EEVblog #1328 - uCurrent OPA189 Measurements

hi in a previous video linkedin down below and up here and at the end if you haven't seen it uh this is part of the series of designing a better microcurrent and the last video we took a look at the opa-189 and how it looked like in most respects it was better than the max 42 38 42 39 that we use in the micro current so i ordered some here you go opa 189 id bvr this is the sot 23 5 package it is available in different packages but i got the sop 23 5 because it is actually directly pin out compatible with my existing micro current so what we're going to do in this video is we're going to actually compare the original design microcurrent to one that uses the uh drop-in replacement opa-189 and measure the uh noise and other performance characteristics of it let's go but it's actually not completely pin out uh compatible because here's the existing max 42 39 it's actually a six pin so you can see it's got three pins on the top here but the opa 189 only has two pins on the top it's a five pin slot 23 package the only thing that's missing is the enable disable pin which we don't actually use on this we just tie it high so we can simply um simply remove the existing chips and solder in the new one doesn't matter beauty so to make this comparison as fair as possible what i've got here is two identical boards from uh actually the same batch just a serial number apart so they use all the same parts that you know come from the same reels and everything so they're basically as identical as you can get but one uses the 189 one uses the original max 4239 now the problem is uh though i can't use the coin cell battery to do the comparison test because the opa-189 uh has 4.5 volts minimum uh supply voltage whereas the uh max 4239 can go down much lower like 2.3 volts or something if i recall correctly is the dropout voltage something like that anyway um but it only goes to 5.5 volts maximum so i'm going to run this from a four and a half volt battery so just enough i've got uh three triple a's here which will give us if we use brand new cells it'll give us just over uh four and a half volts so yeah that'll be good enough for australia right so we're going to start out by measuring the noise now as i said a previous video linked in down below where i've actually uh really quite a while ago this video is where i use my uh hp 35060a dynamic signal analyzer to actually measure uh noise floor of op amps and that's a really great video remember i think it's from about like 20 15 20 minutes onwards in the video is where i show you extensively how to set up a dynamic civilian analyzer to measure noise to match the data sheet noise density actually which is nano volts per uh root hertz uh now to do this of course um you can't just have the boards flapping around in the breeze on your bench you've got to have a nice die cast shielded box like this so i've got that with a b and c output going into our input so we'll put the batteries and everything else inside the diecast box just so we eliminate any external noise any uh you know external 50 hertz all right first thing we're going to do is just measure the uh power supply current because the opa 189 is supposed to take more current yes so let's get the old one the max 4239 i've actually removed the uh dropper resistor for the lead here otherwise the little draw like you know seven milliamps or you know a huge amount of current like that so let's give it a bill this is the max 4239 1.31 milliamps and the 189 oh two point six eight so you know what what one point three milliamps more overall for uh the two op amps and you know there's some residual uh you know other current which will remain the uh same but yeah you know that's it's neither here nor there really unless you're after some ultra low power design so nice now i know that we only have a sample size of one here so take that with a grain of salt but let's just uh see it'll at least tell us if anything's grossly wrong with the offset voltage so i've got the max 4239 i've got it set to uh the 10 milli ohm shunt so it's effectively uh shorted and we're getting about 61 microvolts so they're about offset and that's you know fairly typical for a uh max 4239 mic based micro current and the opa 189 well there you go um 73 micro volts meh neither here nor there good enough for australia 67 look at that oh it's going down it's going down dropping that's plummeting there you go just turn filtering off there filtering back on so yeah um there you go like 53 you know microvolts like in the order of 50 micro volts so yeah um that's fine and we didn't really expect a huge amount of difference because as we saw like on average uh based on the bin in it's basically equivalent to the max 4239 but it does have slightly wider offset margins but as i said sample size of ones but still hey and that's really shouldn't change with our range so that's our it's our next range and our nano amp range whoa and then our range has jumped up but that's because the input's not shorted let me short the input there we go it's back down to 50 micro volts nice and just to check the uh ranges make sure the accuracy is there it's still not you know mucking up uh we're generating one micro amp here with my keithley 2400 source measure unit and we're getting our point 999 eight yeah that's good enough um like i haven't like properly calibrated any of this so like and the max 4239 uh 0.992 so there you go that's more out than the uh 189 is but hey it's to do with all the other stuff involved all the other range resistors everything else and max 4239 generating one milliamp on the milliamp range down or the micro ampere range down there um yeah 0.9999 and the opa 189 1.0011 um no worries that's like you know uh 0.01 percent beautiful generating 1 amp here opa 189 we're getting 1.0023 i know that's like 0.23 but i've had an issue with uh the range resistor um on here and that's what i'm yielding at the moment so anyway um we should expect uh very similar with the with this one as well and the 4239 board 1.0014 look don't worry about these values it's to do with uh the precision of the uh 10 milliohm uh current shunt down here as i said i've been having like batch issues with that so yeah it's all fine i just wanted to show that it's all working and i assume it's working but just want to double check uh to make sure that the split rail is uh working just fine 2.3 and minus 2.3 no worries okay so let's measure the noise first i'm going to just measure the baseline noise of the system here as i've done in a previous video but i'm going to include the box and everything else so we'll whack our lid on there and we need to set it up now this is not trivial now i've covered this in more depth in the previous video but we'll just go over the basics here we're going to i go over the full span here zero hertz to 100 kilohertz here it's 102 but that's to do with the bin in and everything else now of course what we want here is nano volts per root hertz or power spectral density which is uh what is the value which is included in the data sheet and uh by default um this is set up for you know db volts rms that's not what we want that's not a power spectral density so the first thing we have to do is go into measurement data over here and you can see that we're just getting the normal spectrum but we want the psd or power spectral density because if we don't choose that and we just use regular spectrum if we go to scale over here and then we go into our vertical units you'll notice that there is no power spectral density there's no nano volts per nothing per root hertz which is noise density so we have to go into measurement type here choose power spectral density and bingo volts rms per roots hert but we don't want the rms part here so we go back into the scale we go into our vertical units and bingo volts per root hertz we now have that or nanovolts per root hertz but it's it's the same thing it's volts per root hertz it just happens to be nano so we can do that and now we've actually set up our things okay the next thing we want to do is that we want to go into the input type here and we want to do channel one setup and we actually want uh grounded input we don't want it floating i won't go into the difference between floating measurements and we also want our dc as well don't worry about the units there that's just uh the engineering units so we want ground and we should see yep that noise floor drop nice next we want to go into scale over here and we want to auto scale like that so now we get a resemblance of a noise floor and we're looking at well it's jumping around but we're in uh like you know tens of e to the minus nine which is nano volts per root hertz and of course you know you want to clean up all this uh waveform so you want to go over here turn on some average in averaging on how many averages we got we've got 10. let's go to 100. there we go now we can actually start that and it will give us a well it'll average down to a nice waveform so that's actually significantly different to what we got with just the bnc 50 ohm input so let me actually show you that let's restart that there you go that's much nicer and that's what we'll get in in the uh previous video you know around like 30 nano volts uh per root hertz floor but unfortunately um you know just our measurements set up here this is just i don't know what is it iu 58 or something um the coax and the box and everything else not going to be perfect but you can see plug this back in there is a significant difference between the noise floors there so we've got some peaky stuff happening up here i'm not sure why it's tailing up here but hey that's what we've got to work with that's fine because you've got to remember we've got times 100 gain on here so the noise floor of our op amps is going to be multiplied by a hundred so you know we aren't going to be down here we're going to be you know much significantly higher than this so it's neither here nor there and we probably want that on a log axis as well so we're going to scale and log thank you very much there we go that's better let's do that again and as i mentioned in the previous video uh we're only going to get because of our 400 lines of resolution on this thing uh we're going to get large steps like that we're not going to be accurately able to measure over the 400 kilohertz uh bandwidth but hey let's actually run with that so right so just remember that that's our baseline uh noise floor for our setup here okay so let's put in the 4239 first i'll hook up the output okay so i'll put it on the uh 10 milli ohm uh shunt range so it's effectively you know shorted on the input and let's whack that in there and yeah my battery's on and of course it's not just going to be the noise of the op amps because we've got the split rail thing that could introduce noise and you know we're just measuring the whole system here so i'm not measuring the actual individual op amp i'd have to do a you know like a proper jig like i did in the uh previous video to do that i'm more interested in in the actual current micro current application current micro current application yeah i said that right all right so let's not change anything let's start that again and so you know 44 nano volts per root hertz around about there what's that cursor at don't know whoa there we go there we go it's jumped up six micro volts per root hertz but let's get it you know sort of at one of its like lowest points like that there you go you know three and a half micro volts per root hertz let's call it like that so you know that that's what we get over here now you might be wondering what is that spike in there well i can tell you what that'll be around about 15 kilohertz would be my guess 13 13.3 why did i know that was the uh that it'd be around about that frequency because that is the chopper frequency because these are auto zero amplifiers just call them chopper amster is the difference between auto zero and chopper but we won't go into that anyway it's an auto zero amplifier that has a chopping frequency and if you read the max 4239 data sheet it tells you that's typically it is like a spread spectrum kind of thing it's not one fixed frequency it did they add some dithering uh to it but you know it's around about 13 kilohertz and that's exactly what we're seeing there and if you actually sweep uh the microcurrent you can actually see a little bit of low amplitude funny business going on uh you know at around about that 13 to 15 kilohertz mark or whatever that's you know that's a loosey-goosey spec it's not going to be exactly that but that's a roundabout uh the frequency that we're measuring there so there you go so yeah i'm going to call that like maybe three and a half micro volts per root hertz okay let's measure the opa 189 okay the opa189 got to make sure trap for young players that none of this shorts out so okay so we'll whack that in there and once again i've got it on the uh 10 milli ohm current shunt let's run that again shall we so 3.5 micro volts ah it's lower oh our ref what dbm per hertz what did i touch what did i touch okay let's try that again don't here we go now we're talking 800 900 yeah we're talking yeah it's it's lower noise so we went from three and a half micro volts uh per root hertz to around about one mic let's call it one micro volts per root hertz so yeah the max 4239 in this implementation of the microcurrent is about three and a half times worse noise than the opa-189 so yeah it certainly is quieter and the switching frequency the opa 189 that i don't think they tell you exactly but it's up in the like somebody said it's something like the hundreds of kilohertz range they measured onto something unfortunately we can only go to 100 kilohertz with this um and you can see a little something in there but i don't think that's it because somebody else said they've that they've measured it in a project and it was in the couple it was in the hundreds of like 250 kilohertz or something so 25 kilohertz that doesn't sound right so there you go um that is a uh our scale has actually changed a little bit here but our input uh range i believe is our range the same i have to double check that damn it there we go i just set it back to exactly the same range 10 micro volts are per root hertz at the top there and uh you can see yep that one micro volt per root hertz let's call it so one micro volt per root hertz versus 3.5 micro volts per root hertz winner win a chicken dinner for the opa189 nice even right at the top there it's only one and a half so it's still twice as good okay let's just measure the noise on the scope here i've got a 20 megahertz band with limited times one input with the uh coax coming out of the uh shielded box and uh peak to peak we're not that concerned about we're two uh millivolts per division because the sequence has a 500 micro volt uh per division front end but let's just leave it on uh that scale and you know 7.4 millivolts peak to peak but we're more concerned about the rms noise here 940 odd micro volts so that's for the opa 189 if you want to see the noise floor when the microcurrents are disconnected there you go so let's just reset those stats and that's the noise floor with the uh microcurrent completely disconnected inside the shielded box so we're only down around 90 micro volts rms there reset the statistics max 4239 uh see it is uh significantly more uh 12 and a half millivolts peak to peak and 1.6 millivolt some rms noise so there you go it's oh no it's like 50 worse or something like that than the opa-189 so significant uh noise improvements just using the opa-189 in the standard microcurrent circuit configuration nice okay i'm just curious to see um how much noise was actually generated by the uh 200k resistors there and the split rail uh generator basically because we are we have seen noise greater than what you'd expect for the uh baseline noise of the opa189 uh plus the time or multiplied by the times 100 gain so we're going to get some i mean the noise of course you're going to get your thermal johnson noise on your resistors but they are not actually directly coupled into the input as such so you're talking about through the power rails common mode rejection ratio all that sort of stuff anyway i've left the resistors in there because it's a nothing burger but i've removed the lmv 321 op amp because unfortunately you can't just whacking these opa189s in here and a higher voltage you get it to work because the lmv uh 321 op amp that only has a maximum supply rail of like five and a half volts as an a version which i think goes to six or something but yeah basically um yeah you can't just do that you'll have to get another op amp if you want to get uh like operate this thing from say a nine volt battery or something anyway um yeah i've done that and now i'm gonna uh power it from split rails here and we'll see how it goes so it's slightly higher voltage i've got six volts now but yeah that's neither here nor there if we go back to the scope reset our stats there you go uh peak-to-peak what we're getting before i think it has slightly dropped 910 when we getting like 980 or something uh mean rms before so it has dropped somewhat okay i can't remember exactly where we were before on the frequency but it has dropped a little bit in the uh oh it does its auto calibration there periodically so we're a bit under where we were before but uh look there's not much in it um it should maybe it'll go under one here but yeah it's you know there's not a huge amount in it so basically uh the op amp and those resistors weren't really contributing much to that which is as you'd expect because as i said it goes through the power rail system and then its effect on the op amp itself has to do with the power supply rejection ratio so it's not actually coupled into the input as such it because even though it's a float like it's introducing noise into the ground reference the ground reference is still the reference so even if the reference is jiggling around due to the noise it's you know it's not introducing much so there you go that's doing some basic tests on the opa-189 op-amp and as expected i it was you know relatively significantly lower noise than the max 4239 an actual worthwhile upgrade for not much real change in power consumption really and it is effectively a drop in replacement for the micro current although as i said unfortunately that lmv 321 op amp that one will have to be changed to one that supports a higher uh rail voltage so uh offhand i don't know just a regular lm321 um might do the job anyway i need to investigate that but if you've got a microcurrent um and you want to like have a drop in uh replacement for lower noise um and higher bandwidth this looks like a quite a decent option now of course i haven't actually checked the bandwidth yet or rather uh dynamic performance aspects of it um this video is long enough so i just want to measure the noise make sure the op amps work make sure the microcurrent gold like still works it works at dc and its noise is lower and ah everyone's happy so i think we have a pretty decent new candidate and i don't expect any show stoppers in the dynamic performance aspect of this in fact um it could be potentially even better because as we saw the actual chopper frequency is up you know past 200 kilohertz or something like that so in theory you could put a low pass filter on the output as well even mod existing micro current or in a new design micro current which we're working on here you can put in maybe an optional switched output filter so that the performance of it is completely below the uh chopping frequency of the zero drift amplifiers in here because unfortunately the micro current it was uh that you know like 13 to 15 kilohertz or so it's pretty much you know bang on in like the measurement range that you're uh trying to do so even if you didn't want the increased uh bandwidth you could still get the same bandwidth as the existing microcurrent goal but actually put in a low pass filter on the output and have that chopper frequency outside the operational range so right there another big benefit so yeah i'm i'm liking the look of this uh opa 189 it's pretty schmick so anyway hope you enjoyed that found it interesting if you did please give it a big thumbs up and let me know down below how you uh liking this new series in quote marks um which will be just uh random videos going forth it's not some official design project design series it's just i'm doing you know the occasional rando video so this is part three and i'm sure there'll be more parts especially like another one dynamic performance or something so let me know if you want to see that in the comments down below and as always don't forget all my alternative platforms i'm on library i'm on bit shoot i'm on dailymotion i'm on video i'm on my own website you can even download the 720p podcast from my own uh web server on my rss feed and all that sort of stuff so yep there's plenty of alternatives outside of youtube and as always we want to discuss eev blog forum is the place to do it i you know the comments do but forums are better than comments anyway catch you next time [Music] you

Channel: EEVblog

Views: 30,363

Keywords: eevblog, video, dsa, dynamic signal analyser, opa189, ucurrent, measurement, noise measurement, how to measure noise, oscilloscope, siglent oscilloscope, operational amplifier, opamp, op amp noise measurement, opamp noise, bandwidth

Id: UVjLoJMVL0s

Length: 23min 4sec (1384 seconds)

Published: Wed Aug 12 2020