Noise reduction techniques for op amps (configured as differential) - Page 1
Forum section: electronics / projects

author: TrickyNekro
author_rank: Frequent Contributor
timestamp: September 30, 2013, 12:09:55 pm
content: Hallo guys,I am working on a bidirectional current sensor using the LT1999 and OPA2320 and I am struck by a noise problem.I don´t know yet where the noise comes from, whether or not it´s the LT1999 front end or the OPA2320.The noise level is around 40mVp-p and considering that my ADC resolution is around 0.8mV (12bit dsPIC ADC),that kind of noise is very unacceptable. The problem can be easily solved by a "heavy" low pass filter on the output,but I want the circuit to be fast plus I want first to eliminate any over sources of noise and then adjust the output filter.Can you suggest any noise filtering methods? (I will include the schematic below)The sensor will be connected inside and H-bridge, in series with the motor. It´s designed to measure +/- 10A or there abouts,I have already in mind some things that need tuning like providing an external voltage reference for the differential op ampoffset, plus if I operate the H-bridge with a PWM signal of 10kHz a low pass Sallen-Key with the first pole around 100kHz willdo the trick (although sallen-key configuration has such a terrible dependence on component values as far as Quality factor isconcerned so I want to avoid it as much as possible).In the end, I will be redesigning the board, this first board was more a proof of concept. But in the final design the noise figureof the output has to be within +/- 1mV and not more.

author: Paul Price
author_rank: Super Contributor
timestamp: September 30, 2013, 01:06:02 pm
content: Noise figure is not quantified by a single unit of measure such as just using max acceptable noise voltage(such as 1mV max you mention.)

author: penfold
author_rank: Frequent Contributor
timestamp: September 30, 2013, 01:18:48 pm
content: A little bit more information would be nice, shape/form and frequency of the noise would be quite useful.Without looking up any further details on the op-amp etc id speculate it being an oscillation which could be cured by ensuring that there is a sufficient amount of local decoupling to the chips and possibly a compensation cap on the feedback resistor in the opamp.

author: Paul Price
author_rank: Super Contributor
timestamp: September 30, 2013, 01:42:29 pm
content: You are going to use a motor, so you would do well to clearly define the type.I assume you are interested in getting instantaneous motor current readings on a PWM on-time cycle by cycle basis.If you are using PWM drive for the motor, many more different effects need to be considered. To get any where near an idea of the average or RMS motor current you would have to sample the current only during the on period of the PWM pulse and integrate the result by software calculation. The current sense resistor amplifier you are using would not be fast enough to capture these transients, and a 100-KHz low-pass filter would only ring, maybe just smooth out an abundance of motor noise created, while distorting the instantaneous current peaks of interest.Perhaps, instead, a fast settling op-amp connected to the sense resistor would allow amplified readings if synced to PWM on time and would give you the instantaneous current readings you want, if you have a fast enough A2D.Secondly, applying PWM to a motor (and this really depends on the type of motor you are using as well) will result in transients and current surges or exponential rises in current due to ringing, resonance and other inductive and capacitive effects during the PWM on/off conduction period. If you are using an induction motor or a capacitive start type of motor or a multi-phase motor or especially a brush-type motor, the readings you will get from your circuit would not very well reflect motor current.

author: dannyf
author_rank: Super Contributor
timestamp: September 30, 2013, 01:43:37 pm
content: QuoteDon't you need to know what's causing the "noise" before you settle on an effective way of dealing with it?

author: TrickyNekro
author_rank: Frequent Contributor
timestamp: September 30, 2013, 01:49:18 pm
content: Quote from: Paul Price on September 30, 2013, 01:06:02 pmHmm, my bad, ok so the correct form of the above would be DC - 10Hz 1mVp-p noise.

author: penfold
author_rank: Frequent Contributor
timestamp: September 30, 2013, 01:57:57 pm
content: I was actually more interested in the form of the noise you are currently having a problem with.For example, is it by any chance at the same frequency of the PWM switching but not consistent with the expected current waveform, in which case it current be a problem due to common mode rejection. Does it have a dominant frequency, is it sinusoidal etc.

author: c4757p
author_rank: Super Contributor
timestamp: September 30, 2013, 01:59:45 pm
content: Quote from: dannyf on September 30, 2013, 01:43:37 pmI second this. What have you done to investigate the source of the noise? You say you don't even know which chip it is coming from - this is as easy to test as figuring out you have the noise in the first place!The options are 1) extremely sharp LPF, which, as you've already stated, is a bit silly, or 2) eliminate the noise. So... find the noise!

author: TrickyNekro
author_rank: Frequent Contributor
timestamp: September 30, 2013, 02:03:59 pm
content: Quote from: Paul Price on September 30, 2013, 01:42:29 pmPlease excuse my ignorance, but I though that when referring to H-bridge you usually connect your standard brushed motor.This is only a test circuit, as told before, a proof of concept nothing more.I am feeding the motor with a 10kHz signal, if you vary the PWM from 1 to 99% you will see that it´s only worth using harmonicsup to the 10th harmonic. The power of the 10th harmonic is 100 smaller than your main harmonic (according to Fourier series).That´s already small enough no need to push the limits of the design in that extent, and that´s why I suggested a 100kHz 2ndorder low pass filter. What will happen when I connect the motor, that I don´t know. The noise I get is with DC signals applied.You have to get a negative rail to connect any other kind of op amp to that circuit. And that´s the reason I am using the LT part,with a wide enough common mode voltage input range, you get away from needing a second rail.Please note, this is noise level is when applying DC signals to the Rsense. I have not connected the motor yet.

author: TrickyNekro
author_rank: Frequent Contributor
timestamp: September 30, 2013, 02:14:02 pm
content: Quote from: penfold on September 30, 2013, 01:57:57 pmIn order to help everyone to help me :-PThis noise is when I connect DC signals to the Rsense. The signal is clear, it´s from batteries.0A applied 40mVp-p noise, +/- 3A applied the same.It seems that the noise has different coefficients with the main coefficient (the problematic one) is around 1.5MHz,and I suspect that somehow that has to do with the feedback loop of the OPA2320 op amp, I have to read the pdfagain for small letters but I don´t really find a lot of things out of place.The outputs of the LT1999 are clear pretty much but I have to check again don´t want to risk anything.Another thing I am afraid of, is the values of the resistor. Maybe an order of magnitude more in every resistorwould make things better. I am a little bit afraid that the OPA2320 used as buffer can´t keep up thus producing the noise.That´s my biggest bet right now.I will have more details about the noise soon enough, in some hours, I don´t have any tools where I am right now.Thanks for your commends and time,I will try to clarify ass much as I can any questions I getBest Regards, LefterisGreece

author: TrickyNekro
author_rank: Frequent Contributor
timestamp: September 30, 2013, 02:16:05 pm
content: Quote from: c4757p on September 30, 2013, 01:59:45 pmOhhh, don´t get me wrong, that´s exactly what I am trying to do now, and I post here because I need your help,debunking the thing

author: dannyf
author_rank: Super Contributor
timestamp: September 30, 2013, 02:25:36 pm
content: QuoteUnfortunately, you are the only one in the world who can tell, with absolutely authority, where the noise is from.Without that, we are all just speculating for a cure which only causes more problems for you.

author: TrickyNekro
author_rank: Frequent Contributor
timestamp: September 30, 2013, 02:27:49 pm
content: I know, but I don´t have the tools right now, I´ll meet with a friend in some hours, he has a scopeit will make things a lot easier. I just wanted to post the schematics now, in case someone knewif the problem was inherit with my design

author: dannyf
author_rank: Super Contributor
timestamp: September 30, 2013, 02:31:20 pm
content: You obviously have detected the noise - you gave their measurements, using some means.The same means would allow you to tell where the noise is from, step 1 in a long journey.

author: Paul Price
author_rank: Super Contributor
timestamp: September 30, 2013, 02:35:12 pm
content: Depending on your scope, scope bandwidth and way of probe use, the 40mV noise could just be a probe - oscilloscope type of artifact.

author: TrickyNekro
author_rank: Frequent Contributor
timestamp: September 30, 2013, 02:44:09 pm
content: Quote from: Paul Price on September 30, 2013, 02:35:12 pmI have an old analog 15Mhz, and my friend has a 50Mhz Rigol , the basic unit they have. It´s floor noise is about 4mVp-p something like this. It´s ok for that kind of stuff, I am pretty confident for that

author: Paul Price
author_rank: Super Contributor
timestamp: September 30, 2013, 03:05:13 pm
content: Then, that only leaves as the cause the high gain setting of the sense resistor amp and the leads the instrumentation op-amp input acting as antennas. The op-amp input pins of the op-amp must be close as possible to the sense resistor, symmetrical in length, a twisted pair, or otherwise both coaxial cable shielded.But you haven't really described the noise so carefully. Perhaps take a picture of it on a cellphone camera, showing vert and horiz. settings and post it as a .jpg or .png ?

author: TrickyNekro
author_rank: Frequent Contributor
timestamp: September 30, 2013, 09:56:43 pm
content: hmmm, so for the verdict,I did all the measurements and stuff, it seems that there is an inherit noise with the LT1999, something between 4 - 8mVp-p,so it´s getting amplified by the OPA2320 (output op amp), getting some times +70mV peaks. I really don´t know, I mean for+/- 10A with 100 - 200mA steps it´s ok, when the output if unfiltered. With the output filtered it get´s a lot better, with anoise figure of 2 - 4mVp-p with is ok. All the above measurements were made with DC signals on the Rsense.In the end, I think I really need to redesign the circuit with a high precision instrumentation op amp. I already checked INA128,which seems fit for the job, frequency response wise. It get´s a little more complicated with the power rails, and that´s myonly concern, but yeah... In the end it seems that the problem is the noise of the LT1999 so... I another solution is mostly needed.Thanks for your time and patience, I hope I can come up with a working design in the next days!Best Regards,Lefteris, Greece

author: dannyf
author_rank: Super Contributor
timestamp: September 30, 2013, 10:17:30 pm
content: QuoteThat kind of parts would have never passed validation.No, it is something else.

author: TrickyNekro
author_rank: Frequent Contributor
timestamp: September 30, 2013, 10:24:28 pm
content: Quote from: dannyf on September 30, 2013, 10:17:30 pmHmm, take a look at page 9 of the LT1999-10, I think that unfortunately this kind of noise is just there :-/

author: TrickyNekro
author_rank: Frequent Contributor
timestamp: September 30, 2013, 10:50:48 pm
content: Now I feel like an idiot, not the first time and surely not the last but...Still now I can understand why noone had ever posted a circuit with this LT1999.I was browsing through Analog Device´s site, ah well I found this which is like exactly what I needed from the start:http://www.analog.com/static/imported-files/data_sheets/AD8216.pdfIt has unfortunately I much lower gain, but the noise figures are ace in the hole for what I want.I mean, I learned some nice things when designing my project but, I really wasted some good and expensive parts(like 10€ for some 0.1% 25ppm resistors) but still.... I will get it done and post my results! It will take a time, Iordered the samples, but I won´t be in Greece for the next month, but once back I´ll have the thing ready.In the mean while all I can do is check the results with simulators but eh.... that´s not that bad eitherThanks for your time guys, but this whole thing needs a redesign ;-)Best Regards,Lefteris, Greece

author: dannyf
author_rank: Super Contributor
timestamp: September 30, 2013, 10:56:56 pm
content: QuoteUnfortunately you think that way.It is one fine part, trust me and more importantly, trust LT.

author: TrickyNekro
author_rank: Frequent Contributor
timestamp: September 30, 2013, 11:11:22 pm
content: Quote from: dannyf on September 30, 2013, 10:56:56 pmThen why? The pcb design is just as proposed by LT. The opa2320 is designed according to the datasheet.I don´t say it´s a bad part, it´s just not meant for what I wanted and this is my bad, that´s why I feel likean idiot right now. Take a look at that page of the LT1999-10 datasheet, page 9. The noise is about 50mVp-pat the output of the LT1999, it´s commented as output error. It´s there, in the datasheet. Now imagine thatthe output op amp has a gain of about 8V/V. And well, there you have it ;-)The actual noise is less than the datasheet proposed, super! But still not cut for my jobI trust LT, I am the idiot not paying attention to the datasheet, apart from that the phraseRTFM (sorry for my language) exists for a reasonI am not dropping the project completely though, I will see if an improvised, cut traces low pass filter helps.But still, I am not going to get the performance I want.For someone who wants a current average sensor, with a good output low pass filter the above schematicsare perfect. But for someone who wants to extract a current waveform, ah well not reallyAnyhow, mistakes happen and there are there to teach us something

author: dannyf
author_rank: Super Contributor
timestamp: September 30, 2013, 11:16:35 pm
content: QuoteIt is "noise" because you failed to understand its capture: take a digital scope and short the probe. Take a look at the fuzzy line on the scope.is it an accurate depiction of "noise" on the probe?Anyway, the lesson here is that before you try so hard to solve the problem, figure out first what's causing the problem.

author: TrickyNekro
author_rank: Frequent Contributor
timestamp: September 30, 2013, 11:31:27 pm
content: Quote from: dannyf on September 30, 2013, 11:16:35 pmHmm, Ok, I did that already, I shorted the probe and I was getting something between 4 - 8mVp-p noise. I then connectedthe ground of the IC (directly probed almost on the IC) and the output of the LT1999 and the noise was about 12 - 16mVp-p.I did something more also. I putted a 150pf capacitor directly on the output of the LT1999 to get it a little bit unstable, sothe noise would be 16 - 20mVp-p approximately. And the output amplifier successfully amplified that extra noise.The question? What am I supposed to understand from all the above? I can´t be that wrong