EEVblog #528 - Opamp Input Noise Voltage Tutorial

hi welcome to fundamentals Friday today we're going to take a look at op-amp voltage noise now this can be a real big can of worms so I'm going to only open it just a little bit today and we're going to take a look at one of the more confusing parameters on an op-amp datasheet and that's input noise voltage density and important noise voltage if you didn't know well you do now that any op-amp is going to have inherent noise in it just like all components and all wires and everything else has inherent noise within it and the op amp is no different and that's what we're going to take a look at now we're not going to take a look at anything around the circuit resistor nose and other components and stuff like that just what's inherent in the op-amp and to do that we're going to start by taking a look at a typical datasheet now let's take a look at the OP oh seven a typical precision op amp not particularly low noise but it is one of the jellybeans precision devices now it has a parameter here called input voltage noise and that's the noise effectively on the input and the units are very easy they're micro volts in peak to peak and it's called e N or our VN depending on the datasheet could be called other things but they're just typical labels for it and you know that figure might be familiar to you and it's fairly easy to understand okay I've in the case of the Oppo seven we've got 0.35 micro volts peak-to-peak input noise so if we've got a voltage follower like this with a gainer one we're going to get an output noise or an inherent noise in our op amp in our complete amplifier here of that naught point three five micro volts peak-to-peak real easy to understand but there's a catch take a look at the conditions that that value is measured over and it's actually naught point one Hertz to 10 Hertz and with it you might be familiar with this from power specs for example they might specify the output noise of your bench lab power supply over typically a 20 megahertz bandwidth well in this case it's a very small low frequency bandwidth and we'll find out why later it's no point 1 Hertz to 10 Hertz and this is typically how they measure it they've got the op-amp here it may have some may or may not have some gain and the input will be grounded it'll all be shielded of course and then we'll have a bandpass filter of not point 1 Hertz to 10 Hertz we'll have some more gain in there because we're talking about low signal levels that'll go into a scope and they can measure that value and they'll give you a peak to peak or a maximum peak to peak signal and they'll also give you take a look at this also in the does in most data sheets they'll also give you a typical waveform as well once again that's the bandwidth limited to naught point 1 to 10 Hertz very limited frequency range so that's well and good if you're operating down in that frequency range in your circuit fantastic you've got this real-world figure here you understand it it's easy it's a peak maximum voltage and you know what your system noise is going to be at least just you to the op-amp very simple but what happens if you want to actually operate typically over a larger frequency range well get into something a bit more complicated called input noise voltage density you'll notice it's exactly the same but they've added this word density and if we go back to the datasheet and take a look at some typical figures for the oppo 7 what do we get well look you can see that the conditions there there's three different values and these are called the spot frequency values in this case we've got 10 Hertz 100 Hertz and 1 kilohertz and we've got different figures for that ten point three ten and nine point six respectively and you'll notice how it's slightly higher at lower frequencies and that's important which we'll take a look at in a minute but it uses these bizarre units which confuses a lot of people and it's nano volts per root Hertz and here it is once again it's labeled exactly the same ee + VN exactly the same but instead of micro volts peak-to-peak we've now got a value in nano volts per root Hertz what does that mean in a nutshell its spectral density ie the density of the noise over a specific spectrum or frequency range just like our input voltage noise was measured from point 1 Hertz to 10 Hertz it needs a these this unit here actually needs a frequency range over which it's going to be valid otherwise it's a meaningless figured now the confusing part about these units of nano volts per root Hertz is that you go well what kind of units is that well it's just voltage it's it you know even though it's called nano volts per root hurts the / root Hertz part just specifies that it's defined over a frequency range because this is spectral density now so we've basically it is just a voltage that's all there is to it now the datasheet for example for this one at a specific frequency has for example 10 nano volts per root Hertz now it's very important to understand that this is not divided by root Hertz it's per root Hertz and it's actually reference to one Hertz so it's 10 nano volts for every 1 Hertz of bandwidth and that's the key to understanding this thing so if you've only got a 1 Hertz bandwidth then your noise is going to be square root of 1 Hertz which is the same 10 nano volts but you know usually you're not going to be operating over a one Hertz families so let's look at one kilohertz bandwidth and the formula then is f max - f min that's a little bit complicated but it's basically the bandwidth you operate in under so if you operate circuit is operating from zero Hertz up to one kilohertz then you've got a band width of one kilohertz minus zero is one kilohertz 10 nano volts times the square root of 1 kilohertz gives you a final value of 316 nano volts easy that's how much noise RMS by the way this is all RMS noise in your op amp inherent in your op amp just like this value up here it was micro volts but it was specified pita peak this one I'm here nano volts per root Hertz specified in RMS so you can see that the higher frequency range you operate over the more noise you're going to have because it's multiplied by the square root of the frequency if we operate over 10 kilohertz there it's going to be bigger noise once again our 100 kilohertz or a mega Hertz the next important thing to understand is this is what is called input referred noise or equivalent input noise you'll see these terms are various different that types of terminology but it means that this is the noise on the IMP the equivalent noise on the input of the op-amp so what that means is it gets always gets multiplied by the gain of the op-amp in this case we just got a gain of 1 so in the case of this hypo 7 316 nano volts RMS on the input same 316 nano volts RMS noise on the output pretty low noise but if you suddenly whack in a gain of a thousand in there a V equals a thousand bingo you've gone from 316 nano volts to 316 micro volts or 0.3 millivolts much higher noise now if you remember I said this was rms so how do you convert it to possibly a more useful maximum peak to peak value in your system well this one gets a bit fuzzy and you have to introduce probability now what we're talking about here is white noise or you know purely random noise which has your typical gaussian response like this and we won't go into hugely into types of noise but it has that gaussian response now I've drawn a voltage here I've rotated the axes like that so positive and negative voltage noise can always be equally positive and negative doesn't just go positive and basically are the peak value so this is just a typical voltage peak like this over time so as you can see you know the noise is completely random and what are these peak values here going to be this is where you get into that probability term Sigma now if we look at the value of plus minus 3 Sigma there basically what that is that we have a 99.9 percent confidence or close to it that the peak-to-peak noise is going to be within that specific value so that three Sigma value what you to get that that's a typical figure quoted so manufacturers might typically define the convert rms to p2p by using a multiplier of x 6 or x 6 point six six point six will give you 99.9 percent probability the noise falls within a certain range but it doesn't guarantee it as a point one percent chance can be outside that and well it's up to you as the system to designer to determine what probability you need but that's a good ballpark so multiply that value by about six or six point six so in our example of a gain of a thousand here what's our output noise for this op oh seven with 10 nano volts per root Hertz specified well it's going to the output is going to be 316 micro volts RMS around about 2 point 1 millivolt speak to peak with a good confidence level and that is going to be your output noise just solely due to your op amp not taking into account any other components or any other part of your circuit so that's really quite easy to understand once you know just multiply that figure by the square root of your bandwidth and you get your output noise in rms very simple but yeah there's more to it let's go a little bit deeper open that can of worms just a little bit more and yes hold on to your hat we're going into a graph of noise voltage versus frequency on dual log axes so we've got our nano volts per root Hertz here versus frequency and as I said log axes that's important so 10 Hertz hundred Hertz and it's not a linear increase same with frequency 10 hundred 1k and then it's your typical log axes you should be familiar with so the black line there is our noise voltage and you will find this typically find this curve in the datasheet as well and it'll always be in this particular the form and here's where the trick with all this op-amp voltage noise comes in that we've effectively got two different types of noise in our op-amp and they effectively split into different parts of the frequency spectrum the higher frequency say from around 10 Hertz or a hundred Hertz up typically is going to be your Gaussian white noise that we showed before and effectively what we're using up there for our input noise voltage density that's our white noise up there but all op amps regardless of the type are going to have this characteristic response that tails up at low frequencies and this is called 1 on F noise so white noise dominates at higher frequencies 1 on F noise dominates at lower frequencies they're usually you know around about 10 Hertz or lower or that figure that's why our input voltage noise here peak to peak was specified over that 10 Hertz range because they're really looking at the 1 on F noise they're the low frequency stuff whereas our voltage density is looking at the higher frequency noise up here and yeah they are two different things so when we were actually calculating this input noise noise density over here for a 0 to 1 kilohertz range we were actually including this lower part down here but because the frequency range we were working over because it's log axes is so large pretty much you can ignore this tail up end and you know we can stick with the ballpark figures we got over here for our noise voltage density over that entire frequency range and we won't go into specific details of the types of noise because there are quite a few different types but suffice it to say that the white noise the high frequency stuff is made up a combination of shot noise and thermal or Junction or johnson noise as you may have heard it called and the one on F noise is also referred to as pink noise and that's due to what's called flicker noise but it's more typically just called 1 on F noise and that's the trap with components you can't escape this one on if noise it's just inherent in nature there's absolutely nothing you can do about it there are things you can do in the process of manufacturing your devices to you know to reduce the up flicker noise but pretty much you're going to cop it at that low frequency range so you might think these op amps are less noisy at DC well that's not the case as you can see they get much much noisier at DC their lower noise at the higher frequencies it doesn't make sense but hey a lot of things in physics don't make sense right next thing we know we're talking about spooky action at a distance who are now Gaussian white noise like shot and thermal noise has a uniform power density what that means is that it's going to be the same value regardless of the frequency and that's why we get a flat light in there for that but one on F noise is not a uniform power density so that's why we get basically a flat line straight line like that but it has a specific slope 3 DB per octave but we won't go into the details and this all comes back to why our input noise voltage density was specified in the datasheet at three particular frequencies one kilohertz hundred Hertz and ten Hertz it's so that you can do comparisons with other op amps of how this noise changes and how well it performs over a frequency range like that because if you see a large change for example between a hundred Hertz and ten Hertz in for one op amp and hardly any difference for another op amp then you know that that second op amp with the same figure right down to ten Hertz is going to be a better open and that's basically a deciding factor that corner frequency that we've got there that effectively determines how good your op amp effectively is the lower that corner frequency the better your op amp and that's the one you're most likely going to choose all things being equal and as always with data sheets the marketers are going to fudge the numbers to give you the best possible benefit so beware you have to actually go in there and look at the grass look at the individual data and compare op-amps it and it can actually be pretty hard to compare op-amps just from the data sheets not that easy so you've got to be careful and know how to design it into your system and you'll also notice on the datasheet that that's an identical noise spec for current as well so its input noise current density and input noise current and we won't go into that that's the current into the input to the op-amp so at the moment as I said we're only looking at the voltage scenario but hey if you've got significant input counts you have to take the input current noise in your count as well in those really critical low noise circuits but the same sort of fundamental theory applies and yes it's all going to add up with the voltage noise as well so it's got to be careful and by the time you actually practically build the circuit up usually usually the external components are going to dominate your circuit more than the op-amp itself but hey that's why they spec these things because a lot of critical applications you have to get the lowest noise op-amp possible and that's what it's all about frequency range remember how much noise density within that 1 Hertz window and when you extrapolate these two lines here to get that corner frequency crossing point where they intersect if extrapolate that down then we've got there's 10 Hertz there's 20 Hertz so it's somewhere in there let's say about 15 Hertz is our corner frequency for this example we've drawn here then that 15 Hertz point is the point where the value of the white noise is equal to the value of the one on F naught noise and of course if you sum them together let's say it's a 10 then as shown then you don't get 10 plus 10 of course you get 10 times the square root of 2 so you get about 14 point 1 so there you go I probably took a bit longer than I expected and there's a lot more detail in here as well but suffice it to say for your basic op amp like that if you're working from DC if it's all DC coupled in your full bandwidth is from DC to 1 kilohertz for example you effectively do have to take in to account these two different types and noise and you've got to sum them together and when you add noises together it's actually the root of the sum of the squares so it's a square root of this noise here squared plus this noise here squared and you've got to add on together and that gives you total noise but as we said right at the start this is just the noise inherent in the op-amp itself it doesn't include the resistors here which of course have that thermal Johnson noise you might be familiar with that classic equation the higher the resistive value the more thermal noise you're going to get in the resistor and all sorts of other stuff in your circuit so it can get really complicated but I hope you found that real it is pretty easy to understand what nano volts per root Hertz is and how to calculate your noise very simple this is a bit more detailed of how it actually works but let's go and see if we can actually measure exactly this graph to the bench and what tool do you use to measure the input noise voltage of something like an op-amp well you use a dynamic signal analyzer or DSA which we are seen in the previous videos and this is my HP 35 double 600 a DSA they go from DC to about a hundred kilohertz perfect for characterizing the and scene the one on F noise and power spectral density of the noise in something like an op-amp or any other circuit it's the tool of choice but unfortunately this 35 double 608 isn't exactly the world's best performance it's noise floor isn't that great in itself so that's what we do first we'll just measure the noise floor of this unit itself with a 50 ohm terminator on the input of course on channel 1 here and we'll let see what we get but it's not going to be that crash hot but it should be good enough to at least allow us to see differences between different types of op amps so I'll just run through you briefly how to set up a dynamic signal analyzer to measure power spectral density on a low voltage signal like this now when you first turn it on by default here you we've got our frequency spectrum like this it's displaying our frequency spectrum from zero Hertz down here to one hundred and 2.4 kilohertz and we're only looking at channel one so there's a span the record length is three point nine milliseconds for each one of those and on our y-axis here we have DB volts RMS what there we go it's doing its auto calibration and we've got to figure you know down around that hundred and thirty minus hundred and thirty one DB volts RMS mark the first thing we have to do because we're measuring low signal levels go input so I've selected the input button on the front and then channel one range at the moment it's Auto ranging we really don't want that we want it to just be fixed and this thing I'm pressing the up/down arrow keys and as you can see there you go the channel one range up there the highest gain range or the lowest voltage range it's got is minus hunt and minus fifty one DB volts RMS and that's equivalent to I think about four millivolts peak or there abouts next thing we want to do is turn on some averages so I'll press the average button on the front and then we want to turn average on like that because otherwise we'll just get you know we want to smooth align see what happens when you turn the average on there it's set for 10 I'm going to change that to number of averages there and I'm going to enter a hundred averages so now when you press the start button and we start our acquisition there we go it's giving us a bit of a plot already and we can already see that we're getting a result here it is there's our pretty much flat line with the big one on F noise tailing up at the bottom but why didn't it look like the whiteboard well because we haven't plotted the frequency on a log plot yet it's a linear plot it's a linear axis sorry speaking of which we have to go out to the input here set it up and just make sure we've got a DC couple in here we want to go all the way down to DC so to change that to a log graph we press the scale button on the frontier and here it is x-axis there it is currently set to linear we'll change that to log and bingo look at that we're starting to get exactly the response that we now the reason why it's there's not many data points down here because it has to do with the number of lines in the FFT response of this thing now we've got a full span here of 102 point 4 kilohertz and this particular instrument only has 400 lines of resolution so if you divide 102 point 4 kilohertz into 400 you will get if we move our marker across here you'll notice that each step it can only measure at those frequency points there so it's very coarse down there of course and you'll find that the lowest step down there is going to be 1/4 hundredth of 102 point 4 kilo Hertz so 102 point 4 K divided by 400 there we go gives us 256 Hertz where our marker is all the way over there what's that market X there it is 256 Hertz so it can only jump up in 256 Hertz steps because that's all the FFT resolution we've got there and of course that it really shows up when you've got the log X axes like that didn't really show up on the linear one because then it would be stepping in 400 even linear increments across the screen now if I press the measurement data button on the front panel here we're in what's called diet board just normal frequency spectrum mode more correctly referred to as linear spectrum mode and that gives us a voltage response here and as we saw before DB volts RMS there - 123 and if we plug that into the calculator - 123 and then we divide because it's in dB remember if you want to convert it to a voltage then we divide it by 20 and then we take the inverse log of that and we've got ourselves 708 nano volts but what does that mean doesn't really mean anything because that isn't our power spectral density so press the scale button on the front and we'll have a look at the vertical units which we've got here DB volts RMS at the moment and as you see there is no option for that voltage per root Hertz because we're in the linear spectrum mode we're not in a do it we're not actually calculating the power spectrum density but that doesn't mean that this graph isn't correct because it actually is the shape of this graph is absolutely bang-on to what we will get in the spout and power spectrum density except our units are up here aren't correct where DB volts RMS instead of that voltage per / root Hertz so how do we do that well how do we convert it well we can do it manually when do all the math ourselves to convert between the linear spectrum and the power spectrum density but we don't need to do that what we can do is go into the press the measurement data on the front this thing will do it for us that's what these dynamic signal analyzers are designed to do measure this noise specifically and there it is PSD mode or power spectrum density bingo for going to power spectrum density you'll notice that the graph hasn't changed at all and normally when you change mode it rescales things but it hasn't the graph has stayed exactly the same but look what we've got now it's got a little asterisk next to it here and that asterisks means there it is volts RMS / root Hertz and if we go back that's exactly what we want exactly what we saw on the whiteboard and if we go back into the measurement sorry the scale here into our vertical units we'll see because we're now in the power spectrum density mode that we've got root Hertz options here volts RMS square DB volts RMS / root hoods hurts or volts per root Hertz that's what we want volts well we want nano volts but volts per root Hertz is the same thing it'll scale for us so bingo look up what we've got now now what and that value there at 10 kilohertz close to 10 kilohertz is now switched over and it's calculated that it's 28 eetu the -9 that's now now of course nano volts per root Hertz bingo we've now got our DSA to to check its own performance because we've remember we've got a 50-ohm terminator on the front and there it is that's what it is after a hundred averages down here over that well at the moment the full span from zero to a hundred and two kilohertz so as you can see this instrument you know is worse than a basic you know op oh seven op-amp 28 nano volts per root Hertz and as we saw in the datasheet before just a basic of a seven is around you know at a spot frequency in this case ten kilos it only goes up to one I I think but yeah you know because it's flat it's going to be exactly the same it had a figure of around 10 nano volts per root Hertz so this thing isn't good enough to measure the performance of an op oh seven the way you normally do it although it is you could actually use this instrument the way you'll normally do it is use an external extremely low noise purpose-designed amplifier to amplify the noise before it gets into this instrument so you use this instrument you've already gained it up so you bring it way above the noise floor of this instrument and then you can you know if it's got times 100 gain then you can just you know change the units to compensate for that and you can actually measure the performance of an op o seven now if we take our cursor all the way over to the corner frequency down there there once again we're very coarse because we're measuring the whole hundred and two kilohertz bandwidth as it's telling us the corner frequency is about one kilohertz but I know that's not going to be the case what we want to do is change the span so we get more detail down on this one on a fridge and instead of just three crappy three data points and that's easy you just press the frequency button on the front you can see these DSA's are specifically designed for these types of measurements that optimize for it this is what they're designed to do anyway we can just go span like this press the span button and then we can just type in say well night let's do 1000 Hertz will do a kilohertz range and then it's going to restart you can see it's automatically restarted and it'll do the RMS over it takes longer of course because it's lower frequency so it takes a quarter of a second per record length like that but there you go this one has actually dropped off the screen so I think we've done something with our input scale in there so if we press our scale button there we can just Auto scale that and bang that's going to bring it in line like that and look at that look at that we can now when that cursor we can now put it at one kilohertz there you go so it's at one kilohertz there and we're getting a value of about 31 nano volts per root Hertz that's a noise floor of this thing as I said not very spectacular in fact I want to invest in this thing up have a look at the op amps used in this and other components and see if I can actually use modern drop in high performance op amps to actually increase the performance of this thing so I'll leave that to a future video you can see it's essentially flat and it starts to tail up a bit there you can see it just starting to go up so you can see because we are effectively measuring the noise of the input noise of the the input section or the input op amps inside this particular instrument so we'll get exactly the same result if we were measuring an external op amp effectively so the value of one kilohertz here is going to be slightly lower than the value on a hundred Hertz which once again is going to be lower than the value at ten Hertz here and that's why they have those three spot values on the datasheet one kilohertz hundred Hertz and then ten Hertz over here and of course that will be a continued basically completely flat out to that hundred kilohertz we saw last time but you see it pretty much starting to get bad at just under two hundred Hertz there I've put it on one hundred and sixty Hertz for a reason because let's go to the data sheet for this HP DSA and here it is straight out of the user manual on the - 51 DB volt range ie the highest gain range which we've got source impedance of 50 ohms which we got 16 rms averages well we've done 100 you'll notice that it doesn't specify anything under 160 Hertz it's got that hundred and sixty Hertz to that one kilohertz range is - 130 DB volts per Hertz and of course you if you wanted to you have to convert that to the power spectrum density which we can do which we have just done with the instrument itself so there you go that's why they've got a figure of a hundred and sixty Hertz in there because it it's performance really starts under that one hundred and sixty Hertz you know it really starts to be a bit how you're doing and one thing I want you to take note of near fifty Hertz there you'll notice that we're getting no fifty Hertz pickup at all and of course this lab is just swimming in fifty Hertz mains frequency because as we saw in the tear down of this thing it's incredibly well shielded and we've just got a 50 ohm Terminator on the front but as well I think we'll see when we try and measure a practical circuit we're going to get at least some 50 Hertz pickup it's almost unavoidable okay so let's take a note after a hundred averages at our marker frequency of one kilohertz because that's a value we can get from the datasheet for some op amps we're getting thirty one point three nano volts per root Hertz so that's the basic noise floor of our DSA here and of course to measure noise floors like this you need a Faraday cage you need a shielded box one of these die cast alloy boxes absolutely fantastic so the industry standard way to measure these things a little mini breadboard in there with am teal oh seven two on it and I've got two 9-volt batteries now if you look at the data sheet the voltage are the noise for these for all these chips is usually specified at say plus minus 15 volts or sort of maximum rail it's going to be near enough plus minus nine now of course once you put the lid on this sucker there's no way anything is getting in there at all we've got our nice BNC on there we're going to shield a coax all the way to the input Bob's your uncle and of course you do want to use batteries internal to the box you don't want to be using an external power supply or any type of switching power supply or anything like that batteries the only way to do it and you'll notice no I don't need any decoupling on there good enough because we've got the low impedance battery directly and this thing ain't gonna oscillate so we've got our box hooked up with the TL zero seven two in it now I chose the TL o seven two because it's not a particularly out low noise op-amp about eighteen nano volts per root Hertz at that one kilohertz figure straight from the datasheet because it's not designed for noise it only has the figure at one kilohertz it really you know it's not that great doesn't really specify it in depth but here we go so that is the noise floor of our DSA let's press Start and we will get using the exact same parameters we set up before remember 31.3 nano volts per route hurts now of course that is below it so the noise and that we're trying to measure here of this TL o-72 is below the noise floor of this DSA but aha remember that they summit together so we should see an increase there let's press Start and away we go and woohoo look at that one on F noise has gone right off the scale there and look at that bump what frequency do you reckon that is 50 Hertz bang on where are we picking up our 50 Hertz from it ain't through the box it's through the shield of the coax that's the only place it can be getting in I don't know this is a you know rg59 cable or something I don't know just a cheap one I had lying around so yeah you're really even with fully shirako axes and the shield a box we get in our 50 Hertz pickup but anyway look we've got almost got a hundred averages there we go we've gone up from thirty one point three nano volts per route Hertz to thirty eight point zero to eight thirty eight nano volts per route Hertz at one kilohertz so it's gone up by about seven nano volts per route Hertz and what value should have we expected well 31.3 nano volts per route hurts the base noise floor we had there we're going to square that remember the formula we had on the whiteboard for and then we've got some of the squares so we've got to add in the datasheet value typically 18 nano volts per route Hertz at one kilohertz so yeah let's square that and then get the square root we should get around about thirty six point one and we get in thirty eight point one in a thirty eight so you know near enough there you go we were able to see a difference with that TLO seven two now let's get one that's even worse forty two nano volts per root Hertz it's a tail oh six - it's an absolute shocker I put it in there let's press Start and there we go oh-ho we still get our 50 Hertz of course horrible one on F noise gone off the scale here but there we go ah its massive now look at that in the order of you know 75 nono volts per root Hertz all fall there we go after 100 average is 68 point one is that correct I don't know they are what is it 31 at point three squared which is noise floor of our DSA it plus the nominal 42 from the datasheet and then we cannot get that and then we square root that we expected around about fifty two point three and we're well above that so that one's not working out too great really is an ancient chip though trust me it's like 25 years old or something let me check the date code there's actually not a date code on that but this one's actually like I had this one since I was a kid and it was actually D soldered from a board so it's ancient and shocking um but anyway it allows us just to show the difference there what a crappy op-amp can make and how you can measure it and I've now put in an analog devices ad71 - there are the identical r18 nano volts per it hurts of our tail oh seven one so let's give that one a bill and see what we get still get our big 50 Hertz but there we go we're getting yeah about forty odd not too dissimilar to what we were getting with the tail oh seven two and as I said if we really wanted to measure the performance of these op-amp I would have to use an external amplifier in here I'd have to really design it properly and ironically you need an incredibly you know low noise amplifier in that to measure low noise measuring trying to measure the state-of-the-art op amps well it will be very careful in how you roll the input amplifiers and we would still be able to measure it easily once we got you know some gain in that box - I get well above the noise floor there and actually be able to measure properly the absolute performance of the op amps but anyway I hope you found that interesting we were able to see the differences between some ah pants there and if I put in a really schmick op amp in there we would have actually seen it drop to pretty much the same noise floor as this particular DSA so there you go if you want to discuss it jump on over to the eevblog 4 and hope you liked the video and if you did please give it a big thumbs up catch you next time wait hang on I found in any double v 3 for op amp really a good audio low noise audio op amp I think they even use a couple of them in here from what I saw on the schematic anyway and not at the front end I don't think but anyway somewhere in here and that has a noise figure of up for nano volts per root Hertz so let's give it a bill and there we go yep still picking up our 50 Hertz but once again we haven't gone off scale here now and there we go we're not we're almost exactly the same noise floor as we got with the instrument itself what was it 31 0.3 nano volts per root Hertz if we wait till it goes up there we're only a couple of nano volts above that so bingo there you go there's a good quality op amp for you there it is 33 point seven for the record beautiful catch you next time you you

Channel: EEVblog

Views: 123,885

Keywords: Operational Amplifier, opamp, opamp noise, voltage noise, current noise, 1/f noise, flicker noise, Gaussian noise, probability, 3 sigma, rms noise, tutorial, how to, dsa, dynamic signal anayser, 35660a, agilent, Hewlett-Packard (Organization), input noise, noise floor, shielding, noise measurement

Id: Y0jkPLuFdnM

Length: 40min 0sec (2400 seconds)

Published: Fri Sep 27 2013