The HP 3458A DMM

Just a few notes on the Agilent 3458A Digital Multimeter:

The new hp 3458A was described in the April 1989, HP Journal. [1]

Serial Numbers: Note that while a general rule of thumb with hp instrumentation is that the S/N prefix plus 60 gives the year of manufacture, the prefix appears to be more of "series time frame" for many models. So, for example, the hp 3400A prefix series 2415A (some from 1986 and 1990) presumably began in 1984 (24+60).

A particularly good example of the exception to the rule is the hp 3458A, where instead of the traditional four digit prefix, on at least some units, "US280" was the prefix (Not, for example "2803"), with the numbers that followed, the specific units S/N. For example, numbers were over 32,000 by the year 2005. Close inspection of the S/N tags on some units shows that the US280 was separately colored or highlighted on the label. Earlier serial numbers appear to conform to the hp standard numbering system, e.g. 2823A 0nnnn for 1988 or a 1988 series. 2823A 19100 is another example from the mid to late '90s (so the "2823A" series, presumably beginning in 1988, ran through the 90's).

Earlier S/Ns can be identified in part by the fine meshed cylindrical dust filter over the fan opening on the back of the unit. Later units had a simple grill and sheet of filter material. Earlier serial numbers appear to conform to the hp standard numbering system, e.g. 2823A 02nnn.

AutoCal: In our experience, many hp 3458A users do not realize how important the auto-cal operation is. Auto Cal is not a "calibration event". In normal use, Auto Cal is performed regularly, at least every 24 hours, and preferably for every change of ambient temperature > 1 degree c (for highest measurement accuracy, yet another reason why metrology labs are typically kept at a relatively constant room temperature). If you are only working in DCV, our understanding is that it is okay to just do AutoCal DCV (instead of the more time consuming AutoCal "ALL"). If you find you need to use AutoCal more often, you might have a bad A3 board in need of replacement.[2]

Service Notes: Be sure to review the hp 3458A Service Notes!

The amazing! hp / Agilent 3458A (we have had the good fortune to work with many over the years since about 1990), can measure AC volts to a very respectable noise floor in the SetRNDM AC mode. With the input terminals shorted, we typically found noise floor values from about 14 uV to 25 uV. If you get a chance to check yours, please let us know! (joegeller -at- gellerlabs -dot- com)

If you are moving up to a hp 3458A from a hp 3456A, note that it will take some time to get to used to the new command set (esp. via front panel button operations). The command set was very IEEE-488 (GPIB) centric and in our opinion, not very user friendly. However, given time, and good use of the custom programmable front panel function buttons (F1, etc.), new 3458A users will see what a remarkable instrument the hp 3458A is. It is well worth the time to learn it by a combination of studying the manuals and lab practice.

Undoubtedly, some day there will be a 34***A series meter to replace the 3458A from ~1989, however for now, in our opinion, it still stands alone as the greatest DMM ever made in production quantities! So how long will the 3458A be around? The prices (not adjusted for inflation) of three known relatively long lived instruments were plotted versus hp catalog year PDF. The hp 5359A Time Synthesizer (an ultra high accuracy time delay/pulse width "pulse generator") shows a rapid change in price increases towards its last years, no doubt related to the production design style as well as demand. Closer to home (DMMs), the hp 3456A 6 1/2 digit DMM shows a similar increase in slope of the price curve as it came to an end. It is difficult to draw conclusions based on the graph, especially since we are looking back 31 years, past the current generation of managers. Yet, if the slope of the price curve can be used as an indication of product run lifetime, the 3458A might be around for some years to come (assuming no significant competition or another production problem(?) such as indicated by Service Note 3458A-18A [2]).

1. 3458A Digital Multimeter, HP Journal, April, 1989.

2. After we sold our late model 3458A (badly missed, but temporary financial realities prevailed), while trying to understand the 3458A serial number system, I came across the 3458A-18A Time Drift issues Service note from 2007-02-13. Mostly I used our 3458A in recent years in conjunction with the FLUKE 732B after long times of power off, so I did not look into its short term (24 hour stability). However, apparently something went wrong on a series of the late model units (which is remedied by return and replacement of the A3 board). Apparently for a time (inferred from the service note), there was some problem (a manufacturing problem and/or part failure or substitution?) with A3 card production .

Generally, when asked for advice, I recommend purchase of later S/Ns since so many of these fine DMMs were run 24/7 with open input terminals causing high rates of relay state changes (e.g. from autoranging relays changing state with the hi Z input and open terminals).

However, with some later S/Ns as per Service Note 3458A-18A, if the drift rate is greater than 0.43 ppm per day, you might be looking at an additional A3 board swap out. hmm, sometimes newer might not be better. (Also, like the 3456A, occasionally there could be some trouble with the mechanical front-rear terminals mechanical switch, particularly where it was never moved in twenty years. It might be possible to spray some contact cleaner (e.g. Deoxit Gold or D5 then P5) into the switch, not sure.)

hp manuals and related catalog pages are reproduced with Permission, Courtesy of Agilent Technologies, Inc.

COPYRIGHT © 2011 JOSEPH M. GELLER