I attempted to repeat your test the best I could. I am using a few Caddock TK series resistors to make up roughly 1.9M. The two plots shown are my HP34401A (white) compared with the pre-production 121GW (red). I have also included the CSV file off the meter (renamed to .txt to allow uploading). The recording interval is 1 second for both meters.
Looking at the schematic, nothing pops out other than you should have a better reference. A bit odd you don't see the problem in every range.
Added your data set for completeness.
I attempted to repeat your test the best I could. I am using a few Caddock TK series resistors to make up roughly 1.9M. The two plots shown are my HP34401A (white) compared with the pre-production 121GW (red). I have also included the CSV file off the meter (renamed to .txt to allow uploading). The recording interval is 1 second for both meters.
Looking at the schematic, nothing pops out other than you should have a better reference. A bit odd you don't see the problem in every range.
Added your data set for completeness.
Hello Joe,
thanks for your tests.. the measurement of the 1.9M inside the 5450A by a 34465A, or a 3458A at NPLC 10 is as stable to the last digit, as yours with the 34401A, but w/o drift.
I could replicate that instability with other very stable 1M resistors and short, drilled cables also, but sometimes, I also get stable readings.. very strange.
So I wonder, what you mean by 'you should have a better reference', do you mean the 121GW internal reference resistors?
I doubt that these are the root cause for this instability problem.. because the Ohm mode is usually realized by a ratio measurement, so I suppose, that it's caused by the AD1.
And yes, this instability is visible part time in other ranges also, surprisingly in the 50 range, although that's driven by a high 470µA current.
Maybe you still have an old firmware running. In the latest 1-22, the counter register for the sampling frequency might be set improperly. Otherwise, 50 and 60Hz should be suppressed by 120dB @ 5Sa/sec, but 75dB only, when using a faster sampling rate... that could also be the case, as the sampling seems much too fast.
Maybe something inside the Ohm circuit is oscillating, or super sensitive, depending on the hardware version..
I did not fully understand the schematics, either, and also the HY3131 datasheet is really crap., still searching for an Application Note.
Anyhow, I really think, that this problem could be solved by software.
Frank
.. I updated the pre-production meter to 1.02 then saved the calibration data. I then proceeded up upgrade the meter to 1.22.
I remember reading something early on about the firmware may have required the meters to be realigned. This appears to be the case but it should be good enough to see if running the latest firmware will cause the noise problem.
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*********** Update *************
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One last note, it does appear that at least with a 0.5 ohm resistor, with the 1.22 firmware installed, the readings are VERY unstable. The meter will display 0.5 and noise for the other two places. After downgrading, the meter again changes a couple of counts at most. So it does appear that the noise people are seeing could very well be due to the firmware. The other thing I noticed is the display update rate for the low values is much faster with the 1.22 firmware.
Both date and time is on the first row in the file, then it says what interval is used. So it is not impossible, just somehwat cumbersome. Add another column, copy the time from the top row, then next row is the one above+(interval in seconds)*(1/86400). If using excel or the like. Otherwise I suppose a script could be used...
But it would be very easy codewise to implement the timestamp into the log file, so we don't have to mess around like that.
One detail paid my attention: I barely can read red labels. Too dark tone compared to the background.
The default setting is to sample and save points as fast as the ADC allows, hence the interval is not fixed but depends on the mode, range, etc. It is therefore impossible to calculate the time for each point using the default setting.
However it is true that if you change the settings and limit yourself to a slower sampling speed, i.e. not using the full potential of the meter, it does seem like the timing is consistent so that can assume a fixed number of seconds between each point. I tried it over 4 minutes using a stopwatch and I could not detect any drift or dropped points by eye at least. Hence I guess this is the recommended method for now.
In order to allow for faster sampling I guess you need the actual time in a column (seconds since start is probably easiest), as it would make analysis much easier without having to process the time manually every time. Also as a separate improvement I would remove the "mode" from being printed on every row, either print it once in the header or only print it when it changes (if you intend to include mode switching during logging in the future). This would save approx 10-30 % in space depending on the mode used.
I also noticed that the meter does not autorange during logging (it goes into manual ranging when starting the logging process). This is definitely a feature that I would like to see in future updates.
Edit:
More issues I noticed today:
- The claimed "update rate" is 5 samples/s nominal, however my display only updates 1-2 times/s, hence the meter feels slow and unresponsive, is this an error in the manual or is the meter supposed to be faster?
- When manually selecting the 10 A current range, the display tells you that you are in the 1000 A range. Perhaps there is no 10 indicator in the LCD and this is intended, if that is the case please update the manual.
- There is a 1000 V range, however the manual specifies maximum 600 V for the input protection. Perhaps this is the same issue as above? Please update the manual.
- There are no range indicators in the display for Frequency, Period, Capacitance. Perhaps indicators is lacking in the LCD? In that case I suggest to always show leading zeros in these modes when in a manual range. For example the 100 nF range, write 07.6 nF, instead of just 7.6 nF in order to indicate the amount of headroom available.
Arrived this morning. Flabbergasted. Cant use it, NO PROBES and no zippy-up case thing...
Can someone clarify the DC+AC mode? Should the meter show on the secondary display the AC or DC component? Are the segments in red circle for that purpose?\
What other DMM use the HY3131?
Keysight U1282A
AC+DC mode means the meter measured the AC and the DC part of the voltage and then sums them together before showing them.That's not true. To avoid having Dave remove this post as a non-issue, the manual should be updated to show it as a feature and the proper equation should be added as well.
What the AC DC sgments next to the secondary display are for? Many manuals explain all the segments of the screen. It would be nice touch.