which is around 10uV), but, as the output is very low (tens of uV) and swinging a lot, it was so hard to trim it!Any idea?Regards,Tommy

author: Belrmar
author_rank: Regular Contributor
timestamp: February 18, 2018, 11:03:17 am
content: It may happen if its rail to rail performance is not too good, are you powering it from a split supply?

author: Tommy1984
author_rank: Regular Contributor
timestamp: February 18, 2018, 11:17:31 am
content: Quote from: Belrmar on February 18, 2018, 11:03:17 amThank you for your reply!Yes, it's powered by dual supply. (Split supply using another AD8629 to split 3V3).Please check the schematic above!Regards,Tommy

author: Kalvin
author_rank: Super Contributor
timestamp: February 18, 2018, 11:24:32 am
content: Quote from: NANDBlog on February 18, 2018, 10:42:55 amTake a look at figure 6 in this app note.

author: Kalvin
author_rank: Super Contributor
timestamp: February 18, 2018, 11:37:10 am
content: You may also add a small capacitor in parallel with R3 to limit the bandwidth and make the circuit easier to adjust. Try 1 nF or 10 nF. However, getting down to 1 uV offset range is very difficult as the op amps and the resistors are very sensitive to temperature changes. Have you checked any zero-drift op amps yet? They tend to have very low offset (typ. less than 10 uV down to 2 uV or even better) and you can then use the offset trim shown in figure 6 in the app note mentioned above. Please bare in mind that the trimpot may also drift with temperature.

author: Tommy1984
author_rank: Regular Contributor
timestamp: February 18, 2018, 11:44:43 am
content: Quote from: Kalvin on February 18, 2018, 11:37:10 amThank you for your reply!I'll make a cup of coffee and straight ahead to my work benchBut dude, AD8629 has typical input offset voltage 1uV and max up to 5uV!Do I need zero drift op amp?Regards,Tommy

author: StillTrying
author_rank: Super Contributor
timestamp: February 18, 2018, 12:12:40 pm
content: If you're hoping to measure down to 10s of uV, the input VGND, output VGND, and R2 VGND need to be physically VERY close together, any u Volt drops in the gnd between them will appear on the output.

author: Kalvin
author_rank: Super Contributor
timestamp: February 18, 2018, 12:24:19 pm
content: Quote from: Tommy1984 on February 18, 2018, 11:44:43 amAD8629 is a zero-drift op amp, good. I wasn't reading the thread very carefully, but just noticed that you have tried OPA227 but missed you last post.The gain of the circuit is 11 (G=1+R3/R2), so the AD8629 offset voltage will be amplified also by that same gain: the 1 uV typical offset voltage will become 11 uV offset voltage.Adding a 1nF - 10nF capacitor across R3 will limit the bandwidth and reduce the noise somewhat. Using the trimming method shown in the figure 6 of the app note, you may be able to tweak the offset voltage closer to zero. But like I said earlier, the temperature effects in the resistors and trimpot may introduce some drifting as temperature changes, so getting a clean zero output might be a bit frustrating experience.

author: forrestc
author_rank: Unknown
timestamp: February 18, 2018, 12:27:26 pm
content: I think most everyone who has responded missed this part of your initial question:Quote from: Tommy1984 on February 18, 2018, 10:29:39 amYou've got an opamp configured with 11x gain. Even with 1uV of offset, you're going to get 11uV out. Then you get to factor in all of the thermoelectric effects which can easily add a few more uV to the picture. There are applications where this matters, and for those there are ways to minimize these effects. This takes careful design and board layout and similar.Fortunately current sensing (your stated application) isn't one of them. Typically if you're sensing current you're going to introduce a voltage drop into the sensed circuit on the order of tens of mV, not uV. At 11x gain, this produces an output so large that a few uV isn't going to matter, especially given that you're unlikely to be able to introduce a current sensor (aka resistor) into the circuit with enough absolute accuracy that that 10uV matters.If you're willing to share a few more of your application details, we might be able to point you in a more acheivable direction. Depending on the exact specifics, sometimes a traditional opamp isn't even the correct solution, especially when sensing currents on higher voltage sources (i.e. more than a few volts).

author: Tommy1984
author_rank: Regular Contributor
timestamp: February 18, 2018, 12:36:29 pm
content: Quote from: StillTrying on February 18, 2018, 12:12:40 pmThank you for your reply!Good point to think with!regards,Tommy

author: Tommy1984
author_rank: Regular Contributor
timestamp: February 18, 2018, 12:50:10 pm
content: Quote from: forrestc on February 18, 2018, 12:27:26 pmThank you for your reply!I'll start from the beginning then.The goal is to make (or at least trying to make) an ammeter with range of 5A to 1uA without using several shunt resistors to switch between for every range.So across just one shunt resistor, voltage drop range is between 500 mV and 0.1uV. The ADC much be 24-bits to handle this wide range. I chose 10 times gain so my new range is 5000mV to 1uV.As long the shunt resistor is shorting the two op amp inputs without any current going through (without any voltage drop), so the difference between the two terminals must be zero (theoretically) and the ADC must read ZERO on output, which I'm not getting up to now.Regards,Tommy

author: danadak
author_rank: Super Contributor
timestamp: February 18, 2018, 03:34:16 pm
content: What test equipment are you using to measure the offset ?What is absolute accuracy you want in measurement, temp rangeenvironment it will be used in ?An excellent online book on precision, error budgets.......https://books.google.com/books?id=yLMgBQAAQBAJ&pg=PA90&lpg=PA90&dq=opamp+offset+pease&source=bl&ots=XH_TzOqr9Q&sig=MPq_LcggzhBVYLIzvGa-08y4I38&hl=en&sa=X&ved=0ahUKEwjltoSX5K_ZAhUmw1kKHU0ZC4oQ6AEITjAE#v=onepage&q&f=falseRegards, Dana.

author: Tommy1984
author_rank: Regular Contributor
timestamp: February 18, 2018, 03:44:11 pm
content: Quote from: danadak on February 18, 2018, 03:34:16 pmThank you for your reply!I'm using Voltcraft with minimum resolution 10uVDC, so I could able to get values => 10uV.I wanna be able to read down to 1uV +/- 0.1uV @ 25 degrees C.Regards,Tommy

author: Tommy1984
author_rank: Regular Contributor
timestamp: February 18, 2018, 03:57:25 pm
content: Quote from: Kalvin on February 18, 2018, 11:37:10 amIt rather worked!But because my multimeter as I mentioned before has 10uV as a minimum resolution, so I'm not sure what does the 000.00 mV on my multimeter screen meansAnyway, I changed my resistors values to 20 and 180R so I got 10 times gain with less noise and bias current of course.And for the external offset trim I used a 20K resistor and 20K trim-pot and as I said, zeroooo, but I wasn't sure what if it was under 10uV.Anyway, I'll wait for my 24-bits ADC or I have to buy a new multimeter with more resolution.OBS! I didn't use any caps because I wasn't sure about the noise value!OBS! I simulated my circuit on LTspice and got 10uV offset voltage!Regards,Tommy

author: danadak
author_rank: Super Contributor
timestamp: February 18, 2018, 04:15:03 pm
content: I looked at several voltcraft manuals, they shows 100 uV resolution,and accuracy in range of +/- .5% to .6%, read (on 200 mV scale) anaccuracy +/- 1000 to +/- 1200 uV.Regards, Dana.

author: Tommy1984
author_rank: Regular Contributor
timestamp: February 18, 2018, 04:33:46 pm
content: Quote from: danadak on February 18, 2018, 04:15:03 pmSorry, I forgot to mention the model number. It's VC-880.Min resolution is 10uV. You can check herehttps://www.conrad.com/ce/en/product/124609/Handheld-multimeter-Digital-VOLTCRAFT-VC880-Calibrated-to-Manufacturers-standards-no-certificate-Data-logger-CAT-III?ref=searchDetailAnd please read my last reply to Kalvin!Regards,Tommy

author: Kalvin
author_rank: Super Contributor
timestamp: February 18, 2018, 04:42:02 pm
content: Quote from: Tommy1984 on February 18, 2018, 03:57:25 pmYou probably need to increase the feedback resistors in order to reduce the output current, as you are now very close to the maximum specified output current +/-10 mA. I would use something larger values like 200 ohm and 1800 ohm.The trimpot of 20 kohm is okay, but you now also have to increase the 20 kohm resistor to 200 kohm or more: You can simulate the circuit using different values for the resistor from 200 kohm resistor up to 2 Mohm so that offset trim doesn't affect the gain, but you are still able to adjust the offset +/- 15 uV.

author: Tommy1984
author_rank: Regular Contributor
timestamp: February 18, 2018, 04:58:56 pm
content: Quote from: Kalvin on February 18, 2018, 04:42:02 pmIsn't so weird

author: StillTrying
author_rank: Super Contributor
timestamp: February 18, 2018, 05:04:58 pm
content: Quote from: Tommy1984 on February 18, 2018, 12:50:10 pmMeasuring 5A down to 1mA is hard enough, so good luck with that.For measuring a few uV across a current sense resistor a INA226 is going to be hard to beat.www.ti.com/product/INA226

author: Tommy1984
author_rank: Regular Contributor
timestamp: February 18, 2018, 05:07:11 pm
content: Quote from: StillTrying on February 18, 2018, 05:04:58 pmThat helps a lot DUDEThank youBTW, it was between 5A and 1uA, not 1mA, is it still hard enough?Regards,Tommy

author: Kalvin
author_rank: Super Contributor
timestamp: February 18, 2018, 05:23:12 pm
content: Quote from: Tommy1984 on February 18, 2018, 04:58:56 pmI ran the simulation, but I changed R1=200, R2=1.8k, R3=10MEG, R8=100k and R9=96k and the simulation gave offset voltage below 1 uV measured from U1 output.

author: Damianos
author_rank: Frequent Contributor
timestamp: February 18, 2018, 06:23:17 pm
content: Very interesting project for a beginner!An Ammeter with 7 digits resolution, made at home!Using as a reference an instrument having an accuracy of 100 uV in the lowest range, while the requested resolution is 1 uV (accuracy?)!How the amplifier will output 5V with 1.65V power supply is, of course, unimportant!The Op-Amp noise figure is at least five times the requested resolution! (Signal 0.1 uV, Noise 0.5 uV typical)... ...After that, who cares about thermal voltages, interference...With a little more effort, a ten or twelve digits instrument is easy to implement!

author: Tommy1984
author_rank: Regular Contributor
timestamp: February 18, 2018, 06:40:07 pm
content: Quote from: Damianos on February 18, 2018, 06:23:17 pmWell thank you for your reply first.I updated the power supply to 5V so it will be +/- 2V5 so the op amp can handle max 2300 mV (which is 2.3 A).I said at the begging that i'm trying to make this project real.I'm a beginner, beginners make mistake and I don't feel shame of that.But, the real shame is, to hang around and read beginners posts and problems and mention their failures in some way as ooooo I AM F%&KING GENIUS!I bet people like you don't dare to post a question or do anything useful in their life!Cheers,Tommy

author: Kalvin
author_rank: Super Contributor
timestamp: February 18, 2018, 06:59:32 pm
content: I think this is quite useful and educating project for a beginner. At least he should be getting a working instrument and also learn the limits of the real-life components and learn something about physical phenomena like thermal effects and circuit noise, compared to the clean simulated circuit. Typically it is quite easy to build an instrument with 1:1000 dynamic range, 1:10000 is much harder and 1:100000 requires good experience and knowledge (just ask any Voltnut here in the forum). As he is also adding a 24-bit ADC, there is some software development involved which won't be wasted time either, and can be reused in later projects.

author: Tommy1984
author_rank: Regular Contributor
timestamp: February 18, 2018, 07:01:57 pm
content: Quote from: Kalvin on February 18, 2018, 06:59:32 pmThank you

author: Damianos
author_rank: Frequent Contributor
timestamp: February 18, 2018, 08:48:33 pm
content: Quote from: Tommy1984 on February 18, 2018, 06:40:07 pmThanks for your opinion but keep it for yourself. Do you know anything about me? Do you have my CV at hand?You are not able to understand for who was my comment but you can criticize easily!I tried to point out the most basic things, while designing an instrument, that were ignored...Good luck,D.

author: Tommy1984
author_rank: Regular Contributor
timestamp: February 18, 2018, 09:17:16 pm
content: Quote from: Damianos on February 18, 2018, 08:48:33 pmThank you again for your helpRegards,Tommy

author: StillTrying
author_rank: Super Contributor
timestamp: February 18, 2018, 10:47:20 pm
content: In practice I think your output will drift quite a bit, probably +/- 150uV or more if your full scale is ~ 5V.So a 'Set Zero Button' will be needed at least !

author: Marco
author_rank: Super Contributor
timestamp: February 18, 2018, 11:24:38 pm
content: Easiest would be to use acomposite autozero ampand a DPDT push button switch you push before you start using it to zero out the last few uV with the microcontroller.The drift will be more than small enough and the low noise BJT opamp will let you measure something useful at the uV range.

author: David Hess
author_rank: Super Contributor
timestamp: February 19, 2018, 12:17:18 am
content: Quote from: Tommy1984 on February 18, 2018, 10:51:41 amUsually the circuit configuration has DC gain so any input offset voltage is multiplied before measurement. So for instance a non-inverting amplifier with a gain of 1000 will show 10 millivolts for 10 microvolts of input offset which is much easier to measure.Input offset voltage can be adjusted at the input, output, or through the offset null pins.If the offset null pins are used, then they should be used *only* to adjust the input offset voltage of the operational amplifier itself and not offsets generated elsewhere, like offset from the input bias current through the input resistance, because on precision operational amplifiers, trimming the input offset away from zero will increase the input offset voltage drift.Quote from: Tommy1984 on February 18, 2018, 12:50:10 pmThis is going to be a problem no matter how good the operational amplifier is. At low current shunt voltages, the thermocouple voltages generated by the dissimilar metal junctions inside and connecting to the current shunt will overwhelm the output from the current shunt.Low value 4-wire precision resistance measurement gets around this by reversing the excitation current and voltage measurement "chopping" the output from the resistor in much the same way that a chopper amplifier removes its own input offset voltage. The same thing could be done with a current shunt by placing it inside of a transistor H-bridge which is switched with overlapping drive to reverse the current through the shunt but I do not remember ever seeing this done in practice.

author: Marco
author_rank: Super Contributor
timestamp: February 19, 2018, 02:57:17 am
content: The input resistance is 0.1 Ohm, so offset current isn't relevant.Thermal EMF for the resistance wire to copper lead connections,it seems so. Vishay has nice high power bulk metal foil current shunt, the CSM3637P, nowhere in stock though. They also have the CSM3637 which is 2W at 200 mOhm, they are even "guaranteed in stock", unfortunately Vishay seems to speak a language almost like English except the meaning of guaranteed is the opposite.

author: forrestc
author_rank: Unknown
timestamp: February 19, 2018, 11:12:56 am
content: Instead of quoting any particular post, I figure I should start clean here:What some of us are trying to explain is that the dynamic range of what you are trying to do is way too high.Let's start with the shunt resistor.Let's assume at the bottom of your range you're willing to accept a 1uV input from your shunt. This is probably way too low to be useful, but let's go there anyways. Getting 1uV at 1uA takes a 1resistor. At this end of the range the power dissipated by the resistor is negligible. 1e-8 watts.Now, let's go to the other end of your range. Putting 5A through that same 1resistor will now result in a 5V drop (burden voltage), and 25W of power dissipation. That much power being dissipated in the shunt resistor is going to cause serious heating and will cause serious drift just due to the temperature coefficient of the resistor. Not to mention that I'm not even sure you can get a 25W resistor with good enough accuracy/tempco effects (someone will probably let me know of some esoteric part which does).Let's move on from the shunt.Let's say that you instead just want to be able to measure from 1uV to 5V. This is 5,000,000 counts, aka 7.5 digits. In the ADC world, this does require somewhere between 22 and 23 bits of accuracy. 24 bits will give you some reasonable room to move. One would think you could just slap a 24 bit adc on there and it would work fine. But if this were the case, you'd have rigol and the like selling 7.5 digit dmm's for $99.00. There's a reason that even the least expensive reasonable quality 5.5 digit DMM's cost multiple times that.The real challenge is that electronics are really not that stable. We've all been programmed to believe so with modern electronic design, but underneath it all, things drift, things aren't as accurate as you'd like. Everything is affected by temperature. At the lowest voltages even the temperature gradients on the circuit boards matter since the resistance of the traces change causing voltage offsets. And on and on. Getting a 7.5 digit anything done takes a lot of work and a lot of patience. There's a reason why the 7.5 digit DMM's are well over a thousand dollars, even when bought used.The real killer is dynamic range. Although it is definitely quite difficult to read down to 1uV, it is even harder to read both 1uV and 5V without a range switch. Most electronics like their signal to be bounded in a tight little range. This is any thing but tight. So you get to not only deal with the challenges of dealing with voltages down at the microvolt level, but you also have to deal with the challenges of high dynamic range, which is another challenge at least as difficult as the "micro voltage" issues.I agree with others that a high precision project like this is an interesting thing for a beginner. But be aware that a project like this takes a *lot* of attention to detail, and a lot of frustration, and a lot of research and most importantly a lot of experimentation and learning. You may find that it is even impossible as stated, and so your goal may have to change, or even be abandoned. As long as you are treating this as a project you may never finish but instead learn from the journey, then I'd say go for it. But please be aware that every problem solved will likely make you discover a new problem. And another and another. But each of these is a learning experience.P.S. I'm a big fan of nearly impossible goals, as it tends to prevent you from reaching a goal and then quitting. The trick is understanding how to gain satisfaction from each step, instead of from reaching the goal. And if by chance you get to what you thought was an nearly impossible goal, by all means pick another and proceed on.

author: Tommy1984
author_rank: Regular Contributor
timestamp: February 21, 2018, 12:21:57 pm
content: Quote from: forrestc on February 19, 2018, 11:12:56 amYou are right in every single word!Regards,Tommy