HP 34401a measurement time for 100PLC - Page 1
Forum section: electronics / testgear

author: krasimir.k
author_rank: Contributor
timestamp: April 13, 2018, 05:33:20 pm
content: Hi,Recently I bought an old HP 34401a DMM. When I set to measure in slow 6 digits mode (100PLC) I got measurement results every 4 seconds. I expected 2 seconds (on 50Hz power line) integration time according the datasheet plus some processing time (I hope that it should be far below 2 seconds). I measured the same time manually when the "star" blinks on the VFD display or with the RS-232 logging.Anybody to know what is wrong?Regards!

author: LaurentR
author_rank: Frequent Contributor
timestamp: April 13, 2018, 06:28:30 pm
content: Quote from: krasimir.k on April 13, 2018, 05:33:20 pmI assume that's because the 34401A is by default in "autozero" mode where it zeroes out the measurement path before each reading and that step takes a whole 100PLC itself.Look for "autozero" in the manual.

author: HighVoltage
author_rank: Super Contributor
timestamp: April 13, 2018, 06:48:52 pm
content: Yes, LaurentR is right,the 100 PLC = 2sec at 50Hz is only valid for autozero=OFF

author: krasimir.k
author_rank: Contributor
timestamp: April 13, 2018, 07:03:22 pm
content: Thanks for the point!This make sense. I checked the user guide document page 59/60, where is written:"When autozero is enabled (default), the multimeter internally disconnects the input signal following each measurement, and takes a zero reading. It then subtracts the zero reading from the preceding reading. This prevents offset voltages present on the multimeter’s input circuitry from affecting measurement accuracy."The AutoZero is always ON when using the "Front-Panel Operation".I'm going to use the ZERO:AUTO OFF SCPI command to see if this can help to speed-up the measurement in trade off of accuracy.In the Keysight sitehttps://community.keysight.com/community/keysight-blogs/general-electronics-measurement/blog/2017/04/07/increasing-dmm-measurement-throughput, I found the following info :"Therefore, if your measurements are taken in an environment with a stable temperature, or if there are several measurements taken in a short period of time (temperature changes occur over longer periods of time), the improvements in throughput by turning auto zero off will far outweigh any slight compromise in accuracy. For example, with auto zero off in a stable environment, the Keysight 34460A/61A/65A/70A DMMs typically adds only an additional 0.0002% of range +5 ?V for DCV or +5 m? for resistance accuracy specification"

author: Kleinstein
author_rank: Super Contributor
timestamp: April 13, 2018, 08:09:08 pm
content: Quote from: krasimir.k on April 13, 2018, 07:03:22 pmThe Auto zero function not only eliminates the offset and drift error. Ideally it also eliminates much of the 1/f noise of the amplifier and ADC. So there can be a significant increase in noise when not using AZ at long integration times. So 100 PLC without AZ might not make much sense - 50 PLC with AZ would be likely lower noise. It goes so far that many (modern) meters don't do a true 100 PLC integration, but more like average 10 conversions with 10 PLC and alternating zero phase.