No fuss good volt-follower op amp - Page 1
Forum section: electronics / beginners

author: Rick Law
author_rank: Super Contributor
timestamp: June 24, 2014, 04:53:05 pm
content: I need some advice again...To get closer to 0.0 V, I was thinking about upgrading a 2902 doing just voltage following in a 0 to +5V environment. Besides getting one with lower Vos, I was thinking of perhaps "other benefits" from a "better" OpAmp. But I realize I have no clue what to look for in the datasheet other than Vos.I understand the (lack-of) true rail-to-rail. I understand how Vos affects Vout verses Vin, temperature drift affecting amplification (even x1 is multiplying), and how faster slew rate fits in.I know that it is unreasonable to expect Vout=Vin to femto volt accuracy, but what would the error be? How linear will it be... Reading the datasheet gave me no clue why OpAmp X may be better than Y. What if (for the purpose of understanding) it is constant temperature, near zero Hz, assuming perfect circuit outside the OpAmp (so Rg/Rf is exactly 1)... how close is Vout=Vin given this OpAmp's specs?I just finish reading TI's OpAmp for Everyone thinking it may help - it is informative but not for the question at hand. I still have no clue.Two part question:1. Whatwould you suggest as theno-fussgood voltage follower opAmp (single rail and as near to rail-to-rail say in the $3 USD range)and2. Whatpart of the datasheet most interest you when merely for volt following?Thanks

author: katzohki
author_rank: Frequent Contributor
timestamp: June 24, 2014, 07:59:51 pm
content: Would be helpful to understand your application, can you post a schematic?Input Offset Voltage might be something for you to consider looking at.As for a good voltage follower, heck I would probably use a LM358 or whatever's around really. Depends on thefrequencyI expect it to be seeing, but if it's sort of just a buffer or current supply I'd say any old opamp will do. If it was for digital communications I'd go for something more specific.So what are you using it for?

author: chrissmeuk
author_rank: Contributor
timestamp: June 24, 2014, 08:42:09 pm
content: +1 for LM358. Rail to rail, stable and reasonable bandwidth and doesn't hurt you if you use it as a straightforward comparator. TI also like to give me an endless supply of these from their free sample system

author: Rick Law
author_rank: Super Contributor
timestamp: June 25, 2014, 02:32:12 am
content: Quote from: katzohki on June 24, 2014, 07:59:51 pmThe circuit is really nothing much. The four OpAmps voltage follower just front ends an AdaFruit ADS1115 (4 channel 16bit ADC) board which I am experimenting with. One OpAmp follower for each channel. The OpAmp output is where I connect to the breadboard to whatever I would be experimenting.I blew two ADC's on my Mega328 feeding it 20-25 volt by mistake. That is when I started using the OpAmp-voltage follower as quick protection.The OpAmp is powered by an LM317 voltage adjusted to where the opAmp is unable to output over 5V. So even if I mess up and connect the ADC to the wrong spot on the breadboard, the ADC's will never get more than 5V from the voltage follower thus reducing the chance of my killing it by mistake. Apart from a decoupling capacitor on the OpAmp's power, there is nothing in that circuit. Just 4 voltage follower front ending the Adafruit ADC.Powered by the same LM317, I actually have an LM358 doing the same for the AdaFruit 12 bit DAC. So the DAC output is voltage followed by the LM358 and the breadboard circuit never drawn from the DAC direct but drawn from the LM358 instead.I too think the LM358 is a bit better than the 2902 as voltage follower. It sinks less current but lower Vos for sure.I was thinking of replacing them both (the DAC's front end LM358 and the ADC's front end 2902) with "something better" and I have no clue on how to judge - hence the question about what in the datasheet is of most interest for mere voltage following.

author: theatrus
author_rank: Frequent Contributor
timestamp: June 25, 2014, 03:05:54 am
content: You can try an LT1638 op-amp, which is actually specified to have a large "over the top" voltage on its inputs:http://cds.linear.com/docs/en/datasheet/16389fg.pdfIts not real protection, but it does give you a lot of flexibility.

author: Rick Law
author_rank: Super Contributor
timestamp: June 25, 2014, 03:56:06 am
content: Quote from: theatrus on June 25, 2014, 03:05:54 amHey thanks. Just looked over the datasheet. Both the lt1638 and the 1639 are what I want. However, Mouser doesn't have either one; Digitkey has min-order over 100 on one and over 3000 on the other; and Newark doesn't have the PDIP version.So, I will note them on my "like/want" should I come across them in smaller qty.You also brought up a point that was in the back of my mind. I never experimented with way-over-the-top. I just assumed that if the voltage is below the OpAmp max V in, it wouldn't act funny. I should kill a chip or two testing Non-Inverted-input to something like +30v with Vdd at just 6.xV and see what the voltage follower outputs at max.Thanks.

author: theatrus
author_rank: Frequent Contributor
timestamp: June 25, 2014, 03:58:00 am
content: Opamps do different things when the inputs go over V+. It varies.The SO8 package for the LT1638 is readily available. Do you really need DIP? Maybe an SO8 to DIP adapter?

author: Rick Law
author_rank: Super Contributor
timestamp: June 25, 2014, 04:19:50 am
content: Quote from: theatrus on June 25, 2014, 03:58:00 amI was thinking about DIP adapter, but rejected because I would be spending $8 (chip+adapter) to protect $7 (cost of ADS1115). Had it been PDIP, at least I can kind of justify by the "convenience" of pop-in replacement.I think I may get an ADS1115 and an MCP4725 for the DAC. If I blow either out, now I get to do some SMD soldering that counts. I have been practicing on an old cell phone on small devices (sub mm between pins). That would make me feel good if I actually can do a fix on something that small rather than mere practice on a dead cell phone carcass.Good learning exercise - thanks for your generosity in sharing your expertise!Rick

author: theatrus
author_rank: Frequent Contributor
timestamp: June 25, 2014, 05:19:53 am
content: That opamp is a precision unit and carries a premium priceThere are others out there, may be worth looking through the TI, Analog, and Maxim catalogs.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 25, 2014, 05:34:35 am
content: The three general errors are offset, gain and distortion, for example expressing the transfer function Vo/Vi as a linear least-squares best fit line, lumping together all nonlinearity as residual error.Offset is only guaranteed to some range, meaning, what any given part will do when you wire it up and measure. The offset itself is fairly stable, usually varying some with common mode voltage (especially types with complementary input stages -- near the rails, one set shuts off and the offset shifts relatively suddenly) and temperature. If those are fairly stable, trimming is useful for achieving even better offset performance. (One of many reasons why precision equipment is best used once it rises to operating temperature.)Gain is the input voltage difference required for some output voltage change. Obviously, if the output range is ~10V and the gain is >10^4, the input doesn't change more than 1mV for any output voltage. But that's a lot to a precision amp, so they necessarily have higher gain, often over 10^6. In a follower, the overall gain is 1-(1/Av), so a 1ppm gain error is 99.9999% of an ideal follower.Distortion or nonlinearity (same idea) is any departure from this. For example, the inherent transfer function of an amp might be terrible (1mV input gives 5V, 0.2mV gives 7V, 3mV gives 8V...), but it doesn't matter much normally because of feedback. In a follower, this input-referred error is the only  distortion you get.Tim

author: Rick Law
author_rank: Super Contributor
timestamp: June 25, 2014, 06:07:22 am
content: Quote from: T3sl4co1l on June 25, 2014, 05:34:35 amThanks, Tim.I think I understood what you said except the last paragraph. (It is 2am where I am at, so brain needs a recharge first) I will re-read that last paragraph tomorrow.Just want to thank-you first.Rick

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 25, 2014, 06:18:17 am
content: Oops, I put 0.2 when I meant 2. But anyway, see the progression, if it were linear, it would be 1mV = 10V, 2mV = 20V (or saturation, whichever comes first), etc.Tim

author: Rick Law
author_rank: Super Contributor
timestamp: June 26, 2014, 12:19:51 am
content: Quote from: T3sl4co1l on June 25, 2014, 06:18:17 amNow I see... 1mV, 0.2mV, 3mV... I was not seeing the progression. With the correction, now I understand the last paragraph as well!Thanks for the education. Much appreciated.Rick

author: David Hess
author_rank: Super Contributor
timestamp: June 28, 2014, 12:55:54 am
content: Beyond input offset voltage, common mode rejection ratio is what is going to distinguish precision operational amplifiers from general operational amplifiers in voltage follower applications.

author: jmole
author_rank: Regular Contributor
timestamp: June 28, 2014, 01:49:22 am
content: My favorite no-fuss op amp, for a slightly different reason (unlimited capacitive loading without oscillation), is the LM8272:http://www.ti.com/lit/ds/symlink/lm8272.pdfinput offset voltage and RRIO current are pretty good as well.

author: Rick Law
author_rank: Super Contributor
timestamp: July 01, 2014, 06:08:55 am
content: Thanks for all the help, here is what I ended up doing:I moved my "front ends" from breadboard to a protoboard. My "DacAdcAdaptor" host the AdaFruit 12bit DAC and the AdaFruit 16bit ADC. Both are powered by the 7805 on the board (so I am not relying on the USB power). The OpAmps are powered by an LM317 adjusted via the trimpot to output 5.1 to 5.2volt max on either OpAmp.The DAC is front ended by a TI's TLC272 (was LM358). So the DAC outputs to the (1/2) TLC272 and the application drawn from the voltage follower instead of directly from the DAC. I already blew one DAC prior - I think from connecting the output to +5v by mistake. That leads me to add the LM358 as buffer. Now the TLC will "take the heat".The ADC is front-ended by TI's TLC274 (was LM2902n). Each channel hides behind the voltage follower (1/4 of TLC274), so each is "protected" from some over-voltage on the input to the ADS1115. Their literature keeps warning about keeping ADC input to V(in)<Vdd+0.3v and Vdd<5.5v, I suppose the margin is small.This attached picture is my "DacAdcAdaptor" with pin header connected to my other experiment (yeah, if you spot it in the picture, all 5 inputs are shorted together for now so the 4 ADC channels are merely reading the DAC's output. My adaptor on the left, and other experiment connected to it on the right - not pushed all the way in with the pin header but just to show how I intended to use the adaptor).Result:DAC:The LM358 was doing an adequate job preventing over-sinking/over-drawing. I connected the output (first the LM358, then the TLC72) to a 5V spot (simulating a mistake) and the DAC didn't die. Same with gounding the output to simulate mistake. It didn't kill anything. However, it increased the minimum output.The DAC set at 0 for min output natively outputs 1.x mV. The LM358 increased that (minimum) by just over 1mV. The TLC272 increased the min by less then 0.2mV. (I was expecting 1mV from the datasheet, that is better then expected). I am not sure about other changes/errors yet, but I expect better consistency across the entire 0 to 5V output.I also expect my "over-sinking/over-drawing" problem solved by the LM358 is similarly solved by the better TLC272 as well. TI's datasheet said: "The output stage of the TLC272 and TLC277 is designed to sink and source relatively high amounts of current". Ithinksince LM358 was adequate, I should not be in too bad a shape here.ADC:I simulated mistakes by connected the adaptor's input (ie:via OpAmp as buffer) to a +12V source. The TLC274 (like the LM2902 it replaced) at 6.5V-ish Vdd took that 12V at In+ and the follower outputs 5.1V. So, it prevented over-voltage to the ADS1115 at least for a short time. I did that for about 5-10 seconds only - should be long enough for me to discover the mistake had it been a real mistake.My UT61e serves as the "correct" voltage for now (calibrated to match the DMM plus at exactly 5V), and using the on-same-board DAC's output as test voltage. I tested the ADC with all 5 PGA setting. Natively, the ADC will track my UT61e at +- 2mV max for all ranges up to range max and typically matched the UT61e's reading within 0.5mV.With the 2902 as voltage follower as input buffer, the ADC tracks my UT61e at +- 6mV. It doesn't give meaningful reading until double digit mV. With the TLC274, it tracks my UT61e at +- 2mV. (Typically, the TLC274, it is within +-0.5mV of the natively no-buffer reading.) It also gives meaningful reading down to just over 1mV (at DAC@0) as oppose to merely down to >9mV for the 2902.So, the new OpAmp increased range down to 1mV from 9mV, and took 4mV error off the voltage follower - the price of some "protection". (I prefer to look at it as adding typically 0.5mV error and 2mV error worst case as price. Same math, different perspective.)I was hoping to identify something better and you guys helped me made my decision. After reviewing your suggestions, it helped me conclude where the trade off should be. It makes no sense to "protect" a $7 chip with a $8 setup. The TLC274 is not as good as I like, but do an adequate job and way better than the 2902. If I make the small mistake over volting the input, I am out a $1 opamp instead of a $7 chip. I can just drop a replacement in.Thanks for your input. Valuable in my learning...