Hello. I've been lurking around on this board recently and caught a bit of the noise bug
I noticed that the noise on the new Keithley DMM7510 does not go down as fast as it should for longer integration times, like 10 PLC, especially on 10 V scale. Here is a comparison of the time trace for DMM7510 and Keysight 34465A on 10 V scale, 10 PLC with autozero, continuous triggering. These data are saved to internal buffer, then transferred to USB stick, no computer connection.
Even though the rms noise is similar, it is clear that DMM7510 data have much larger flicker noise component. So, I started to look at Allan variance of the voltage (with shorted inputs) for my own meter as well as using the data that others have posted on TiN website. Allan variance tells you what is the rms scatter of the data on a particular time scale. It should go down as 1/sqrt(time) until the instrument reaches offset drift noise floor. Here are the plots for 10V range, using PLC of 1 and 10. All the data are with autozero on. Turning the line sync on or off does not make a difference.
It is clear that DMM7510 has a problem, where the Allan variance does not go down until about 30 sec timescale. It is present for three meters measured by different people. Keysight 34465A has the expected normal behavior, it goes down as 1/sqrt(time) and then reaches a floor given by the drift. One can obtain similar plots from any other long raw data files posted on TiN webste (having all the data available is really great!) For example, it would be interesting to compare to a nice long HP3458A file (I can do that if you tell me which one to use).
The really weird thing is that the noise in 7510 changes depending on measurement trigger time. I noticed that first from DrDiesel's data taken at 1 sec intervals (instead of continuous, as most other data). I found that I can get the best performance on my meter using 0.1 sec trigger and 1 PLC. One can also notice that the noise decreases faster than 1/sqrt(time) after 30 sec. This indicates a quasi-periodic noise source, I suspect some kind of clock interference.
For reference, here are my measurements of the RMS noise for various ranges and speeds for the two meters:
Range | Speed/Average | 34465A ppm | DMM7510 ppm |
100 mv | 1PLC | 2.6 | 0.43 |
100 mV | 10 PLC | 1.2 | 0.29 |
100mV | 100PLC From 1 PLC | 1.1 | 0.21 |
100 mV | 100PLC From 10 PLC | 0.97 | 0.2 |
1V | 1PLC | 0.32 | 0.081 |
1V | 10PLC | 0.14 | 0.055 |
1V | 100PLC From 1 PLC | 0.103 | 0.04 |
1V | 100PLC From 10 PLC | 0.098 | 0.033 |
10V | 1PLC | 0.13 | 0.081 |
10V | 10PLC | 0.04 | 0.051 |
10V | 100PLC | 0.016 | NA |
10V | 100PLC From 1 PLC | 0.014 | 0.043 |
10V | 100PLC From 10 PLC | 0.014 | 0.038 |
10V | 1000PLC from 1PLC | 0.01 | 0.019 |
10V | 1000PLC from 10PLC | 0.0063 | 0.016 |
They are generally consistent with earlier measurements for these meters reported here. DMM7510 is better at 100 mV and 1V, but mostly worse than 34465A for main metrology range of 10V.
There is also a number of programming issues I found on the Keithley DMM7510 meter. It gave me a blue screen of death several times (even Chinese instruments don't really do that). When using internal trigger timer, it tends to hang up and stop triggering after a few thousand to tens of thousand of points. When saving to USB, it does not warn if the file already exists. Overall, 34465A is clearly a great deal with 7.5 digit performance. DMM7510 is questionable if its worth 2-3 times higher price, perhaps it can be improved with firmware.