Low level AC measuremets with HP 34401

torsdag den 23. december 2021 kl. 02.54.33 UTC+1 skrev Reinhard Zwirner:

google says, ..

That DVM has a huge amount of noise kicked into the front end from the VF display. They hid that by zeroing AC measurements much below 1 mV.

I like the Fluke better.

Reinhard Zwirner wrote: ==================

** Amplify your signal prior to the DMM. By 10x or 100X using say a wide band op-amp.

I use a 1000X pre amp for low level noise measurements.

..... Phil

It's also available in HTML format at:

Danke,

Try:

** FYI:

I have a 4.5. digit ( 20,000 count ) DMM that employs an AD536 rms-DC converter - cousin of the AD637 in the HP. The converter IC has bandwidth inversely proportional to the input voltage, so 10% of range is a good place to stop thinking it very accurate. Using one with a 4.5 digit meter is pushing it, so 6.5 is off the map.

Suspect the HP already has 10X and maybe 100X gain stages internally, prior to the converter. In any case, an external pre-amp is the go.

..... Phil

The 34401A was my first bench DMM. Bought it in 1998.

I have never calibrated it, could not afford it

This year I went the consultant route, so bought 3 Siglent DMMs. Just for fun, I connected the same references voltage to the 3 Siglent and the 34401A They measured the same except for minor last digit difference

Not willing to spend on calibrations, the next best thing is to compare to other instruments. Quite unlikely that they would drift in the same direction ?

By the way, I paid a lot for the 34401A, now I can get 4 Siglents for the same price

I like the 34401, even with some of the faults. As you've seen they're basically accurate forever. Not sure how the new keysigh stuff compares for long term reliability.

At the place I worked before they used it per default. I asked the instrument responsible and he said when they were sent for calibration they were never out of spec. Not even a single instrument. So they prolonged the calibration interval to 2 years

We use mostly Keithleys in our test stands. They're OK.

I really like my Fluke 8845A. It's the winner when you want to measure a few microvolts.

Calibration is mostly just a racket, although I completely agree you need to check instruments to make sure they're not broken. I have the hardest time with stuff like capacitance meters.

As for calibration of things like rulers, I confirmed that yes, I can hand draw a 1 foot ruler that's only 11 inches long on a paper towel and sent it it for calibration at a metrology lab, and that with the calibration chart it would be be considered accurate and for a few bucks more I could claim NIST traceable for my fine instrument. Would it be useful? No, as long as all the errors are documented, it's better than a Starret scale with no paperwork, at least in the eyes of retarded places ike NASA.