Category Archives: HP 3455A Digital Voltmeter

One of the first microprocessor controlled digital voltmeters ever – and still in service!

HPAK 3455A Digital Voltmeter, 34401A Digital Multimeter: switch-on behavior

It is often assumed that high precision instruments are better not switched off but left on running 24/7, just because it is to tedious to wait for the specified “warm-up” period. Mostly, this seems to be more of a habit than a real requirement, at least for multimeters and their build-in references. Let put it to a test.

With all good references back in Germany, we first need to fabricate a little substitute, a +5 V reference based on a MAX6350. This has roughly 0 ppm tempco around ambient temperatures, and +-0.02% initial accuracy.

max6350 5 v reference

Some caps were added to keep noise down.

With this reference fully ‘warmed up’, two instruments were used to monitor the output, a 34401A, with current calibration, and the recently repaired 3455A, with about 4 years old calibration.

The result:
3455a vs 34401a switch on

The 3455A provides 10 µV resolution, in the 10 V range. The 34401A – the displays shows 6.5 digits, same as for the 3455A, but the build in resolution, when read through GPIB, is about 1.3 µV.

As you can see, there is really no warm up. It is stable to ppm level, after a few seconds.

The offset of the two meters:

3455a switch on

About 4 ppm difference – not too bad; certainly, well within spec of these two instruments.

HPAK 3455A Digital Voltmeter: some cleaning, and a really low drift reference

The 3455A is a really nice unit if you are looking for a digital voltmeter/multimeter that is really accurate, and if you don’t want to spend too much. These units go for about USD 100 to 150, not bad, considering their performance – 6.5 digits.

This unit just needed some de-dusting, and was missing some screws. Build some time early 1979 (judging from the data codes, rather than the serial number), this unit doesn’t seem to have hand any issues over the last 35 years – while in service at Northrop Grumman.

Last calibration:
3455a cal sticker

Did a quick check vs the best reference I have in the lab, a 34401A.
3455a check

For a bit more through test, the 3455A was monitored for about 1 hour, with a low noise 9 V source at the input, and the differences logged.

3455a cal offset

Apparently, the 3455A is about 6.7 ppm low, vs. the 34401A. Which one is right, I can’t tell – the 34401A stability specification is about 20 ppm, after 90 days, far worse than the difference of the two meters.

The 3455A has a 11177B reference assembly. It has option S01 – no idea what option this is. HP P/N 03455-66520 Rev B.

The active part, a HP 1902-0926 thermostated Zener – very similar to the LM399, but not in the typical isolated (white) case. One thing is for sure, this part is now well aged!

3455a ref assy

3455a 11177b s01 ref assy board

The reference assembly has some remarkable 0.01% tracking resistors, with 1 ppm tempco – it doesn’t get much better even nowadays, and the resistors are in hermetically sealed cans, with glass seal, except one 1.0020 k +-0.01% resistor in the red resin package.

AC measurements were also carried out in the 10 Hz to 500 kHz range – it seems the 3455A even outperforms the 344401A for flatness, especially at the low end of frequencies.