Electronics > Metrology

What is the next best ratio measurement device after the 3458A?

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Dr. Frank:

--- Quote from: zlymex on September 20, 2016, 12:19:27 am ---
Good advice. A very common failure of 3458A is the 'big bad chip'(U180 on A3 board). Here is another photo of collection of the replaced U180, most of which result in halt of the meter.
--- End quote ---

Hello zlymex,

can you please unfold, which error messages are related to the failure of U180?
What do you think, is the real root cause inside U180 to let the 3458A halt?
Maybe excessive drift of its internal resistors, so that certain ratios of these are outside limits?

Is error 202 'Slave Test: Convergence' also related to such a failed U180?

Thank you
Frank

Dr. Frank:

--- Quote from: e61_phil on September 22, 2016, 02:58:25 pm ---
Just to get it right: If I would build a bridge with a AD5791 (specified INL 0,19LSB ~ 0,2ppm) and measure the residual voltage. My uncertainty in a voltage transfer (for example the voltage measured on a 10k resistor (well known) and a 1k resistor (unknown) 1:10) would never be smaller than 2ppm? Is it that simple?

--- End quote ---

Sorry, the AD5791 is specified as 1ppm INL plus 1ppm DNL; both have to be taken into account.
It's the stability of the INL only, which is 0.2ppm.

Please check hpj4/89, page 22 and following, about the 10:1 transfer calculation related to DNL and INL errors.
Therefore, you can't expect better than about 11ppm 10:1 transfer from that AD5791.

If you characterize some ratios with a 3458A, I really doubt, that this would be a stable solution, due to varying code, noise, temperature influence (specified???), and so on.

So, if you need a 10:1 ratio only, instead of random ratio numbers between 1.0 and 0.1, better build yourself a simple, fixed 10:1  Hamon  divider, with self-adjust capability. (6 stable PWW resistors required, only)

This will probably give better 10:1 transfers (at low voltage levels) than even a 3458A, <= 0.2ppm of output is possible.

Frank

zlymex:
Hello Dr. Frank,

There are 11(out of 13) chips were marked "??" in Chinese which means dead/halted in the photo I posted. This is the most serious error that U180 may cause to an 3458A. The halt may be occasional or only on DC range(according to the hand writing on the chips).
There is one marked as "AC measurement Ok", which probably means DC will be halted as well.
There is only one saying "workable but large tempco", I assume there is no error message associated with it.

I've just asked the one who provided the photo(thy888), he said they were all halted with error message 114, and the auxiliary message being either Multi-slope ADC error or Time Out error.

There are two root causes, one is the severe mismatch of resistors, the other is the bad input switching JFETs of the ADC. There are two paths for the signal, one is the high resolution one(50k resistor) the other is the high speed one(10k resistor).

I once try to fixed an 3458A with large daily drift but failed, there is no error message, just like mine. At the end, I suspect it was due to the U180, had it replaced, and the meter is Ok. There was a thread at 38hot that record my work: http://bbs.38hot.net/thread-436-1-1.html  http://bbs.38hot.net/thread-4104-1-1.html

For that error 202(Slave Test: Convergence), it is caused most probably by open JEFT(cannot be closed) of the high resolution switch for 50k resistor, but the high speed JFET switch will probably work, this can be verified by setting the NPLC to a very small value(so that only high speed path is used) and see if the meter will work. And yes, those 10k/50k resistors and JFET switches are all part of the U180 chip that cannot be replace alone easily.

Edit: add a pic from HPJ 1998-04

Kleinstein:
Likely they will not reuse the old U180. It's failure prone and likely quite expensive. Today would likely not us a single converter for high speed and super good INL, as there are rather good fast converter chips (LTC2378.. and similar) that could be used for the very fast tasks and AC.

They may still end up with a new similar chip though, but if they don't need high speed this could be a simpler version or than could get away with separate resistor array and switching chip, maybe even of the shelf parts. My guess would be more like continuous integration, integrator with AZ OP and than an ADC chip for the residual charge - so more like in between the 34470 and the Datron 1281. One thing they should really add is a temperature controlled fan and better RF shielding (way more cell phones and WLAN today).

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