Agilent U1253B Input Impedance Problem


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!

7 responses to “Agilent U1253B Input Impedance Problem”

  1. Amigo Brian ,que mas lindo puedo tratarte
    Si te cuento que la empresa que vende estos equipos (U1253B9) en Ecuador le notifique del problema y me devolvieron 500 dolares que di de anticipo para la compra.
    Gracias por este gran aporte de experiencia y concimientos al mundo.
    Livio Ortega M

  2. Amigo Brian gracias por estas obserbacines técnicas.
    Estoy comprando un multimetro Agilent modelo U1253B para reparaciones electrónicas, puedo usarlo con las puntas de fluke 80K-15 para medir altas tensiones en tarjetas inversoras de TV.
    Si el U1253B no tiene una impedancia de 10 M no lo tomo o lo devuelvo a la fabrica.
    Livio Ortega

    • Thank you for your comments Livio.

      I would not recommend that you purchase the Agilent U1253B for use with a 80K-15 high voltage probe. If you try to use this probe with the Agilent U1253B then you will experience measurement errors. If you look at the technical specs for the 80K-15 you will see that it requires a measuring instrument that has a 10M input impedance. Most good multimeters have a reliable 10M impedance right across their measurement range, but the Agilent U1253B does not.

      Another really annoying problem with the U1253B is the battery life. You will forever be picking it up to use it and then finding it needs re-charging. The combination of light-emitting display and poor capacity PP3 battery power is a bit of a disaster to be honest.

      I hope this helps.

  3. I also experienced measurement errors with a Meterman 37XR and it turns out that its input impedance is 9MΩ, not the 10MΩ stated in its specifications. And this time, there was no fine print to say otherwise.

    So watch out folks! Always make sure you know what your multimeter’s true input impedance is!

  4. Nice catch, I have both meters too but don’t work over 250V so would never had thought to reconfirm the input impedance. Another reason when in doubt, A Fluke industrial meter is the meter to reach for. I still do this now when I doubt a measurement with my 3 Agilent meters, they have a reputation to live up too.

    • Yes indeed. The Fluke is my meter of choice actually. I like the Agilent too but the battery life is a real pain in the neck.

      Unfortunately my Fluke was away for calibration at the time. Otherwise I’d have found the problem sooner. As it was I spent quite a long time blaming the circuit under test for poor performance!

      But I learned a valuable lesson: never allow yourself to become complacent about instrument input impedance! Assume nothing!

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