HP34401 - AC current range damaged non-linear - Page 1
Forum section: electronics / testgear

author: floobydust
author_rank: Super Contributor
timestamp: June 11, 2017, 06:12:08 am
content: I've seen several (3) damaged HP34401's where the AC mA range is non-linear. Here's a graph of one.They read zero even with a few mA, then read low until about 6-10mA, beyond which they start to read properly.The silly units passed third-party calibration because that is a zero/full scale check only. Drove me nuts to find out what was going on.I tossed the units on the "fix on a rainy day" pile. Suspect any overload/ blowing the A fuse can also damage the meter's internals. There is a op-amp clamp circuit that might be where the anti-zener is.

author: Dr. Frank
author_rank: Super Contributor
timestamp: June 11, 2017, 08:18:40 am
content: Is this measured on the 1A AC range?Then you are using the instrument completely outside of its specification, as the reading is < 1% of full scale.AC measurements (ACV, ACI) are defined for >5% of full scale only.Between 1% and 5% of F.S., add 0.1% of range, that is up to 10% additional error.This is due to the limitations of the AD637 AC/DC converter.Additionally, in these older HP DMMs, below 1% of F.S., the firmware artificially drives the reading earlier towards zero.This is no defect, but intended by HP.Joe Geller found that out long ago:http://www.gellerlabs.com/34401A%20AC%20zero.htmIf you want to measure such low AC current, you have to enable the hidden 10mA AC range, by sending the command DIAG:POKE 25,0,1 to the instrument.Frank

author: HighVoltage
author_rank: Super Contributor
timestamp: June 11, 2017, 08:44:13 am
content: I also learned about this low AC limitation of the 34401A and the hidden 10 mA range from this Forum in 2014.https://www.eevblog.com/forum/testgear/agilent-34401a-hidden-10ma-ac-current-range/msg399472/#msg399472This was also one of the reasons for me to get a 34410A, which is much more stable and linear for low AC readings

author: nfmax
author_rank: Super Contributor
timestamp: June 11, 2017, 09:34:16 am
content: Interestingly, the34970A, which uses an almost identical DVM subsystem to the 34401A, exposes the 10mA & 100mA AC current ranges. The accuracy specification on the 100mA range isworsethan that for the 10mA range, because it uses the same 0.1 ohms sense resistance as the 1A range: the 10mA range uses a 5 ohm sense resistor (which is also present in the 34401A).I find I get a useful reading from my 34401A down to about 6mA or so (0.6% of range), below that it starts to seriously under-read, much as in the OP's plot.

author: tszaboo
author_rank: Super Contributor
timestamp: June 11, 2017, 09:35:56 am
content: It has the following spec for 1A 10khz (best for AC): 0.10 + 0.04 (reading, range)So +/-40mA on the 1A range is perfectly within spec. Also:Quote

author: floobydust
author_rank: Super Contributor
timestamp: June 12, 2017, 04:02:22 am
content: OK so people are saying this is normal for the HP34401:AC 1A range (using front panel buttons) and reads zero with a few mA AC flowing, then 2mA low until 10mA input.Checking the manual, the accuracy spec is derated for 10-50mA AC (1-5% of range) by an extra 0.1% and 0.04%of range. Below 10mA certainly worse as I'm seeing.So it appears the meters are operating fine.I've never seen a few AC mA disappear before with benchtop meters.