Agilent U1253B - curious input impedance problem. - Page 1
Forum section: electronics / general

author: Thermal Runaway
author_rank: Contributor
timestamp: September 29, 2012, 11:05:40 pm
content: Hi everyone,I have recently been experiencing some problems whilst taking some high voltage measurements with an Agilent U1253B and a high voltage probe. This has lead me to investigate its claimed input impedance. If you're interested, have a read and let me know what you think! If you can provide any explanation your thoughts would be most welcome:http://www.brianhoskins.co.uk/website/agilent-u1253b-input-impedance-problem/

author: Unknown
author_rank: Unknown
timestamp: September 29, 2012, 11:39:51 pm
content: Would note 3 on page 195 of the U1253-90035 manual explain your results?

author: Thermal Runaway
author_rank: Contributor
timestamp: September 30, 2012, 12:09:31 am
content: Good spotting - I missed that!I think that might explain it actually, yes! But... if I've understood that correctly... they're basically saying that the input impedance is only 10M if the measured voltage is between -2V and +3V. Otherwise it's effectively 5M.So why say it has a 10M input impedance on the VDC range? For the vast majority of the range it only has 5M! That's silly.Thanks for the response - appreciated.Brian

author: tom66
author_rank: Super Contributor
timestamp: September 30, 2012, 12:13:10 am
content: Quote from: alm on September 29, 2012, 11:39:51 pmHmm. I'm not sure if I like a multimeter whose input impedance varies with input voltage!

author: Thermal Runaway
author_rank: Contributor
timestamp: September 30, 2012, 12:24:49 am
content: Quote from: tom66 on September 30, 2012, 12:13:10 amYes and it actually has caused me a practical problem. I spent quite a while blaming my circuit for poor performance before I realised that the multimeter was lying to me. The thing is I verified it with another U1253B so I had ruled out the instrument as being the problem. Doh! It wasn't until I measured it with my Fluke that I realised something fishy was going on.This also explains why the Agilent and the Fluke agreed with each other up to about 3KV.The moral of the story is:Never assume your instrument's input impedance!!! It's not necessarily 10M! And in the case of Agilent, even if theyquote10M you have to make sure you read the fine print because it could be misleading ;-)

author: SeanB
author_rank: Super Contributor
timestamp: September 30, 2012, 06:02:38 am
content: I have an ISO Tech A9 clamp meter that advertises the low input impedance, and that it will vary according to range. Then again it has a 30 sec limit on voltage reading, as it is a 40k input impedance. A plus is you cannot blow it up on ohms by applying it to mains, it has no range switching, below 2V it does continuity and above 3 it does voltage. Leave the leads open it does current. Has a single button to turn on and off and select hold. All else autorange, which makes it great for electrical work.

author: DavidDLC
author_rank: Frequent Contributor
timestamp: September 30, 2012, 08:21:11 am
content: I hope people understand that these new Agilent models are not very good designed, I had a problem with my Agilent multimeters ( explained in a separate post ). If we stop buying them, maybe Agilent will take a deeper look and improve their instruments.David.

author: T4P
author_rank: Super Contributor
timestamp: September 30, 2012, 08:26:00 am
content: No need to stop buying, just write to them. They will surely help youAgilent ain't fluke, bro.

author: Thermal Runaway
author_rank: Contributor
timestamp: September 30, 2012, 09:35:06 am
content: I actually like Agilent instruments - in my experience they are quality, trustworthy instruments. In this case I think they've been a bit of a let down though, mainly from a user manual point of view. It's like they've tried to brush over the fact that the instrument is basically only 5M input impedance (for the vast majority of its VDC range) by claiming 10M and then making a confession in the fine print.Since -2V to +3V is such a tiny part of the 5V to 1000V range, I think they should quote the impedance as 5M and be done with it. Especially for the ranges that are greater than +3V because they'realwaysgoing to be 5M aren't they!Brian

author: DRT
author_rank: Regular Contributor
timestamp: September 30, 2012, 09:54:07 am
content: Some Flukes do similar things too, although far less likely to cause such measurement issues. The 8840A is >10 GOhm up to 20 V but 10 MOhm above that.

author: Unknown
author_rank: Unknown
timestamp: September 30, 2012, 10:09:55 am
content: I also find the -2/+3V limits for the 1000 V range very odd, and I disagree with the claim that it's 10 Mohm, since it's only for 0.5% of its range. Sounds like they're using a secondary more sensitive circuit in parallel that's being clamped above these voltages. Input impedance variations with range are acceptable (like the very high impedance FET inputs at lower DCV ranges), but variations depending on the input voltage are poor design in my opinion.

author: Thermal Runaway
author_rank: Contributor
timestamp: September 30, 2012, 11:03:35 am
content: Quote from: DRT on September 30, 2012, 09:54:07 amThe fact that the 8840A has 10GOhm up to 20V is more of a feature though. Having a higher impedance for some of its range is an advantage. With the Agilent U1253B it's the other way around - they've stated it as being a 10MOhm input impedance (which is pretty much the standard) but then they've confessed in the fine print that actually, for the vast majority of the range, it's only a 5MOhm input impedance - andthat'sa disadvantage.The reason it caused measurement issues for me was because I tried to use the instrument with a 1000:1 high voltage probe and these devices (the multimeter ones, anyway) are designed to be used with a 10MOhm impedance. If you try to use it with a 5Mohm impedance, which I ignorantly did, then you're going to experience a 'gotcha'.

author: Thermal Runaway
author_rank: Contributor
timestamp: September 30, 2012, 11:05:27 am
content: Quote from: alm on September 30, 2012, 10:09:55 amAgreed.

author: saturation
author_rank: Super Contributor
timestamp: September 30, 2012, 01:30:12 pm
content: Great catch, it would be a major nuisance as many dividers/probes expect a fixed impedance from the meter throughout the ranges.I just check my 1252a specs and its worse with the dual display, but I've not measured it directly. Worth keeping in mind."Input impedance: >1Gohm for 50 mV to 1000 mV ranges. For U1251A, inputimpedance is 10 Mohm (nominal) for 5 V to 1000 V ranges. For U1252A, inputimpedance is 10 Mohm (nominal) in parallel with 1.1 MÙ at dual display."Per the manual in the 1272a series the impedance is 10Mohms throughout its ranges.Note, these are for DC specs, add the <~100pf in parallel for AC.If I've time to check its actual values I'll post back.