Using scanner cards for precision 10V measurements.

[mimmus78]

Hi guys,

I got one of this old 3488a scanner cards controller with a few multiplexer cards inside to monitor my LTZ1000 references.

The problem is that all this is engineered more for static setup and you are not supposed to fiddle to much with the wiring.

Connections are on the rear of the instrument, you need to undo two screws for getting out the connector, than you have to use the screw driver again for each wire.

To get my life easier I was thinking of building a connection box with copper binding posts, and connect the multiplexed output on the read input of my 3458a.

Unfortunately it seems I have hard time in finding copper binding posts for a cheap price and forking few hundred EUR for 20 or 40 copper banana binding posts seems not a good idea.

Any suggestion how to get it simple without spending too much?

[TiN]

Bit unclear why you want to fiddle with the wiring, or add any more uncertainty by extra connections?
You feel like measuring DCV to <10ppm accuracy is not already hard enough? 

Here's little breakdown:


Test 1, Keithley 7168 + LTZ modules + 2x3458A differential. Start from March 13 till March 20.


Test 2, Keithley 7168 + LTZ modules + 2x3458A differential. Start from March 21 till March 30.
Using same wiring as test 1.

Test 3, from April 4.. wiring change to shielded wire.


Looks all nice and pretty, yea? Well, look closer. Let's compare same modules readings, with some channels swaps:

LTZ1000ACH,       HP 3458-A9 STD board, LT1013, 15K/1K standard = T1: 7.18462793, T2: 7.18462959 (+0.23 ppm), T3: 7.18464116 (+1.61 ppm)
LTZ1000ACH Jumpy, KX B1 board,          LTC2057,13K/1K          = T1: 7.07259966, T2: 7.07259549 (-0.58 ppm), T3: 7.07258331 (-1.72 ppm)
LTZ1000CH,        KX A0 board,          LTC2057,12.5K/1K        = T1: 7.13666880, T2: 7.13666914 (+0.04 ppm), T3: 7.13668100 (+1.66 ppm)
LTZ1000CH,        KX B1 board,          LTC2057,13K/1K          = T1: 7.13663661, T2: 7.13664779 (+1.56 ppm), T3: 7.13666121 (+1.88 ppm)
LTZ1000ACH,       KX A0 board,          LT1097, 13K/1K          = T1: 7.12982613, T2: 7.12982551 (-0.08 ppm), T3: 7.12982924 (0.52 ppm)

So from above should be clear that we can pretty much forget about ppm-accuracy with scanner, even though readings well in 0.5ppm window if you don't mess with scanner/wiring/setup and just let it sample data

And this is with specialized nV-scanner card with JFETs & isolation, with copper-copper connections using single freshly cut wire (soldered to LTZ module side, crimped to card input port on scanner side). I'd expect general purpose scanner with relays would be 10 times worse at best.

[Assafl]

What about connecting channels in parallel? (At least on the 7001 it is possible to turn on groups of relays).

As far as relay contact resistance - that should go down; but as far as PPM? Will EMF & other noise go down?

[e61_phil]

Did you check this site: http://blog.scottlabs.org/?p=21 ?

He uses a setup where he can change polarity to reduce the effects of thermal EMF

[ManateeMafia]

Unfortunately, the scottlabs equipment is no longer in use. I emailed him once and I think some other issues took priority. He was helpful in allowing access to his data.

Depending on how accurate you want to measure the references, the setup can certainly be difficult to properly measure.

I am currently scanning 8 10V references (soon to be 9) using a Windows PC, Data Proof 160A, and a HP 34420A. The data is not what I consider metrology grade but you can see from the plots that noise and sudden issues with power and stability can wreak havoc on your data. I am tweaking the setup. I would like to get a 3D printer to make some shields that fit over the binding posts to block air flow.

When I first set it up, I had an issue with extra outlets. I plugged the 34420A into a separate outlet and had strange results. Moving the cord to the same outlet fixed that issue.

The 732B that is drifty has been that way and is in possible need of a "referencectomy". I have some spare parts for that and it may turn into a teardown/repair. The bad 732A is in need of some monitoring of the reference oven temp. It almost looks like the oven is not working which is a common fault with these models. I will have to write some code to add a 34970A and measure the oven thermistors. I should have done it sooner.

The scaling may not be what you are used to and the program has been a WIP for the last 12 months. The dashed lines are 1ppm divisions. I already have a request to change the plots for improved detail so that will be addressed.

[mimmus78]

TiN I think maybe you need to reconsider the most classical (rel)way.

I measured max 2uV between all 10 channels of a 444470A mux module. Most of the channels are +1uV above what I can measure by connecting my LTZ1000 reference directly to 3458A ... pretty down the noise.

Settling time is also good, if you don't switch very often, the third read at 100 nplc shows that channel already settled to uV magnitude. What I reproduced also confirms the specifications of this module reporting a max offset of 4uV.

I know all this convoluted setup can upset your measurements, but I can trade some accuracy for a more handy setup. Especially having the possibility to compare many many modules is quite interesting to me at this point.

Anyway if you want the best measurement even connecting directly the LTZ1000 to 3458a may not be good enough (remember me to pimp down my 3458a reference, this 1 or 0.5 ppm random histeresys I have is pretty annoying).

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