FLUKE 87V High Z mode input Impedance value

[kg4arn]

I can't find any documentation on the input impedance value for the FLUKE 87V in the High Z mode (600mV) range.

In a previous thread:  https://www.eevblog.com/forum/chat/i-need-to-know-the-history-of-fluke-87-(-1998-2010-)/msg3907/#msg3907  it was said to be greater than 4000 Megaohms.

Is this the correct number?  I can measure the loading effect of the 10Meg input impedance but I am unable to measure the loading effect in the High Z mode, so I would believe the number quoted in the thread above.

[alm]

As far as I know it's not specified by Fluke, but you can expect it to be >> 10 Mohm since it's just a FET input with some protection circuitry. Typical specs for similar meters would be something like >1 Gohm (note how they only give a minimum).

[kg4arn]

Thanks alm

[walter.aranda]

I confess that I have also had the same question, I even asked to ChatGPT IA, he told me that it is approx 10G Ohm (I attach the image, - it is in Spanish -) if it is possible using the "S" conductance reading function in nS (Available in Fluke 8X, 18X, 18X series), I have measured it, in Vac: 0.06nS = 16.67G Ohm, in Vdc: 89.36nS= 11.19M Ohm, in mVDC:43.55nS=22.96M Ohm. Which demonstrates consistency, since this function is generally useful for measuring very high voltages. Regards

[tooki]

Remember that ChatGPT is not a search, and doesn’t know math, it’s just a statistical probability engine for what the next word is likely to be.

[newbrain]

For the last picture, also consider that if the display shows overflow, you are quite probably measuirnig the conductivity of the non linear protection circuitry, I.e., not the input impedance.

[tom66]

Remember that ChatGPT is not a search, and doesn’t know math, it’s just a statistical probability engine for what the next word is likely to be.

And it's very, very good at 

[newbrain]

Remember that ChatGPT is not a search, and doesn’t know math, it’s just a statistical probability engine for what the next word is likely to be.

And it's very, very good at 

Just like humans, then?

[walter.aranda]

I ask to ChatGPT only just a curiosity, since I did not find the answer by looking for it traditionally on Saint Google, of course it gives wrong information, for example the other day he told me that the 189 can measure in low pass filter, I questioned him and he apologized (I send in attached to laugh a little, -it's in Spanish too, sorry-)
About the non linear protection circuitry, it's true, I didn't think about that, I'll do test with resistor loads, maybe it helps to give an idea. Thanks!

[gamalot]

I don't have an 87V but I do have an 83V.

I applied 1.2V to two 1G ohm resistors in series, and then used the mV range of 83V to measure the voltage drop on one of the resistors.

The 83V reading slowly rises from 0 at the beginning, until it stops around 600mV, which feels like charging a capacitor.

Once the charging is over, you can say that the input impedance of 83V is now much much higher than 1G ohms. 

---

I noticed that when the input voltage is higher than about 2.5V, the input impedance of 83V will decrease. For example, when the input voltage is 3V, the current is about 2.8uA. I guess the protection circuit should be working.

[bdunham7]

It isn't really helpful to think about the input characteristics of a Hi-Z meter like this in ohms.  The small current going through the meter and how it responds to applied voltage depends on a number of characteristics put together--the input capacitance, the leakage of the protection clamps and the bias current of the input circuit, etc.  The bias current may be more or less constant, the leakage current will depend strongly and non-linearly on input voltage and in the case of a non-steady voltage, the current through the capacitor will depend on how the voltage is changing.

In cases where all of that is so small that it doesn't matter, then things are great.  If you start trying to use a Hi-Z voltmeter to look at G-ohm voltage dividers or circuits with picoamps of current, you'll discover all of what I just said above to be true.