Hi,
The other day when I was checking my 5uA DC current standard with a Fluke 87-V I noticed something really strange. When I compared the high-resolution values with the standard resolution values things didn't add up. I know that going into HighRes mode on this DMM does not make it more accurate, it just gives you more resolution which is fine with me. But what I did not expect to see was that the values did not agree when rounded. I then ran an experiment where I connected some of my DMM's in series with a Keithley 236 (set to a current source). The table in the attached image file shows my results. As you can see the high-resolution values have an offset of about 0.15uA when compared to the Keysight 34461A readings. However, in standard mode (one decimal less) the Fluke 87-V values are spot on...What could be causing this discrepancy? Are the Fluke 87-V calibrated separately in the standard mode and high-resolution mode? Thanks in advance!
Thank you! Yes, I am not concerned about being in or out of spec. It was just so confusing to see that while it is not more accurate in HighRes mode, it is actually LESS accurate than in normal mode... Good to know! Still love this meter!
From a user's perspective it would seem logical that the HighRes mode are the internal values and that they get properly rounded before being shown in normal mode. But that does not seem to be the case here which caught me by surprise. Thanks again for sharing your insights!!
Interestingly, if you manually set the 6000uA range, then go to Hi-Res mode, it gives a different reading to the 600uA range to the tune of 200-300nA for a 10uA nominal reading, when technically it should be identical.
It does it in both polarities.
I really think it must be how the firmware configures/uses the ADC.
Interesting! Will have to try that too. As you can see in my images, I used auto-range on all DMM's. Thanks for sharing!
I like the way it comes up with a negative value in hi-res mode where it had a positive value in normal mode.
I like the way it comes up with a negative value in hi-res mode where it had a positive value in normal mode.
The 20K/6K thing has always seemed weird to me and seeing the behavior here is even weirder, in spec or not. Clearly this is an offset issue, but I've no idea exactly how the 6K mode and 20K mode differ overall. The 6K mode was able to read spot-on, so in this case at least, the 20K mode doesn't seem to be worth paying extra for!
I use several 87-5 but I don't think I ever switched it to 20K mode.
I like the way it comes up with a negative value in hi-res mode where it had a positive value in normal mode.
Not sure how exactly the hi-res mode works, but keep in mind that 0.1uA on the 600uA range represents an absolute value of ~0.0167% of that range. On the lowest range of this meter...
I'm actually quite impressed that it really reads 0.1 in standard mode in the first place.
I recently snagged a faulty Keithley 197A that just needed some TLC, and as luck would have it was calibrated only about 1 year ago. This thread seemed like a good way to take it for a spin. Results attached.
I thought this was a pretty well known issue. All the 87V I've come across have a slightly different offset in low res vs high res and it can drift over time, you just don't notice the drift as much on 6000 count mode as it only effects the LSD.
With nothing connected, one of my 87V reads 0.3uA in low res and 0.17uA in high res, so it's more accurate in high res... If you want LSD accuracy, high res and use of the rel function is your best bet.
keep in mind that 0.1uA on the 600uA range represents an absolute value of ~0.0167% of that range. On the lowest range of this meter..
0.0083% of the ADC range because the range is from -600uA to +600uA
I recently snagged a faulty Keithley 197A that just needed some TLC, and as luck would have it was calibrated only about 1 year ago. This thread seemed like a good way to take it for a spin. Results attached.
Very interesting results! Thanks for sharing! Your results confirm what I am seeing on my end with the 87-V: the standard resolution value is not just a rounded value of the high-resolution value for small currents. In your case, though, the accuracy is actually higher in the high-resolution mode. Perfect!
To dig a bit deeper into this I also added my Fluke 87-III to the mix. As you can see in the results that are added to this post, the 87-iii does not seem to have this issue. Its values match nicely!
I should also say that what I am observing here is related to
small currents on the uA range. I also tested for small voltages on the mV range and neither my 87-V nor my 87-iii show rounding issues with those measurements. Attached is a picture that shows my results when all the DMM's are connected in parallell to a 5mV voltage source (87-iii and 87-V are in HighRes mode).
I thought this was a pretty well known issue. All the 87V I've come across have a slightly different offset in low res vs high res and it can drift over time, you just don't notice the drift as much on 6000 count mode as it only effects the LSD.
With nothing connected, one of my 87V reads 0.3uA in low res and 0.17uA in high res, so it's more accurate in high res... If you want LSD accuracy, high res and use of the rel function is your best bet.
Interesting. I was not aware of this. Great idea about using the relative function in situations like this! Thanks for sharing!!
To clarify, I didn't use relative mode on the Fluke 87-V in any of my tests. Using relative mode on my specific sample yields about a -0.20uA offset throughout the readings so isn't usable.
I obtained this 87-V as a new in box return and it came with a calibration certificate which stated 330.0uA for an "as left" value.
Same story on the 287, but with a value of 500.00uA.
No data for the Keithley 197A but after some additional tests I would say it is between 0.001uA and 0.003uA low from 0 to 5uA. Running on up to 200uA, it ultimately is about 0.025uA low, which is still well within spec: 199.785uA to 200.215uA.
Was surprised to find my Keysight U1461A has a "hidden" 0.0001uA display and the digits appear to be meaningful. Just discovered this later today so no extensive tests yet.
Wow - that is quite the discovery! I will have to take a closer look at that too. I'll let you know what I find out on my end. Good find!
Connect the leads through a decade resistor box instead of a short/DUT and run through the tests again. Try 100-500 Ohms. I got some interesting results.
I thought this was a pretty well known issue. All the 87V I've come across have a slightly different offset in low res vs high res and it can drift over time, you just don't notice the drift as much on 6000 count mode as it only effects the LSD.
With nothing connected, one of my 87V reads 0.3uA in low res and 0.17uA in high res, so it's more accurate in high res... If you want LSD accuracy, high res and use of the rel function is your best bet.
keep in mind that 0.1uA on the 600uA range represents an absolute value of ~0.0167% of that range. On the lowest range of this meter..
0.0083% of the ADC range because the range is from -600uA to +600uA
I have 3 87 V here and they seem to agree with yours.
Good point regarding the shorting ports. I also did not short anything in my video as my two leads are not connected in the other end so the meter is seeing an open - with and without leads connected to the ports. But as you say, the magic happens when you insert the banana plug in the uA port. The COM port has nothing to do with this at all.
FYI, on the BM786 it's zero with no lead, and then when the lead is plugged in it just up for a second or two but then settles back down to zero.
On the 121GW with no lead there is an offset, but it drops to zero with the lead.
The offset in Hi Res mode on my 87-V jumps a bit between -0.02uA and -0.05uA. (Naked plug in mAuA jack only.)
My somewhat cryptic comment about connecting a decade resistor box is pointing out that there is probably current flowing between the COM and mAuA jacks. You can see this in the 100-500Ohm range, as the readings change quite a bit.
The best I could do to measure this at the moment was 1kOhms through the Keithley 197A which resulted in about -0.020uA and 3kOhms through the U1461A was -0.0007uA. I did what I could by using shielded leads, swapping polarities at both ends, etc. but maybe someone with some metrology-grade equipment could jump in.
