Fluke 289 AC Floor? - Page 1
Forum section: electronics / testgear

author: Phoenix6478
author_rank: Contributor
timestamp: August 24, 2018, 11:00:15 am
content: HelloI have just received my brandnew Fluke 289 and i have some questions about it:If I short the leads out and set the meter to the AC Volts mode in the 1000V Range/LowZ mode it shows around 1,6V. If i set it to the 10A AC Range it shows around 0,100A with the leads shorted out. In the µA AC Range it shows about 1,00µA with the leads shorted out.Fluke states the following in the manual of the 289:QuoteSo, I guess everything is right with my 289? The 1,6V in the low Z only equals 160 counts; the 0,100A reading equals 100 counts like the 1,00µA reading.I believe that this AC Floor reading has to do with the True-RMS chip. In the DC-Range it goes to 0 just fine. I also own a Agilent U1252B and it is bang on with the Fluke 289 in terms of AC accuracy. On the calibration sheet the 289 is also well within spec. But the U1252B alway zeroes out in the AC mode. Could it be that the U1252B is not really a True-RMS Meter?GreetingsPhoenix

author: Phoenix6478
author_rank: Contributor
timestamp: August 24, 2018, 06:20:21 pm
content: I have just received a response from Fluke. They say it is normal, because the meter is only designed to measure 2% to 100% of the range in the 1000V setting. This fact is also written in the manual. The meter is not counting the reading as an offset, because then my measurements would be off by 1.6V, which they are not because i checked them with an other meter.I am very surprised by how fast the Fluke customer service responded to my question. It only took 1h to get a response. Huge thumbs up for that

author: Kleinstein
author_rank: Super Contributor
timestamp: August 24, 2018, 07:10:57 pm
content: The classical analog true RMS chips are limited at low amplitude. This is not just the Fluke meter, but also applies to many higher grade bench-top meters. The error may not be the same with all units. Some may read "negative" and software will "correct this to a stable zero. So a meter that reads a solid zero may not be any better, just an error to the other direction.There is a better, digital sampling technique, that could extend RMS reading to a smaller fraction of the range. However this is more like found in some higher grade bench top meters (e.g. Keysight True-Volt).I would guess this problem is already on the DMM FAQ list, just like drifting voltage reading with an open high Z ( > GOhms) voltage input.

author: coppercone2
author_rank: Super Contributor
timestamp: August 24, 2018, 07:37:40 pm
content: Won't the digital technique have more of an error with ugly crest factors compared to the analog solution (some kind of multiplier) which has lower dynamic range due to nonlinear behavior of the transistors at low input levels?I never really realized this, might be another win for thermal RMS meters.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 24, 2018, 07:49:45 pm
content: It depends on the ADC used, how well the digital sampling could work with large crest factors. The analog ones usually specify something like a crest factor of 3 at full scale - this is not that difficult to handle in digital. So a 12 bit converter might be already good enough to get comparable results to the analog way. Performance would be a little different though. The "offset" is not the only weakness of the analog way. The digital way tends to be limited BW.

author: Phoenix6478
author_rank: Contributor
timestamp: August 24, 2018, 08:05:01 pm
content: @KleinsteinThank you very much for the detailed explanation

author: David Hess
author_rank: Super Contributor
timestamp: August 24, 2018, 09:41:45 pm
content: Any AC to DC conversion whether analog or digital and averaging or RMS is sensitive to noise which will always produce an uncorrectable offset. The high input impedance dividers and circuits used by multimeters only make this worse.The noise is not a problem for DC measurements because the input is typically integrated over a whole number of power line cycles producing a sin(x)/x frequency response which significantly attenuates any AC components of the signal including noise.The AC to DC conversion on the other hand picks up all of the noise within its input bandwidth and converts it to a DC signal. If this is done digitally, then it also includes ADC quantization noise and noise which aliases into the Nyquist bandwidth from the sampling ADC's input bandwidth.