Hi, my name is Mike Hawes, and I'm an Application Support Engineer for Agilent Technologies.
In this short video I'd like to describe the NULL function and how that works with our
DMMs to eliminate the lead resistance that we get when measuring with a two-wire ohm
resistance. So let's move to the whiteboard and I can diagram how that works.
The diagram on the whiteboard shows how a DMM makes a simple resistance measurement
using the current source and the internal volt meter. If you're not familiar with this
technique, please see one of the other DMM basics videos entitled "How Agilent DMMs Measure
Resistance." To use the NULL function, we primarily want to do this when there are two
things that are true. The unknown resistance is about the same value that our lead resistance is,
or it's close, and we want to cancel the lead resistance and only be measuring
the unknown resistance.
The other situation is that this lead resistance must be consistent. We're counting on it being
a constant value. So the way the NULL function works is when using the DMM, you remove the
unknown resistance, you short the two leads together, and then you push the NULL button,
and that actually makes the resistance measurements that includes the lead resistance of both leads,
and that is remembered or retained in memory in the DMM so that when you reattach
the unknown resistor, and make measurements, these lead resistances are subtracted from
the calculated resistance.
So again, using the NULL function, the unknown resistor is actually going to be the calculated
resistance minus this NULL function. In our case, it’s two times the lead resistance.
So you've eliminated the lead resistance and have a more accurate measurement.
Let's use the DMM and see how this is actually done on a DMM.
In this short demonstration we're using Agilent 34410 DMM, and we have it on just the default
power-up volts mode with the high and the low lead shorted with just a paperclip.
So you can see in voltage mode it's reading a short. I'm going to go ahead and push the
two-wire ohm mode, and you can see that we're getting approximately a tenth of an ohm,
a hundred milliohms, but with just a paperclip you know that most of the resistance has probably
dropped across leads. So let's use the NULL function. I'm going ahead to disconnect the
paperclip, and I'm going to short the two leads together. So they're shorted. And you
can see there's only about 73-74 ohms of this 100 milliohms are accounted for in the leads.
So when I push the NULL button, you can see now it is nulling out and subtracting that
73-74 milliohms and we're getting pretty much zero ohms. So we can now un-short, and we
go overload because we're not connected to anything, but I'm putting the paperclip now
on the leads again, so now we have that paperclip re-established, and you can see that the actual
resistance of that paperclip is about 11 or 12 milliohms, so this is a nice way of using
the NULL function to eliminate the leads.
If we want to stop using the NULL function, I can just toggle the NULL function again,
and you can see now the value is back up, not quite up to the 100 milliohms, but it's
up in the high 80s, 88-89 milliohms. So this concludes the demonstration of how to use
the NULL function for Agilent's DMMs.