Also - whan a 3458a is sitting in a semiconductor test rack, I suspect it is using its other skill - super fast lower resolution measurements. The ADC errors won't show up there.
But that was in the days when you could get a replacement A3 board.
Also - whan a 3458a is sitting in a semiconductor test rack, I suspect it is using its other skill - super fast lower resolution measurements. The ADC errors won't show up there.
That is also what I think. Most people wouldn't even notice an ADC problem. They will run ACAL before use and everything will stay very well within the 8ppm specification (and a 3458A can do much more than 10V measurements and for the other ranges the ADC drift is even less important).
......
If the new A3 board is really 100% compatible, and it sounds like it is, I would guess you can buy new boards for your old instrument soon.
Best regards
Philipp
I'd be curious to know what a new A3 with CAL from Keysight would cost? I'm guessing it's not a normal hobbiest fee? At least when they sold A3 boards you could buy one put it in and then work a hobbiest CAL with some help from other voltnuts or against your own 10V references and 10K Ohm reference.
Bill
The drift is only the first visible symptom of a failing U180. Changes are a drifty ADC will after not so long time (e.g. a year) show error messages about ADC convergence. The faster conversions are actually more sensitive to the resistor ratios - so DNL errors may be become visible too. I would not be surprised if this would be earlier with the fast modes than with the higher PLC modes.
The reports are that keysight is currently not selling individual A3 boards, but just replace it as part of a repair. This could be to check for possible compatibility issues with older / mixed units. For serious use of the 3458 one would need a new CAL after the ADC board replacement anyway. So sending in the meter for repair is not that much extra hassle for most customers.
The drift is only the first visible symptom of a failing U180. Changes are a drifty ADC will after not so long time (e.g. a year) show error messages about ADC convergence. The faster conversions are actually more sensitive to the resistor ratios - so DNL errors may be become visible too. I would not be surprised if this would be earlier with the fast modes than with the higher PLC modes.
The reports are that keysight is currently not selling individual A3 boards, but just replace it as part of a repair. This could be to check for possible compatibility issues with older / mixed units. For serious use of the 3458 one would need a new CAL after the ADC board replacement anyway. So sending in the meter for repair is not that much extra hassle for most customers.
Switches
A last major concern for the ADC design was the switches
required to control the inputs and the slopes. Because the
switches are in series with the resistors, they can add to
the temperature coefficient of the ADC. A custom chip
design was chosen so that each switch could be scaled to
the size of the resistor to which it is connected. This allows
the ADC to be sensitive to the ratio-tracking temperature
coefficient of the switches and not to the absolute temper
ature coefficient. Another advantage of the custom design
is that it allows the control signals to be latched just before
the drives to the switches. This resynchronizes the signal
with the clock and reduces the timing jitter in the switch
transitions. The result is a reduction in the noise of the
ADC.
He tried to understand the problem better, but did not arrive at a definitive conclusion about what is the typical problem inside a bad U180.
My 3458A is in mint condition. It had been advertised as never used. Upon receiving it, all original parts and packaging were present. Not one mark, ding, scratch, or dust in the filter. But, it seems to fall short of the "golden units" described by TiN and others on the forum.
My voltnuttery has led to the construction/purchase of four LTZ1000 references. It would seem they are noticeably better than the DMM. A spreadsheet and graph are included below.
The DMM uncertainty (36.4 uV @ 10V) is specified in the calibration report that was part of the package. The meter is branded Agilent and the date of manufacture is 2011.
QuoteHe tried to understand the problem better, but did not arrive at a definitive conclusion about what is the typical problem inside a bad U180.I did arrive to definitive conclusion, but I thought it's obvious enough already and didn't feel repeating it. If one cares for meter to meet spec, SN18 is official as it gets. If one wants better than specs, well, you are on your own hunt for magical 0.0ppm/yr system.
Feeling like every year people here refresh and have this kind thread about u180, repeating same things over. I remember Dr.Frank explained these same things at least 3 times by now.
My 3458A is in mint condition. It had been advertised as never used. Upon receiving it, all original parts and packaging were present. Not one mark, ding, scratch, or dust in the filter. But, it seems to fall short of the "golden units" described by TiN and others on the forum.
My voltnuttery has led to the construction/purchase of four LTZ1000 references. It would seem they are noticeably better than the DMM. A spreadsheet and graph are included below.
The DMM uncertainty (36.4 uV @ 10V) is specified in the calibration report that was part of the package. The meter is branded Agilent and the date of manufacture is 2011.
We have been using HP 3456A for years and years. Those don't have that ACAL procedure to check the ADC, so all ADC deviations enter as errors into the measurement. The ADC has to be very stable and it is (except some small temperature coefficient). Long term stability is something like +/- 1 or 2 ppm per year. Our instruments are old and well aged.
Reading this forum one may get the impression that a 3458A is a must-have for metrology, because it's ADC has the best linearity. But then we read that its ADC contains a part that may cause tremendous drifts. Drifts up to 157 ppm/year are officially considered "normal", as long as the ACAL procedure takes care. So in my eyes the 3458A series is a similar kind of retarded progress as the 3457A series. Just think about the fan that came back after it vanished from the 3456A and the 3457A.
I think people are asking over and over again, because all this is hard to believe and difficult to understand. What i take from this discussion is that a metrology grade 3458A is something special that money won't buy. Maybe that's the reason why people try to be friendly with Agilent/Keysight.
Regards, Dieter
When to Use Autocal
For maximum accuracy, we recommend performing ACAL ALL once every
24 hours or when the multimeter's temperature changes by ± 1°C from when it was last externally calibrated or from the last autocal. (We recommend that the calibrator store the multimeter's internal calibration temperature using the CALSTR command; this can be read later using the CALSTR? command.)
More information about my measurements:
1/ NPLC 100
2/ Readings taken every 24 hours
3/ DMM warmed up at least 4 hours before each reading
4/ DMM turned off each day
5/ Autocal performed immediately before each session
6/ Lab is temperature and humidity controlled (plus/minus 1 degree C)
Thanks Soulman and Dr. Frank!
My conclusion is that the DMM internal reference is moving a bit ... perhaps due to the on/off cycling. The external LTZs are powered constantly. Autocal shows little to no change. That would indicate that the A/D board is solid?
TiN, how is "superstable" LTZ1000 reference project coming along?
More information about my measurements:
1/ NPLC 100
2/ Readings taken every 24 hours
3/ DMM warmed up at least 4 hours before each reading
4/ DMM turned off each day
5/ Autocal performed immediately before each session
6/ Lab is temperature and humidity controlled (plus/minus 1 degree C)
Thanks Soulman and Dr. Frank!
My conclusion is that the DMM internal reference is moving a bit ... perhaps due to the on/off cycling. The external LTZs are powered constantly. Autocal shows little to no change. That would indicate that the A/D board is solid?
TiN, how is "superstable" LTZ1000 reference project coming along?