limitation of opamp in the single supply differential mode - Page 1
Forum section: electronics / projects

author: VanitarNordic
author_rank: Frequent Contributor
timestamp: April 11, 2019, 08:31:24 am
content: Hi,I have a sensor which is connected to +12V and it shows some small variations at the output.I have an opamp which I have configured in AC input, inverting amplifier. therefore I should supply it with the load voltage and then find the best value for + rail (by default the half of the VCC which is 6), something like the figure below, but the opamp maximum supply is 3.3V. I am bounded to use this opamp and I can not change the sensor voltage.is there any method to fix this problem?

author: Zero999
author_rank: Super Contributor
timestamp: April 11, 2019, 09:12:39 am
content: I don't fully understand the question.Are you asking who to use an op-amp powered from 3.3V to amplify the signal from a sensor powered by 12V?If do, the answer is yes. You might need to add some protection diodes to the op-amp input, if they're not already built-in, to prevent spikes when the AC coupling capacitor charges, on power on.

author: VanitarNordic
author_rank: Frequent Contributor
timestamp: April 11, 2019, 09:33:02 am
content: Quote from: Zero999 on April 11, 2019, 09:12:39 amNo, the question is when the input is +12 referenced, the suitable voltage for the + rail could be something of course above +3.3V. in this circuit I don't see a correct output except I use a +12V opamp and adjust the + rail to see the desired wave at the outputI have already used the diode and input is AC coupled

author: Zero999
author_rank: Super Contributor
timestamp: April 11, 2019, 11:18:58 am
content: Quote from: VanitarNordic on April 11, 2019, 09:33:02 amIf the AC coupling capacitor has the correct polarity and isn't faulty, once the steady state condition has been reached, the DC voltage on the input should make no difference, to the output voltage of the op-amp. Please post a complete schematic showing the sensor, the op-amp and supply voltages clearly marked.

author: StillTrying
author_rank: Super Contributor
timestamp: April 11, 2019, 12:36:57 pm
content: It's not very differential, we've been here before.

author: Ian.M
author_rank: Super Contributor
timestamp: April 11, 2019, 06:46:36 pm
content: Sim it. Test what happens when you tie the input (to the 0.1uF cap) to +Vcc then apply a small signal to both, on top of the 12V DC bias!It isn't differential, and it has worse than absolutely lousy common mode rejection.

author: StillTrying
author_rank: Super Contributor
timestamp: April 11, 2019, 07:14:55 pm
content: It might be something like that, I haven't the time to play with it, inside the box is guesswork.

author: Ian.M
author_rank: Super Contributor
timestamp: April 11, 2019, 08:03:16 pm
content: For the classic single OPAMP differential circuit:to have good common mode rejection, R1/Rf must equal R2/Rg. When capacitors are involved in the input circuit, you must consider their impedance so R1 + a series capacitor becomes Z1 and you must add a series capacitor to R2, unless the capacitive impedance is negligible with respect to R1 at the lowest frequency of interest.If you hav capacitors in series with both R1 and R2, and split Rg into two resistors each of double its value as a potential divider between +3.3V and ground, it maintains the same Thevenin equivalent resistance for Rg, but biasses in+ to mid-rail, allowing the OPAMP to function normally.Input clamping is left as an exercise for the reader - if R1 and R2 are large enough your OPAMP may not even need it - see datasheet for details of what input protection it may have built-in.

author: gf
author_rank: Super Contributor
timestamp: April 11, 2019, 09:02:20 pm
content: Quote from: Ian.M on April 11, 2019, 08:03:16 pmSuch a divider leads to a modest PSRR wrt. the +3.3V supply rail, though.

author: VanitarNordic
author_rank: Frequent Contributor
timestamp: April 11, 2019, 10:22:10 pm
content: Yes, I did some mistakes in resistor values. I'll modify them and see what happens

author: Ian.M
author_rank: Super Contributor
timestamp: April 11, 2019, 10:33:37 pm
content: Quote from: gf on April 11, 2019, 09:02:20 pmYes. That's the price you pay for trying to do too much with just one single supply low voltage OPAMP.If the 3.3V rail (or bias supply) isn't clean, your signal is going to get FUBARed unless you can cancel out the noise in further processing, e.g if the next stage also uses a matching divider to set its reference voltage. Otherwise, you'd do better returning the bottom end of a single Rg to a properly stabilised and filtered low impedance reference voltage that's within the rails with enough room for your amplified signal swing above and below it.

author: StillTrying
author_rank: Super Contributor
timestamp: April 12, 2019, 01:39:04 am
content: If you've got the op amps +Ve input AC de-coupled to the 0V and its -Ve input AC coupled to the +12V you're just x100 amplifying the noise of the 12V supply.Old thread. "www.eevblog.com/forum/projects/lf357-opamp-replaced-by-an-ad797-subsititude-must-work-better-but-it-dosn't!!!/msg1503595/#msg1503595"

author: Zero999
author_rank: Super Contributor
timestamp: April 12, 2019, 07:37:39 am
content: Quote from: StillTrying on April 11, 2019, 12:36:57 pmI'm not sure it's supposed to be a differential amplifier, despite the thread title.I thought it was just an inverting amplifier, AC coupled to a signal floating at 12VDC, with the op-amp run off 3.3V.

author: Ian.M
author_rank: Super Contributor
timestamp: April 12, 2019, 08:23:03 am
content: Read the other thread (andto O.P. for not mentioning this is a followup). You've been there before!If the O.P. has to use a ground-referenced low voltage OPAMP doing it with anything other than a gated precision differential amplifier configuration will be like listening for a mouse fart in a hurricane. He's interested in mV or uV level signals immediately after a 1.9V pk-pk pulse. That's going to make life interesting (in the Chinese curse sense).

author: VanitarNordic
author_rank: Frequent Contributor
timestamp: April 12, 2019, 08:44:46 am
content: Quote from: Ian.M on April 12, 2019, 08:23:03 amThank you, Mr. Sherlock Holmes, for being a detector of my postsI don't want to accept your insults, please do not reply to this thread, unfortunately, I can not remove this thread otherwise I can not stop people like you to talk like this.I don't know how to deal with super professional designers here who fight with each other to prove their words instead of telling their technical opinion and leave.

author: Zero999
author_rank: Super Contributor
timestamp: April 12, 2019, 08:47:21 am
content: Quote from: Ian.M on April 12, 2019, 08:23:03 amWhich thread? I've just looked through the original poster's recent threads and can't find anything similar on the first page of hits.I remember another recent thread about differential amplifiers but I think they might've been started by someone else.

author: Ian.M
author_rank: Super Contributor
timestamp: April 12, 2019, 08:58:00 am
content: StillTrying linked to the middle of it above. Go back to page 1 and re-read. Unless the O.P's requirements have changed in the past 11 months, the small signal, and artificial constraints (+12V referenced sensing coil and 3.3V ground referenced single OPAMP supply) will make it very difficult to get usable results.@VanitarNordic,If you fail to provide pertinent information like other solutions that have been tried and failed and discussed here, and that you are interested in very low level signals preceded by a high amplitude pulse, you can expect a certain amount of opprobrium. I haven't questioned your intelligence, sanity or professional integrity, so if you feel insulted it must be because I linked a Wikipedia image to clarify the constraints on impedance ratios for a true differential OPAMP configuration. As you have decided to put personality before engineering, I'm just going to sit back and watchGood luck - you are going to need it!

author: VanitarNordic
author_rank: Frequent Contributor
timestamp: April 12, 2019, 09:42:02 am
content: Quote from: Ian.M on April 12, 2019, 08:58:00 amNo, I never ever feel insulted by the technical talking or challenging the information. we learn from discussion. but please check you wasted how many hours to investigate someone else posts and make angry comments like that just because maybe the topic or content is repetitive or information is not enough.Generally, engineers never support each other and they have the habit to ruin each other work, that's why their hard work or high working stress gets paid much less than the people in medicine who make daily mistakes in their routine job and get high salaries just because they support each other, even if their colleagues kill somebody

author: Zero999
author_rank: Super Contributor
timestamp: April 12, 2019, 09:56:59 am
content: Quote from: Ian.M on April 12, 2019, 08:58:00 amThat was nearly a year ago. I didn't think it could be related to this thread.Quote from: VanitarNordic on April 12, 2019, 09:42:02 amIt's not like that where I work. Perhaps you should consider another job?Anyway, back on topic. The op-amp should be run off the 12V rail. An AC coupled differential amplifier could be used. Awhile go Audioguru did a schematic showing the inverting and non-inverting op-amp configurations, both DC and AC coupled. Here's my version, showing the differential amplifier configuration: bottom right. Since the inputs are DC biased at more than half the supply voltage, the polarity of the input capacitors will need to be reversed.

author: gf
author_rank: Super Contributor
timestamp: April 12, 2019, 10:51:55 am
content: Quote from: Zero999 on April 12, 2019, 09:56:59 amThere also do exist 3V single-supply capable instrumentation amplifier ICs with CMRR of > 80dB, which is likely better than the CMRR achievable with a differential amplifier based on a single regular opamp. In order to fit into the amp's common mode input voltage range, only AC coupling is an option then, of course. The AC ripple on the 12V rail must not exceed the amp's peak-to-peak common mode input voltage range either.[ The first search result I found with Google is e.g.https://www.analog.com/media/en/technical-documentation/data-sheets/AD8223.pdf,but there are many others. ]

author: Zero999
author_rank: Super Contributor
timestamp: April 12, 2019, 11:06:05 am
content: The link was broken. Here's the correct one.https://www.analog.com/media/en/technical-documentation/data-sheets/AD8223.pdfThe AD8223 is a good idea. Its common mode range goes to 150mV below the negative rail, so if the peak supply ripple + signal is under that, both inputs can be biased to 0V via suitable resistors, say 1M to 10M and the input AC coupled to each input, via a capacitor.

author: SiliconWizard
author_rank: Super Contributor
timestamp: April 12, 2019, 01:57:45 pm
content: I've used an AD8223 for a thermocouple amplifier on a 3.3V single supply and it's great.I suggest still checking that you're using it in its operating range. The datasheet will help, you can also use this tool:https://www.analog.com/designtools/en/diamond/#difL=-0.08&difR=0.08&difSl=-0.08&gain=100&l=-6&pr=AD8223&r=6&sl=-6&tab=1&ty=1&vn=-10&vp=10&vr=0You may have to use a Vref > 0.

author: VanitarNordic
author_rank: Frequent Contributor
timestamp: April 12, 2019, 02:34:52 pm
content: Quoteso you confirm my word that when the sensor is VCC referenced (+12V), then the opamp should be at least 12V and a 3.3V opamp cannot do the best job

author: gf
author_rank: Super Contributor
timestamp: April 12, 2019, 04:06:58 pm
content: Quote from: VanitarNordic on April 12, 2019, 02:34:52 pmWhat is the peak-to-peak voltage swing at the 12V level?a) common mode (-> basically the power supply ripple which you do not want to get amplified)b) differential (the sensor signal itself which should be amplified)And is AC coupling OK? You do not need DC coupling, do you?

author: gf
author_rank: Super Contributor
timestamp: April 12, 2019, 04:14:30 pm
content: Quote from: SiliconWizard on April 12, 2019, 01:57:45 pmSure, if the input is AC coupled, then it is definitively necessary to set an output offset > 0 in order to avoid clipping of the negative lobes.

author: ebastler
author_rank: Super Contributor
timestamp: April 12, 2019, 04:38:14 pm
content: Quote from: VanitarNordic on April 12, 2019, 09:42:02 amYour approach is a bit strange, if I may say so. Instead of two posts complaining about Ian's reference to your old thread, you could simply have said "Yes, it's the same design challenge" or "No, this is about a different requirement".And how do you know that Ian "wasted how many hours" to find that old post of yours? He might simply have remembered you. I can see how that could happen...Finally, if you come here asking for help with your design, you have to be prepared for criticism. If there was nothing wrong with your design, why would you have asked for help in the first place? So I don't understand the second paragraph of your quoted post.

author: Ian.M
author_rank: Super Contributor
timestamp: April 12, 2019, 04:55:00 pm
content: For the record: I didn't go looking for O.P's old topic on my own, StillTrying posted the link to it, and it took me no more than 10 minutes to skim through it for an overview of O.P's problem.Doctors mistakes kill patients one at a time. Engineers mistakes kill people by the plane, train or bridge full.

author: StillTrying
author_rank: Super Contributor
timestamp: April 12, 2019, 04:58:16 pm
content: Quote from: VanitarNordic on April 12, 2019, 08:44:46 amIf it wasn't for me remembering and eventually finding the old thread none of us would have much idea of what you're trying to do!Getting a clean <mV signal AC coupled off a noisy +12V is certainly going to be V hard.Years ago when I did similar I did it like below, the high side switch is a minor problem, but the single supply opamp is simple - no differential or AC coupling caps needed. It's so long ago I can't remember what I used for the high side switch or the opamp, probably a CA3130, or something.Quote from: ebastler on April 12, 2019, 04:38:14 pmLOL. I well remembered the other thread, but was to busy with other things to find it (there's 2!) and choose a good post to link to.

author: VanitarNordic
author_rank: Frequent Contributor
timestamp: April 12, 2019, 05:22:05 pm
content: Quoteif you don't have a technical point, please stay out of this. I did not ask you to teach ethical points. That was my comment just for Ian.

author: VanitarNordic
author_rank: Frequent Contributor
timestamp: April 12, 2019, 05:28:50 pm
content: I already got the technical answer and solved the problem. Thanks for all technical contributions

author: ebastler
author_rank: Super Contributor
timestamp: April 12, 2019, 05:49:35 pm
content: Quote from: VanitarNordic on April 12, 2019, 05:22:05 pmSorry, but you made a very general, sweeping, moral statement about the bad behavior patterns of engineers "in general". That's what moved the discussion into the realm of "ethics" for me, if you want to use that term.Quote from: VanitarNordic on April 12, 2019, 05:28:50 pmFair enough. See you next year!

author: VanitarNordic
author_rank: Frequent Contributor
timestamp: April 12, 2019, 06:00:30 pm
content: QuoteYou don't reply in that case and stay out of this. you have made no contribution here. if you want to continue to your moral things, do it, but I'll not reply anymore.

author: Zero999
author_rank: Super Contributor
timestamp: April 12, 2019, 07:57:16 pm
content: Quote from: VanitarNordic on April 12, 2019, 02:34:52 pmI was a bit confused when I made that statement.If the peak to peak value of the ripple on the 12V supply plus the input signal you're trying to measure is under the op-amp's input voltage range than it should be fine.In other words, assume the op-amp is run off 3.3V and will work with its inputs down to -0.3V and up to 3.6V (300mV beyond either rail is pretty common for most rail to rail op-amps), then it should work, as long as the ripple on the 12V plus your signal is under3.9V peak to peak. There should also be protection diodes between the op-amp inputs and its positive power supply rail, assuming it doesn't have them built-in.

author: mikerj
author_rank: Super Contributor
timestamp: April 12, 2019, 08:41:13 pm
content: Quote from: VanitarNordic on April 12, 2019, 06:00:30 pmYou seem like a real piece of work. You are the one that came here asking for help remember?