Valhalla 2720GS

[maxwell3e10]

Well, this makes it easy: 25 60 02 12 58

[Kleinstein]

A capacitance measurement should tell which shield is close to which winding: e.g. the shield to PE should have the largest capacitance to the primaries.

[branadic]

The primary winding is the inner one, so you can tell which of the screens is the inner one too (the closest to the core), which has to be connected to earth. The other two screens are physically tied together on the mainboard, so it doesn't make a difference to which solder point you are connecting them.

-branadic-

[branadic]

Here are results of my Valhalla2720GS-HSR-ADR1399 running for almost 2200 h.
One might be suprised by the bad behavior, but that can be explained. The measurements suffered from EMI of what I called a scream cube (a 1 m³ fine particle filter that was running near one of our test benches) nearby. This scream cube was switched off 11 days ago and we can see how temperature in the lab stabilized and output noise went down to a reasonable level since then. We can also see some jumps that resulted after performing internal cals of the Valhalla.
Since the scream cube will not be turned on again, the related project has ended, I guess it makes sense to repeat the noise measurements, but also to further observe how stability turns out from here.

-branadic-

[branadic]

An update of my Valhalla 2720GS/HSR-ADR1399. I'm approaching 5000 h of operation after my repair, modifications and upgrade. Since removing the "scream cube" from the lab the extreme spikes disappeared and never showed up again by now.
At some point I had to readjust the oven settings of my copycat reference board 4, as the reference board was continiously turned off by the unit due to an "oven high" failure. The components and thus the voltages had drifted quite a bit. Since I made the adjustment the board was never turned off again.
What I observe are some large intermittent deviations, which I think are due to the units internal intercomparison of the references, as they appear at least at around 12:00 pm. I can tell, as ACAL of 3458A happens every day at 8:00 am, hence the gap in the data.
The location of the equipment near the AC in the middle of the room is far from ideal and the south side of the lab with its mirrored glass window front isn't perfect either, but it is how it is. There is clearly some temperature depence that needs to be ruled out.

I also received data of an INL measurement of another 2720GS against a Datron 1271 from some fellow voltnut that I attach. Again, results are disappointing given the claims Valhalla made: "The 2720GS delivers the performance required to calibrate the world's most accurate 7.5 and 8.5 digit DVM's yet does even more."

-branadic-

[branadic]

As already stated, I replaced all 6x LM399 references in my unit and also equiped my copycat reference board with them too, so a total of 8x ADR1399. Unfortunately, noise of the instrument didn't improve as I have hoped for. So I looked into the schematics https://xdevs.com/doc/Valhalla/2720GS/Valhalla_Scientific_2720GS_Vol.%202_sm.pdf and noticed IC202, IC315 and IC4 in the chain, all three of them being ICL7652CPD. So I today performed the exercise of removing them one after the other, replaced them with a socket and fitted a single AD4523-1 in each position, with the following outcome:

IC202: noise didn't change at all
IC315: noise went up by a factor of ~2.5
IC4: noise went up by a factor of ~10

The instrument is now back on ICL7652 and noise is back where I started (8-9 µVpp at 10 V output, 0.1 - 10 Hz). The only thing that was successful today, while I was at it I tightened the Cal keyswitch.

-branadic-

[Kleinstein]

The ADA4523 is a rather extreme case: low voltage noise, but really high current noise and thus not suitable for more than some 10 K of source impedance. In addition the ADA4523 may be more picky about supply decoupling and a bootstrapped supply and has a slight chance to oscillate. The higher speed could also be an issue. A more moderate replacement would be a LTC2057 or AD8638.

The ICL7652 are not very low noise by todays standard, but should still be way better than 8 µV_pp for a 0.1-10 Hz band. It is still not clear how the noise was measured and especially the upper BW limit.


A point to check could be the noise with different voltage settings. E.g. with the output set to 0 V still is the upper range should not include ref. noise. One could also look at the DAC output directly, without the output amplifier/buffer.

[dietert1]

You may have a look here once more: https://www.eevblog.com/forum/metrology/valhalla-2720gs/msg5040739/?topicseen#msg5040739.

Regards, Dieter

[branadic]

It is still not clear how the noise was measured and especially the upper BW limit.

Output set to 10 V, measured with a 0.1 - 10 Hz LNA, 80 dB amplification by pipelie.

You may have a look here once more: https://www.eevblog.com/forum/metrology/valhalla-2720gs/msg5040739/?topicseen#msg5040739.

Thanks, that is something I could look at.

-branadic-

[branadic]

When looking at the Valhalla schematic i found the 10 KOhm averaging resistors bad. In my LM399 arrays i have been using 274R resistors and i think one should replace the 10K resistors by 1K. I think they used 10K since it is a 2-wire circuit, but one doesn't need full precision for the averaging coefficients.

Did that, but without a change in terms of noise at the 10 V output (13 V range that is). Haven't yet checked the 6.5 V range, but this sort of proofs that the noise is generated somewhere in the chain after the references have been averaged.

-branadic-

[chuckb]

The data sheet for the ICL7652 lists the current noise at 0.004 pa / rt Hz. This number is most likely not a measurement. It may be a shot noise calculation based on Bias current.
The LTC2057 looks like a good replacement with 0.2 uVpp noise. I have not calulated the output noise from the opamp current noise of 0.12pa / rt Hz. But the LTC2057 has the lowest current noise, of the low voltage noise chopper opamps.
The ouput noise could be from residual PWM current spike noise or another grounding issue. My latest project took several revisions to get the grounding correct. Just going from memory I believe the Datum 4910 PWM filters had more sections.
Good luck!

[Kleinstein]

There are OP-amps with less current noise than the LTC2057. It still has a good combination of current and voltage noise, especially among the higher voltage ones.
Chances are the OPA186 should be lower current noise (at least that is what the data-sheet says), but is also higher voltage noise.
The ICL7652 should be low current noise, maybe not all the way to 4 fA/sqrt(Hz), but it may not be that much worse.
Current noise specs with AZ OP-amps are anyway not very reliable: quite some false ones, calculated from the bias current and even the real value can depend on the conditions of use. There is a good chance to have correlation in the current noise for the 2 inputs - so a single number is not enough to describe it fully and there are different ways (resistor at one side of both) to have the resistance to test.

The reported noise still looks higher than expected for the ICL7652 - so there are likely other sources.
There are different stages that could be followed to see if the higher than expected noise is alread there at the reference, the PWM DAC or only at the output stage.  Ripple in addition to noise a possibilty.

[branadic]

The 0.1 - 10 Hz at the 6.5 V output appears to be similar to the noise of the 10 V output (13 V range). So one of the things to look at is the noise of the averaged reference voltage.
Someone suggested to stop the DAC by pulling the jumper PL201 connected to the 16.777216 MHz clock at the reference generator.

-branadic-