HP/Agilent/Keysight 34420A NanoVolt/Micro-Ohm Meter repair

[plesa]

BTW anyone have an actual 34103A short that they don't mind taking the case off? I'm reluctant to pay $100 extra to have someone solder a couple of copper links into a FVN.1S.304.CLYC37Z Lemo. I was expecting this to be a crimp connection, but it looks like 'solder bucket' to me.

TIA, Alan

Of course. It is soldered. I cannot check the solder used, maybe later. For investigation I have also the input connector which is also soldered.

[alanambrose]

Ah thanks so much - all the contacts are joined rather than just the two pairs?

(edit) Duh, of course, for 4W ohms...

[Goff]

Hello,

I'm new to the forum here and

already have a difficult question.

Recently bought a "new" 34420A directly from Keysight Germany.

Question: The meter has an inherent noise,

it buzzes (not loud but not quiet),

can anybody confirm this?

Here are a few dates:

Keysight 34420A

- Green Display, white Keysight Logo

- Date of first Calibration: Jul 2016 in Malaysia

- Revision 10.0 - 5.0 - 3.0


Thanks in advance for any answer!

with friendly greetings from Germany,

Goff

[HighVoltage]

Hello Goff and welcome to the Forum.

The 34420A should not make any noise. The instrument does not have any fan (like the 34401A) and the noise that you describe might be resulting from a noisy transformer.

I would contact Keysight in Böblingen and I am sure they will exchange to instrument to your total satisfaction.
 

[plesa]

My meter is silent.Does not matter if new one or 20 years old one.
 Maybe the transformer shielding is not properly fixed.

[Goff]

Hello plesa,

thanks for the information / answer.

... HighVoltage - Yes, the complaint was made.

[MegaVolt]

Good afternoon.
Is there a description of the change history of firmware 34420a?
Has anyone seen on the network firmware files newer than 5-5-2?
Are there any instructions and successful experience updating the firmware?

I would be grateful for any information.

[HighVoltage]

My two 34420A have following revision numbers:

Agilent
SN:   MY42004xxx
REV: 10.0-5.0-2.0

Keysight
SN:   SG42000xxx
REV: 10.0-5.0-3.0

I would have expected a unit with higher SN to have a newer FW Revision.
However, I do not see any differences in the functionality of the two instruments.

[MegaVolt]

My two 34420A have following revision numbers:

Agilent
SN:   MY42004xxx
REV: 10.0-5.0-2.0

Keysight
SN:   SG42000xxx
REV: 10.0-5.0-3.0

I would have expected a unit with higher SN to have a newer FW Revision.
However, I do not see any differences in the functionality of the two instruments.

I met mention of a maximum belief of 10-5-5.

Do you have the opportunity to read the contents of the flash?

[branadic]

Sorry to pick up that old thread, but has anyone ever created a firmware dump? I'm particulary interested in the firmware Rev3.0.

-branadic-

[branadic]

I was sensitized to check my 10 V range with a battery powered LTZ reference to see if everything works as it's supposed to work. There was strange behavior reported by someone on a unit that was in for service. So I monitored the unit as suggested and the unit I have seems to do well. No excess noise, no strange jumping around, everything seems to be fine which indicates that my unit was a bargain.

Edit: The test was performed at 10 NPLC and the filter turned off.

Code: [Select]

hp34420A.write("CONF:VOLT:DC 10;:SENS:VOLT:DC:NPLC 10;:INP:FILT:STAT OFF;")  # 10V range, NPLC 10, Filter off
-branadic-

[HighVoltage]

Looks nice!

Now you need to do the same test on the other side of the spectrum in the nano volt rage with a stable 1 nV source.

[alm]

a stable 1 nV source.

I think that's called a short

[Kleinstein]

The stability looks OK. Much of the lower frequency noise is likely from the LM399 reference in the 34420 and thus the popcorn type jumps of some 0.4 ppm.
The 10 V range is a bit noisy, but that is a limitation of the ADC and there is not much that can be done there. This is not really a problem for the low ranges (1 mV and 10 mV), where this meter is good for.

A test at the other end of the range would be either a short, or a 1 mV/ 10 mV source to be near the end of one of the lower ranges.
The more interresting part would likely be the short and maybe some extra series resistance (e.g. 10 or 100 K) to see if there is drift / noise in the input current.

[HighVoltage]

a stable 1 nV source.

I think that's called a short

The Keithley 260 Nano Volt Source is your friend for such measurements.
Range: 0.01 nV to 1.11 V

I can dial in 10 nV stable on my 34420A with the Keithley 260.

Maybe we can call 0.01 nV a short.

[TimFox]

The last voltage divider stage in that Keithley source is made of copper, despite its temperature co-efficient of resistance, in order to minimize thermal emf.

[alm]

What's the noise like at 10 nV from the 260? The 34420a is specced for 1.3 nVrms, which I imagine is under absolutely ideal conditions re thermal gradients and cleanliness of connections.

I actually had the Keithley 260 in the back of my mind when I wrote it, but I was in doubt even that can deliver a stable 1 nV outside a physics lab.

[MegaVolt]

Since both resistors in the dividers are made of the same metal, their temperature coefficient is compensated.

The noise level is determined by the output impedance. On the nV range, this is only 1 ohm. This is very little noise.

[Dr. Frank]

Two years ago, I had a 34420A for repair on my bench.
Using the same configuration and settings as branadics, w/o filter, it showed random excessive noise, triple the usual level, i.e. rms 7.6µV vs. 2.6µV.
It had the same FW version 2.0.
I measured stability of its reference, at the output of the input amplifier, with different GPIB adapters, and so on, and in the end could not identify the root cause. With branadics measurement, I now can rule out a FW error.
The 2nd graph shows the calculated digital filter, like it would be implemented in the 34420A.
Frank

[HighVoltage]

What's the noise like at 10 nV from the 260?

I don't remember and would have to measure this again.

The Keithley 260 is well built inside for low EMF and low noise.
Look at these impressive copper connections..

[okti]

Here's my trusted old HP34420A at DCV10 NPLC10 FILTEROFF (source: LTZ1000) measurement fresh from the oven:

[okti]

Same meter, DC1mV NPLC10 FILTEROFF with short:

[branadic]

The 10 V range is a bit noisy, but that is a limitation of the ADC and there is not much that can be done there. This is not really a problem for the low ranges (1 mV and 10 mV), where this meter is good for.

Would you mind explaining why you think that the ADC is the limiting factor here instead of the reference? To put some numbers to it, LM399 is typ. 7 µVrms, max. 50 µVrms noise (values for LM399, not LM199 in LT datasheet). Take that times ~1.4 for the +/-10 V reference voltage in the unit, which is the typ. 10 µVrms that we are familiar with. The noise I see is smaller than that, indicating the noise of LM399 is smaller here.
Has there been any report to the noise limit of a multislope III ADC, like feeding it with lower noise reference such as LTZ, that I've missed?

-branadic-

[Kleinstein]

The lower frequency noise (after the 50 points filtering) is to a large part the LM399 reference, though there could also be some 1/f noise from the resistors.
The higher frequency noise seen for the single 10 PLC conversion looks quite high for a LM399 reference:
I have looked at my test with an external LM399 reference and LM399 ref. at the ADC and there I get around 6 µV_peak to peak in between the popcorn events even with 1 PLC and some 12 µV peak to peak including the popcorn noise / longer time scales. 2 x LM399 reference at both ends would be comparabele to the 7 V scales up to 10 V. I have some reference filtering, but this would no longer be effective for 10 PLC and only 1 reference.

The reportet noise for the 34401 that I found is somewhere around 2-2.5 µV RMS ( 12-15 µV peak to peak) for 10 PLC and I don't see much of the noise coming from the input stage, though there is a little. So this would explain much of the observed noise.
I would be easy to measure that noise with a short.

I have not seen much theoretical analysis on the 34401 type ADC noise, though I have just done some calculations on that. There are a few uncertainties, and my calculations still get a slightly lower noise ( some 1.4 µV RMS) from the noise sources I included. The noise is not just one large noise source, but more like a combination of many sources. From my analysis the worst offenders are the current noise of the OP27 in the integrator, higher frequency reference noise, jitter and quantization noise. In theory one could improve on the current noise and a little on jitter, but not with many other noise sources, especially not the quantization noise. 

There is one unknown in how the ADC is handling the first / last reference steps - that is the reference signal just before the auxiliary ADC is read. This part naturally will not be 100% effective and there is a chance that this factor is not perfectly treated in the firmware. This would be some extra discrete size errors, that can look rather random, though also changing with the input voltage. So some readings may be effected more than others.
Ideally there would be a procedure in the factory calibration to minimize this error part, but I am not so sure they actually do this and if so, that factor may also change over time.

[branadic]

I searched the web and found that report here: bbs.38hot.net/thread-69139-1-1.html where someone replaced the LM399 in 34401A by an LTZ1000 and noticed an improved noise behavior as well as improved t.c.. As 34420A is a somewhat pimped 34401A chances are that replacing it with a lower noise ADR1399 will slightly improve noise in 10 V range without to much hassle.

-branadic-