Fluke 8845/8845 - what does autozero do? - Page 1
Forum section: electronics / testgear

author: alank2
author_rank: Super Contributor
timestamp: December 07, 2015, 05:10:02 pm
content: It is barely mentioned in the user manual with no UI way to change it. The programming manual shows how you can read or set it. Does anyone know what it does?BTW, I have been playing around with the LAN interface on it to make a remote interface. It isn't polished at all, but I can send commands to it and it can update the measurement as fast as 14 times a second using the read? command.

author: dom0
author_rank: Super Contributor
timestamp: December 07, 2015, 05:24:01 pm
content: I don't know what auto zero does in your multimeterspecifically, but in general it either shorts the AD or the input amplifier input (he) to ground, does a conversion and substracts that measured offset from the following conversion.

author: eas
author_rank: Frequent Contributor
timestamp: December 08, 2015, 08:38:54 am
content: Yeah, typically, it corrects for short-term drift. On my Keithleys, it can be disabled remotely to get higher measurement rates, but they advise to explicitly auto-zero outside of the time-critical measurement sections.

author: HighVoltage
author_rank: Super Contributor
timestamp: December 08, 2015, 10:13:08 am
content: For the Agilent 34401A, the AUTOZERO is used to offset any voltages that are not part of the DUT.This can be thermal EMF voltages on terminals.I would think it is the same on the Fluke metersHere the text from an older Agilent paper I had stored:AutozeroWhen autozero is enabled (default), the multimeter internallydisconnects the input signal following each measurement, and takes azero reading. It then subtracts the zero reading from the precedingreading. This prevents offset voltages present on the multimeter's inputcircuitry from affecting measurement accuracy.When autozero is disabled, the multimeter takes one zero reading andsubtracts it from all subsequent measurements. It takes a new zeroreading each time you change the function, range, or integration time.Applies to dc voltage, dc current, and 2-wire ohms measurements only.Autozero is enabled when you select 4-wire ohms or ratio measurements

author: HighVoltage
author_rank: Super Contributor
timestamp: December 08, 2015, 10:21:29 am
content: Here is an Agilent video, explaining it

author: eas
author_rank: Frequent Contributor
timestamp: December 09, 2015, 05:33:24 am
content: Quote from: HighVoltage on December 08, 2015, 10:13:08 amHow could that possibly adjust for thermal EMF on the terminals?According the video, the short is made right after the switch to choose between front and rear terminals.For the Keithley 2000 and similar meters, the measurement cycle integrates the signal from the DUT, then integrates the zero voltage, followed by the reference voltage. These values are all used to calculate the reading. With autozero turned off, only the source signal is integrated. When calculating the reading, the last recorded zero and reference values are used for all measurement cycles.So, it seems the basic principle is the same, but different vendors have differing implementations, with, no doubt, differing characteristics.