Tip:
Highlight text to annotate it
X
Hi, my name is Mike, and I work for Agilent Technologies. I'm an Application Support Engineer.
This short video will describe how the AUTOZERO function is used to eliminate offset voltages
in your DMM. In our first slide here we've got a partial block diagram of the 34401A DMM,
and I want to use this to show where the offset errors are in your DMM. The first section
of the block diagram, No. 1, is where we switch between the front and rear terminals.
Section No. 2 is where we actually change the direction of the signal path depending upon
whether you're measuring DC volts, AC volts, resistance or current.
If you happen to choose AC volts, you'll get switched into Section No. 4 where we have
our amplifier and RMS converter. Or you may pass into Section No. 3 where we have our
current sources for ohms, DC amplifier. And finally, we get into Section No. 5 which is
the voltage reference and our analog to digital converter. That's where the signal is converted
from an analog signal into digital information and then it's passed into our logic.
The area highlighted in blue is where we can potentially have offset errors in our DMM.
These offset errors can be created by bias currents that are going when we have the instrument
powered up. Bias currents all over the amplifiers and circuitry will go through small resistances
that develop very small voltages and these are offset voltages that we need to try to
eliminate if possible. Additionally, we have thermal EMF or thermal electric voltages and
these create errors whenever there are dissimilar metals that are connected, and this is a
function of temperature.
So as we warm up the DMM, the temperature is going to be changing so these thermal EMF
errors, even though they're very small, in the microvolts, they will be changing and
therefore the offset errors will be changing as our instrument warms up. So we want to be
able to eliminate these errors because they degrade the performance of the DMM.
The AUTOZERO function allows us to do this. First let's take a look at that section that we described
previously in the block diagram where the front and rear terminals come in.
Not only do we switch between those terminals, we also have a shorting relay in here, and
what AUTOZERO basically does is, it will make a measurement that you have asked for from
either the front or the rear terminals, and then it will disconnect those terminals,
it will make a short internal to the DMM, and it will short out the whole section,
Subsection No. 2, 3, 4, 5 so that we can eliminate all of those offset errors, and it'll make another
measurement. And then that measurement is always subtracted from the measurement that's
made from the front or rear terminals, eliminating all those offsets in that circuitry.
So this is the AUTOZERO function. We have three choices. The first is AUTOZERO on, and
this is the default, the most accurate, and this will make a zero reading for every input
reading that is made. So you're basically making two readings -- the input that
you're asking for, and then an internal zero measurement that is subtracted.
The second choice is AUTOZERO off. If you do not want to make two measurements, you've got a fairly stable thermal
environment for your DMM. Then you can choose AUTOZERO off, and it will only use the previous
measured zero reading and subtract it.
So the subtraction still happens but the measurement is not made unless you change the function,
the range, or the integration time of your DMM. The third alternative is AUTOZERO once.
This is very similar to AUTOZERO off except for when you change that function, range, or
integration time, it will make sure that it's making its zero measurement with one power
line cycle to get good noise rejection for its zero reading, and then it will return
to the previous specified integration time. Very similar to AUTOZERO off.
So there's a trade-off here that you have to realize when you're using your DMM.
If you want to take full capability of the AUTOZERO function and use AUTOZERO on, you'll actually
be making two measurements for every one that you want to make; an external one, as well
as the AUTOZERO or the internal zeroing measurement. This will limit the DMM reading rate, and
you may not want this situation.
In that case, a good compromise is to use either AUTOZERO off or once, and I would suggest
using AUTOZERO once since it makes a solid zero reading using integration time of one
power line cycle. So this concludes this short video of how our AUTOZERO function is used
to eliminate offset errors in the DMM.