Gotcha!

I’ve experienced a problem recently whilst measuring high voltages with a 1000:1 high voltage probe and an Agilent (U1253B)/Fluke (28II) multimeter.  The problem is that the two meters don’t agree with each other!
The Fluke (my meter of choice) returns voltage readings within expectation all the way up to 12kV (12V as displayed on the instrument).  But the Agilent meter only seems to agree with the Fluke up to about 3kV, after which it starts to drop off.  By the time we get to 12kV the Agilent is reporting a voltage that is more than 1000V less than expectation.  I tried another Agilent U1253B and experienced the same drop off. What is going on here?

Know your input impedance

So I started to think about input impedance.  The high voltage probe is designed to work with a 10MΩ impedance.  Both these high-spec handhelds will be 10MΩ, right? That’s standard for handhelds these days.  Is this a fair assumption?
It turns out, no it isn’t!!!

Firstly, RTFM.  Both the Agilent and Fluke claim 10MΩ input impedance for the D.C. voltage range in their manuals.  However, the Agilent has a fancy dual display mode whereby you can measure two different properties (say, A.C. and D.C. voltage) simultaneously.  In this mode each display presents as a 10MΩ impedance so you end up with an effective impedance of 5MΩ in total.

…but I wasn’t using the dual display mode, so I should expect 10MΩ, right? Well, that’s what the manual says.  But let’s measure it!

Measure the Agilent’s Single Display Input Impedance Using the Fluke

Firstly we connect the Fluke up to the Agilent and take a resistance measurement of its inputs.  We should expect to see 10MΩ, and sure enough we do:

Measuring the Agilent’s single display input impedance using the Fluke.

Measure the Agilent’s Dual Display Input Impedance Using the Fluke

Next we set the Agilent to dual display mode and take the measurement again.  We should see 5MΩ, right? Yes! So far so good…

Measuring the Agilent’s dual display input impedance using the Fluke.

So far we seem to be doing well.  The Agilent’s input impedance is as expected.

But there’s a problem…

There’s more than one way to measure the Agilent’s input impedance.  We can use an insulation tester.  The difference is that the Fluke is applying a constant current and then using the measured voltage drop to calculate resistance, where as an insulation tester does it the other way around – it applies a constant voltage and, I presume, uses a measured current to calculate the resistance.  It’s six of one and half a dozen of the other – both types of measurement should agree with each other.  But do they? Let’s find out:

Measure the Agilent’s Input Impedance Using the Insulation Tester

Here we connect the Agilent up to the Insulation Tester.  I tried it at various test voltages and they all agreed with each other, but there’s a surprise in store – the insulation tester reports 5MΩ input impedance for the Agilent’s voltage measurement range.  And this measurement is reported regardless of whether the single or dual display mode is used!

Measuring the Agilent’s Single Display Input Impedance Using the Insulation Tester

What is going on here? Why is the insulation tester reporting 5MΩ input impedance for the Agilent’s single display mode? And could this explain my measurement problems with the high voltage probe? I think it could! But in that case, what can we say about the Fluke’s input impedance? Let’s measure it, first with the Agilent and then with the insulation tester:

Measure the Fluke’s Voltage Range Input Impedance Using the Agilent

Okay so we connect the Fluke up to the Agilent and measure its input impedance using the Agilent’s resistance range.  We get 10MΩ as expected:

Measure the Fluke’s voltage range input impedance using the Agilent.

Measure the Fluke’s Voltage Range Input Impedance Using the Insulation Tester

Now we measure the Fluke’s input impedance using the insulation tester.  We should get 10MΩ:

Indeed we do get 10MΩ.  So, to summarise:

• The Agilent’s single display input impedance measures 10MΩ using the Fluke’s resistance measurement, but the insulation tester says it’s only 5MΩ – and this is regardless of the display mode – both the single and dual modes look like 5MΩ to the insulation tester.
• The Fluke, on the other hand, looks like a 10MΩ impedance to both the Agilent multimeter and the insulation tester.  This is what you would expect.
• I tried the measurements again with another Agilent U1253B and I experienced the same thing.  I also experienced the same voltage measurement problems when using the high voltage probe.  This rules out a faulty instrument.

So what is going on?

This is a very good question! Why does the Agilent look like a 5MΩ impedance to the insulation tester? Why is it not 10MΩ as stated in the manual? And why is there a discrepancy between the insulation tester measurement and the multimeter measurement? This discrepancy isn’t seen when we measure the Fluke.

This input impedance problem provides an explanation for the voltage measurement errors I’ve experienced.  The high voltage probe I’m using is designed to work with a 10MΩ multimeter, so a lower impedance instrument is going to present a problem.  This is what I’ve experienced in practice.  The Fluke, on the other hand, works with the high voltage probe no problems at all.

Misleading Impedance Specifications

Here’s a copy of the input impedance specifications from the Agilent U1253B user manual:

As you can see, they are quoting 10MΩ for each VDC measurement range, from 5V to 1000V.  However, there’s a problem with this! Refer to note 3 in the fine print below the table.  That’s right – the input impedance actually varies with input voltage! So, even though they quote 10MΩ input impedance, it’s actually only 10MΩ for input voltages between -2V and +3V! Outside of that it’s only 5MΩ.

To put that in perspective, -2V to +3V is less than 0.3% of the instrument’s total range.  So for 99.7% of its range, the impedance is only 5MΩ.  Despite this, they somehow think it’s informative to quote the input impedance as being 10MΩ.  That’s a bit bizarre.

Anyway, this fact explains why the insulation tester and the multimeter disagreed over the input impedance.  The multimeter’s constant current stimuli yields a voltage that is <1.5V so it comes in on the 10MΩ impedance zone.  The insulation tester’s minimum voltage stimuli of +50V is well into the 5MΩ impedance zone.
Also, the fact that the instrument has 5MΩ impedance above +3V explains why it starts disagreeing with my Fluke after about 3kV.  The high voltage probe is designed to work with a 10MΩ impedance so as soon as the Agilent’s impedance changes over to 5MΩ, erronous measurements are returned.

The Moral of the Story Is:

Never assume your instrument’s input impedance! It’s not necessarily 10MΩ! And, in the case of Agilent, even if the manual quotes 10MΩ make sure you read the fine print because the might have been misleading you!