Hi I thought I'd Show you a little trap for young players here about how the input resistance of your multimeter can really have an effect and really matter in some specific circumstances. Now what I've got here is my little microcurrent. uh, test jig for my microcurrent. Just plugs on the top here like this and I can plug a no one current into here and it's just got a window comparator as we'll see past fail results.

It's got some trim pots to allow me to trim the exact uh specification pass fail specification that I need. So let's take a look at it, take some actual measurements, and see what we get. So what I've got here for the circuit is a Precision voltage reference here LTC 65 precise 1.25 volts like 0.025% Accurate. Really quite nice.

And uh, then I've just got a resistive divider down here like this and what this is doing is just generating two Precision reference voltages ref plus and and ref minus here. So that's all I'm doing and it's basically I'm going to try and set them today to Plus .25% or 1 volt, 0.025% above 1 volt which is 1.025 volts and 0.025% Below 1 volts. There you go. So we're trying to set some really precise values here and to do that, of course, we're going to use our 6 and 1/2 digit uh, you know, really? Uh, topof the line agilant mm to measure our voltage across our two pots and you'll notice very low impedance in here I Mean we're only talking uh, 10 Ohms here and we're talking a 2K pot across that 10 Ohm resistor.

Why you put a higher value pot across a lower value resistor like this? Basically stability. Because I we're generating a very basically this Center tap here is going to be 1 volt. Uh, I've chosen the divider resistors to actually give that from our 1.25 volt reference and then we're uh, using the low value 10 Ohms in here to give us a precise value above that. and then we're using a higher value pot.

In this case 2K across that. the reason why you wouldn't put a very low value pot in there is because the wiper resistance is really going to have an effect on the stability of that. So it's much better design practice to put a higher value pot across a lower value resistance in this case. Anyway, that's just a little aside.

we're going to generate and measure two precise voltages here. and then we're going to test it and see if it all works. So there, these Uh two pots here, which we're going to adjust the two trim pots on there and we'll measure the wiper voltage on there. Now this, the wiper voltage for these two.

Uh, the two Precision references Uh voltages go into our standard window comparator here or window detector reference voltage plus reference voltage minus and basically the lead comes on if the input value coming from the banana plug down here. the input banana Plug: We got uh, ground and banana plug input which we can feed a precise DC voltage in to check it. If that's with inside the margin of these two values I.E it's within spec then the inspec lead here will light up. That's the plan.

let's give it a go. So here we go: I've actually adjusted that trim pot I'm measuring the Uh wiper voltage on the positive side here and I've set it for 1.025 and you'll notice that uh I'll just show you how I can actually tweak that Here we go see, we can actually tweak that pretty well so we'll get that right up to 0.25 Near enough. There we go. Fantastic! And likewise with the other pot down here 975 and you'll notice that we can pretty much find adjust that sucker as well.

This isn't a proper screwdriver for doing this. You need one of those proper captive adjustment uh, screwdrivers and you can see that's uh, pretty stable too by the way. Yes, there will be some uh, temperature coefficient and stuff like that little bit of uh, drift with temperature, but you know, really, nothing serious here. Um, unless we're talking huge temperature swings so everything should stay pretty stable when we disconnect this.

Now let's feed in an external voltage into here and see if we can, uh, get within that pass fail window. It should work all right. So what I'm doing now is I've got my Uh konight EDC Precision voltage standard here and I've dialed up one volt precisely on it. sorry I haven't actually let it warm up at all I've just switched it on anyway.

I've got it. uh, measuring. I've got our agilant meter here measuring the output as well so that's what we'll actually go by. We won't go by the exact dials on here yet cuz it needs to stabilize.

but we're going by the agilant meter which we set it to before so we're using the same instrument and it should be within spec. We should and look there it is is. Tada Our green lead has come on with Point Well, let's let's tweak that up, Shall we? There you go? That's pretty close There we go. We got our 1 VT and our inspec lead is on so everything's working fine.

now. let's test this top voltage reference. So this ledge should switch off at 1.025 volts. Does it 25? No.

Look, it's still on Three, Three eight. What's going on? The lead is still on. Look, it doesn't turn off until basically look 47 48 Something like that. So look, it's almost doubled what's going on.

We set that voltage on that pot to precisely 1.025 Vols. So let's test the low side now and see what we get. Remember, we set it to Tri 975 so the Leed should stay on for anything above that value. it should go off for below it.

So let's go to8 No look, it switched off. The lead has switched off. at 986. That's higher than our reference value there of 975.

So what's going on? What value does it actually switch off at 9926? Look at that crazy on off look that's just unbelievable. There is something wrong with the either our comparative circuit or our uh, triming of those resistors cuz nothing has drifted in the meantime. Trust me, the temperature hasn't changed, so that's quite strange. We're feeding in our Precision 1vt on the uh, well, our our adjustable voltage from our reference standard on the banana plug.

here. the input the center input to the window detector. here. We've precisely set our reference value values here plus minus of 0.025% But then when we input the Uh value here, it doesn't seem to match the lead, doesn't match those values.

And you might be thinking, aha, the offset voltage of the Op amp. You got to have really precise upams here. Well, I'm using an Opa 2376 and it's got a nominal a typical offset value of only five microvolts and 5 microv volts in 1 volt isle. 05% at that 1.

So really, you know, um, it's not causing an issue before we're we're not causing an issue at all cuz we're looking at 0.025% this is 05% So the error of that opamp is not contributing, so it must be something else. And no, the input impedance of these opamps isn't going to matter cuz these are really precise fed input opamps. We're only talking like pico amps into. uh, the input values here.

so the loading on there doesn't matter. Although it even if it was much higher, it wouldn't matter anyway because we were measuring the precise value on there, which would have compensated for any input bias, current loading on that trim pot. And if you want to try and figure it out for yourself, it's on the screen there. So stop the video, try and figure out what's going on and we'll find out in a second.

and I Hope you figured it out and you should, because it's the title of the video. This um, Agilant, Uh 34 61a has a Um High impedance input mode, but currently it is set by default to the standard 10m input impedance there. Aha, but hey, 10 Meg should that matter? I Mean take a look at this circuit, right? We've only got 2K on here. What is 10 Meg going to do on that? 2K You might think it's many orders of magnitude above.

It shouldn't have an effect at all. Most multimeters have a typical digital multimeters have a typical input impedance of 10 mohms and really on 2K That shouldn't do much at all. And if you actually do the math and figure out what 10 Meg uh, you know can how it can affect the 2K It depends where the wiper pot value is and all that sort of jazz, right? But we're only talking about 0.02% Well, you know, on a Precision circuit like this, that can really matter. But aha, we the 10 Meg resistor is not across that pot.

there. it's from this reference point, the wiper of the pot down to G. Like this, and look, we've got an extra 10K in there. so that 10 Meg is really going to upset this entire voltage divider thing.

here. You can't do it when you're down at this sort of precision level. That 10 Meg really matters even though it's in a couple of orders of magnitude more than the impedance of the circuit you're trying to measure. And I can show you the effect of that.

Live by using another multimeter putting it in parallel with this uh, uh, Agilant one. I've actually got it. Uh, hooked up back to the Uh pod again there. So let's actually, uh, probe this in parallel.

We'll measure the same voltage of course, but let's see if it changes TDA Look at that. it's dropped just by putting another 10 Meg in parallel. It's affecting our reading because the impedance of that circuit is now changed, so our voltage divider ratio has changed. So any good Precision multimeter will have a high impedance mode.

Sometimes on, uh, like handheld multimeters, it's only on the Molt range. For example, you may not get it on the voltage range. this uh agilant, uh meter, uh, 6 and 1/2 digits doesn't do it on some of the higher voltage ranges like the hundreds of volts. but I think up to the 10vt range.

It allows you to have uh, effectively infinite input impedance and it says Auto here, but impedance? Let's change it and look the value Has Changed By going from our 10 Meg input impedance to effectively infinite, it says in the data sheet I think it's like greater than 2 gig or something like that. But effectively, it's just the input, uh, effective input resistance of like, uh, the Fet input amplifier. That's pretty much, um, all it is. So it's now changed.

so you can see that the adjustments we did before were completely wrong because we forgot to use and a multimeter that had a high input impedance mode 10 Meg is high, Yes, But for precision circuits like this, Nope, it ain't So we'll adjust this again in our high impedance mode 75. Is it? Yep, there we go. That's pretty spot on. We'll do the same for the positive side, and there we go.

We'll tweak that down to 25. That's pretty darn spot on, all right. I've hooked it back up, feeding the output voltage from my Uh generator here into the BNC inputs and also measuring that with the meter here. Um, once again, because this is a low impedance Source Now from my Uh generator, it doesn't matter.

Look uh, whether I'm using 10 Meg or whether or not I switch over to high impedance. Makes no difference whatsoever because we're at low impedance source as opposed to the high heent Source we had in our circuit with the pots down in there. Anyway, let's have a look to see9. We expect it to go out and we it's just under.

So the technically in theory the ledge should be out. but look I mean there it is. It's just on the border. It's just flickering a bit.

but there we go. That's pretty darn close to what we programmed in 975. In fact, our the offset voltage. as I said of our Op Amp down here, the 5 microvolts could be starting to come into play there cuz it may actually be higher than 5 microvolts.

eh? could be, you know, slightly off or something like that. But anyway, that's pretty darn close. Let's try it. for the uh, positive side.

There we go. Our lead is still on at 024. We expect it to turn off at triple 025. In theory there's going to be some no just over, but we go up one.

Oh, it's just starting to flicker. There's a tiny bit of noise on there and bang, it's gone. So there you have it. We're now spot on, so that can be a real trap for young players.

Just keep it in mind next time you're well measuring anything. Really. Is the 10m input resistance of my meter affecting my measurements? At first glance, you may not have thought so. with the lousy, you know very low impedances here 2K things like that.

But as you can see when we're talking about precise settings like this, it really made a dramatic difference. So that's a real practical example where you can come a Guta by not being aware of what effect the 10 typical 10m input impedance of your multimeter is going to have. So any good lab should have a multimeter that has a high impedance voltage mode just for doing these sorts of precision measurements. Where that 10 Meg can matter if you had a very high impedance.

uh, Source circuit. Here, you know, tens of K like we had here 10K hundreds of K even. you know, up in the Megs voltage divider for something incredibly low power or something like that, then wo your 10 Meg is not going to be good enough. and in some circumstances, really, really high impedance circuits.

Even the infinite in effectively infinite input impedance of your meter can actually matter cuz it's not going to be infinite, there's going to be something there. There's going to be some charge on the Gate of that input fet or something like that. But anyway, that's for another time and that's for really really Niche applications. But even for General ones your 10m input impedance on there can really matter.

Keep a watch out for it next time. Hope you enjoyed the video and if you did, please give it a big thumbs up. And if you want to discuss it, jump on over to the Eev blog. Forum The links are down below.

Catch you next time.