DMM Noise comparison testing project

[Kleinstein]

The Alam variance curve looks a bit odd, especially for the short times, below some 2 seconds. The values there are surprisingly low. This makes me think the first values could be "wrong" and not the maximum at some 2 seconds. With "wrong" I mean an issue in the meter that causes correlation between the readings (e.g. a hidden rolling average, possibly just for the zero readings).

[dietert1]

I thought about it, too. Of course there could be some cheating inside the K2700, who knows.
When debugging the fiber ethernet interface i saw a K2700 timing pattern that shows an extra cycle every eight readings, so about once per second. Probably the autozero. Yet another additional cycle happens every 10 seconds. Probably an autocal type adjustment using the LM399 reference. A 5 PLC reading with line sync becomes 120 msec. The observed reading delays are 120 msec, 240 msec and 360 msec.
In my understanding the "flat" part of the Allen diagram above one second is due to autozero. Maybe i can repeat the test without autozero.

Regards, Dieter

[zrq]

This is the noise Allan variance of L4411A (UI less version of Agilent 34410A/11A) with input shorted, AZ ON, and set to 1PLC. Plotted either in volts or ppm of range.
It have a very low noise in the 10 V range that easily reaches 0.1 ppm of full scale at 0.1s averaging time and <0.02 ppm in 2s (close to 100 PLC), but the 1V and 100V ranges have slightly lower noise compared to the older generation 34401A.

[Kleinstein]

I thought about it, too. Of course there could be some cheating inside the K2700, who knows.
When debugging the fiber ethernet interface i saw a K2700 timing pattern that shows an extra cycle every eight readings, so about once per second. Probably the autozero. Yet another additional cycle happens every 10 seconds. Probably an autocal type adjustment using the LM399 reference. A 5 PLC reading with line sync becomes 120 msec. The observed reading delays are 120 msec, 240 msec and 360 msec.
In my understanding the "flat" part of the Allen diagram above one second is due to autozero. Maybe i can repeat the test without autozero.

Regards, Dieter

The classic AZ mode should suppress 1/f noise but add white noise: 2 readings and thus 1.4 times the noise and half the reading rate.
The resulting white noise should give an slope of -0.5 in the log-log Allan variance curve. A flat part and than going up is from additional low frequency noise, e.g. thermal fluctuations / drift in the front end and protection.

Getting the different reading times can be confusing and points to a not so simple AZ mode, like doing a zero less often and maybe additional averaging for the zero. For the Allan variance this is a bit "cheating" for the shorter time scale and thus a better result there. It however comes with more noise on the slightly longer times. The more complicated AZ mode can make some sense in some applications, but it would be nice to also have the simple, classic version as an option as a constant reading rate can also be a big plus.
A non AZ test could also make sense. One could kind of simulate the classic AZ case with these data, though withough the switching transients.

The L4411A noise looks very white with very little extra low frequency noise and drift.

[dietert1]

I thought about it, too. Of course there could be some cheating inside the K2700, who knows.
..

The user manual describes data processing and its configuration in great detail. My Allen diagram happens to document the K2700 behavior in its default configuration after power up (filter on, window = 0.1 %, 10 readings, moving average). One can turn off the filter with the expected effect on noise at averaging times less than 3 seconds.
I also verified that the observed reading delays are gone when autozero is disabled. Rate becomes a steady 120 ms per reading. In the manual they mention that their autozero employs a moving average and needs some time to settle.
Remains to check the effect of temperature at high integration times.

Regards, Dieter

[Alex Nikitin]

Here are some results for the HIOKI DM7275 voltmeter, shorted input, NPLC1 with 50ms intervals and NPLC10 with 500ms intervals, 1hour each run, 100mV, 1V, 10V and 100V. The vertical scale in Volts, obviously.

Cheers

Alex