Multimeter noise vs NPLC vs Filter - Page 1
Forum section: electronics / general

author: rodpp
author_rank: Frequent Contributor
timestamp: June 20, 2020, 06:22:30 am
content: Hi!In the "Ultra Precision Reference LTZ1000" post @dr.diesel posted a graph showing that the lowest noise is achieved by the 7510 multimeter using an aperture between 1 and 5 PLC:I remember reading in others topics the use of 100 PLC in the 3458A, to get better measurements.I have a Keithley 2002 and it's maximum aperture is 50 PLC, with the pre configured HIGHACCURACY setting equal to 10PLC.The K2002 also has the digital filter configuration with the maximum of 100 measures average.My questions are:1- Is the better aperture to lowering noise equipment dependent, like 5 PLC in the 7510, 100PLC in the 3458A, 10PLC in the K2002, etc?2- Is an aperture of 10PLC combined with a digital filter (average) of 10 measures equivalent to an aperture of 100PLC?Other considerations are very welcome.Regards,Rodrigo.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 20, 2020, 10:02:46 am
content: There are 2 PLC settings, one is the length of a single integration and the other is the total lenght with averaging (digital filtering).The graph with noise vs PLC setting is somewhat typical / schematic for the lenght of single integration.For the graph they may be comparing the noise density, not the absolute noise.For this reason meters like the 3458 and K2002 normally do not use integration at a piece much longer than 10 PLC in stead they do averaging of several conversions. So the 3458 and AFAIK also the K2002 50 PLC would be 5 x 10 PLC. For the 3458 one can force a single longer integration, but it increases the noise.For this question we can divide the noise in 3 categories: 1/f noise , white noise (relative to the input) and noise from measuring the final charge. A limited resolution / quantization would also act like noise for the final charge.For the white noise it is equally good to do longer integration or averaging. The noise of the reading is expected to go down like the square root of time.For the 1/f noise longer integration does not help reducing the noise and if there is some 1/f² part it can even get worse.The final charge noise, goes down like 1 over the integration time, so longer integration is better for this part.For very short integration this noise dominates and usually make the noise go up like 1/time to shorter times (e.g. < 0,1 PLC).Where the lowest noise is, depends on the ADC details. Due to mains hum, one is kind of limited to integer PLC numbers. Besides noise, there can also be some INL effects that get better with longer integration. So it may be more than noise to take into account. Some DMMs (e.g. Datron 1281, Solartron) use an Az OP in the integrator and this way get very little 1 / f noise and they can thus use very long integration at a piece.Quite a few Keithley meters (K2002, DMM7510, DMM6500) seem to have some extra trouble with longer integration, that makes the noise go up. I don't know the details, but I suspect some software problem with the auto zero mode and averaging over more zero readings than is good, so that 1/f suppression does not work well.

author: David Hess
author_rank: Super Contributor
timestamp: June 20, 2020, 05:50:05 pm
content: Kleinstein covered what I might have answered except:Quote from: rodpp on June 20, 2020, 06:22:30 amThey are not the same thing for a couple of reasons:1. Digital filtering has lower quantization noise, or at least it better be lower.2. 10 measurements of 10 units will usually be longer than 100 units because of dead time between measurements increasing low frequency noise.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 20, 2020, 07:39:33 pm
content: Usually a single long integration will have less quantization noise. Quantization goes down as 1/T. Filtering by averaging reduced the quantization noise with the square root of N, so slower.10 conversions of 10 PLC will need a little more time than once 100 PLC. However at 10 PLC the difference is usually small (e.g. some 100 µs to mybe 1 ms for the rundown).At 10 PLC and longer one would normally use an auto zero mode, thus alternating between one reading of the input. The relevant frequency is determined by the lenght of the single conversions. So the 100 PLC at a piece version will react to lower frequencies and will usually be more sensitive to 1/f noise.Without auto zero the little extra time would cause slightly more 1/f noise and more quantization noise.Because of the lower noise some meters (e.g. 3458, 34401 ?, K2002) use 10x10 PLC instead of 1 x 100 PLC by default. One the other hand the long integration at a piece is needed in simple meters like the Solatrons, that don't use a fine slope. To avoid the extra 1/f noise, they have an AZ OP in the integrator.

author: rodpp
author_rank: Frequent Contributor
timestamp: June 26, 2020, 08:27:52 pm
content: I would like to thank you Kleinstein and David Hess for the compreensive answers. A lot of new information for me. The manuals are very suscint in this topic, sugesting only a direct relation between the aperture time and resolution, more time better. Disconsidering that graph in the first post, I could not find one mention in the multimeters manuals to disadvantages of using more aperture time, except of course the slow rate of measurements.In the last days I was in the Hospital because a little health problem, so please forgive the delay in answering your posts.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: July 01, 2020, 10:29:29 pm
content: Quote from: rodpp on June 20, 2020, 06:22:30 amNot certainly in that way. Everything turned out to be much more complicated.For 0.1V and 1V ranges, this is true - 5NPLC is the smallest noise. But looking at the graphs, there is a suspicion that digital averaging of samples with 1NPLC will give better results.For the remaining ranges, the larger the NPLC, the better. Because the device allows you to set only 12NPLC then further noise reduction can be obtained with digital averaging.