Agilent U1253B input impedance issues - Page 1
Forum section: electronics / general

author: mzacharias
author_rank: Frequent Contributor
timestamp: November 29, 2012, 01:36:42 pm
content: I have a 1253B and really like it, but this is disconcerting, to me at least.http://www.brianhoskins.co.uk/website/agilent-u1253b-input-impedance-problem/Comments?

author: madworm
author_rank: Frequent Contributor
timestamp: November 29, 2012, 01:53:13 pm
content: Return to sender.

author: Baliszoft
author_rank: Frequent Contributor
timestamp: November 29, 2012, 02:04:25 pm
content: I dont care about it as it is not a concern for me (and i bet for most of us). It gives me fast and accurate measurements (i work mainly on low voltage stuff) and that's the only thing what matters to me. On the other hand, the 87-5s default AC mode for the current measurement is what pisses me off.

author: saturation
author_rank: Super Contributor
timestamp: November 29, 2012, 02:36:43 pm
content: Its has 2 concerns: if you use an attachment that is calibrated for 10Mohms, it reads in error. Also effectively the input impedance is 5 Mohms, not 10, so it does load circuits more than others; in return you get more functions than a Fluke.BTW: I have used Fluke and Agilent HH meters over the past years. The Fluke remains the standard and I prefer its ergonomics, the Agilent has some flaws but offers more functions; depending on the country of your purchase, the price is near the same and makes the Agilent a better choice if youdon'tuse non-Agilent attachments and are aware of its flaws, even if you don't need all extra Agilent functions right now; e.g. data logging. If Fluke drops the price of the 87V to $300 list, that would mean street price of $200+, it would again be an unquestionable meter of choice in a new price for basic DMM because I rarely use any of the added functions of the 1252a which is just a 1253 in blue casing. Luckily I got the Agilent's either for free or at a close out discount.

author: DavidDLC
author_rank: Frequent Contributor
timestamp: November 29, 2012, 08:30:22 pm
content: Another Agilent DMM problem. Those meters are not well designed.

author: mzacharias
author_rank: Frequent Contributor
timestamp: November 29, 2012, 08:38:26 pm
content: Quote from: saturation on November 29, 2012, 02:36:43 pmThe U1253B does have "Smart Ohms" function - I don't believe that is in the 1252A or 1253A Models.I wonder if that functionality could be added via firmware?

author: Unknown
author_rank: Unknown
timestamp: November 29, 2012, 09:12:47 pm
content: I believe this was discussed before on this forum. The input impedance is 5 Mohm in some modes. It's odd, but it is shown in the data sheet, so at least it's well documented. Some HV probes put a much smaller value resistor in parallel with the meter, so the impedance of the meter is not as important. You should always be aware of the input impedance of your DMM when using basic divider probes, some meters might have a very high (Gohms) impedance at low DCV ranges.

author: c4757p
author_rank: Super Contributor
timestamp: November 29, 2012, 11:15:29 pm
content: Out of curiosity, is there any practical reason why a high voltage probe like this couldn't have its own internal, full voltage divider with a buffer so it doesn't depend on meter input impedance? I know the usual method is simpler, but really, one more resistor, a low-power, high-input-impedance op amp and a battery isn't a whole lot of added complexity. That would avoid problems like this, and additionally you're keeping the high voltage out of the meter if there's something wrong with the input circuitry and the voltage divider isn't connected (though I'm sure that doesn't really matter with the massive series resistance one of these must have).

author: grenert
author_rank: Frequent Contributor
timestamp: November 30, 2012, 01:15:41 am
content: I would imagine the reason for this is safety. If the battery is dead, maybe you get no reading even if you are hooked up to kV!

author: saturation
author_rank: Super Contributor
timestamp: November 30, 2012, 12:09:35 pm
content: You're right, and here's something worse, just checking the manual the input impedance in dual mode on the 1252a is 1 Mohm! I have the smart ohm functions on the 1272a but again, most of the time in the lab I don't use anything but the basic functions of the DMM, so in the end the Fluke gets as much use as the 1252a.Quote from: mzacharias on November 29, 2012, 08:38:26 pm