Keithley DMM7510 Autocalibration - Page 1
Forum section: electronics / general

author: splin
author_rank: Frequent Contributor
timestamp: February 18, 2020, 04:08:42 pm
content: I have difficulty understanding the DMM7510 autocal enhanced specs, specifically with respect to the temperature coefficient.The principle behind autocalibration should be straightforward enough - the meter measures every range in every mode possible to correct for drifts compared to the meter's own internal autocalibration standards, the LTFLU voltage reference and (likely) a VHP203 resistance standard. (From Dave's teardown). The accuracy of the autocalibration will depend on the DMM's ratio accuracy mostly determined by its linearity which is specc’d at 1ppm reading + 1ppm range.Taking the DC current range for example, the current shunts range from 0.1 ohms to 1k ohms (the 10A range has a 5mohm shunt but isn't autocalibrated).Issue 1)The 3A, 1A and 100mA ranges all use a 0.1 ohm shunt, but the temperature coefficient for the 100mA range is (50ppm + 5ppm) compared to (10 + 10) for the 1A and 3A ranges. Does that mean the 100mA range uses a different, cheaper shunt rather than the expensive 10W Vishay foil VCS302 shunt? I wonder why they would do that? Perhaps it was more convenient routing/switching wise to do so, and they decided that autocal would restore the accuracy anyway?Edit: just noticed that the 24h specs for the 100mA, 1A and 10A ranges are considerably different at (50 + 18), (150 + 50) and (200 + 40) respectively. The latter are surely the same shunt, the VCS302, so perhaps the different specs reflect the extra self heating at 3A. But why would the 40ppm of range part be lower at 3A than the 50ppm at 1A?Issue 2)The 10mA range has a TC of 10ppm + 3ppm, but the enhanced TC is (0.15 + 0.1). What does that actually mean given that the enhanced specs are only valid when Toper is within Tacal +/- 5C? The enhanced TC is described (note 40 in DS) as from Tcal +/-5C whatever that means.Eg. Tacal was 40C. For the enhanced specs to be valid Toper must be within +/- 5C of Tacal or 40C +/-5. But in that case the TC is not relevant as the accuracy is directly specified in the enhanced accuracy table (ie. (60 + 9) for 90 days onwards).If operating outside the Tacal +/-5C and Tcal +/-5C limit then presumably the basic accuracy values apply, even though autocal will still have corrected most of the long term drift? (This is probably not very important as it would be sensible just to repeat the autocal if possible).In what circumstances would you ever apply the (0.15 + 0.1) TC spec?

author: Kleinstein
author_rank: Super Contributor
timestamp: February 18, 2020, 04:53:22 pm
content: For the current measurement there are several types of error, the TC of the shunt is just one part. There is also a possible self heating part, especially for the 3 A range and there could be self heating related thermal EMF. Another part is the error of the amplifier (including the resistors to set the gain) used for the current readings. The amplifier offsets give some error proportional to the range. In the higher ranges this can be less, as voltages are higher in the 3 A range. The combination of errors of (50 + 18), (150 + 50) and (200 + 40) is still odd: 40 ist way more than 1/3 of 50 and why should the absolute error part be so much better for the 100 mA range. I would more expect the opposite.Accuracy specs are to a certain degree design goals and test limits that are set relatively arbitrary. There is no direct way to measure the specs - it more like a combination of test limits and specs of the parts used. If in doubt they may choose higher error limits to be on the safe side. So those error limits may not be logical at all.The error due to ACAL can be lower than expected for the INL specs. ACAL often only uses one polarity, usually the one that is working better. In addition the readings may be in very fast sequence and this can exclude some thermal effects. In some cases there may be additional correction factors (e.g. from factory calibration), so that constant parts of the INL may be included and only changes in the INL would really cause errors. They could also check the initial ACAL accuracy and this way the INL at the few points relevant for ACAL. So the accuracy specs for the ACAL steps can be better than the general INL value.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: February 19, 2020, 08:45:52 am
content: Quote from: splin on February 18, 2020, 04:08:42 pmThe ranges of 10 mA ... 10 μA do not have a shunt. They work on the basis of a transimpedance amplifier.Quote40 Add per degree from TCAL ±5 oC.

author: splin
author_rank: Frequent Contributor
timestamp: February 19, 2020, 01:48:40 pm
content: Quote from: MegaVolt on February 19, 2020, 08:45:52 amInteresting point - is this unique (for DMMs) to the DMM7510? But it's worth noting that the Keithley datasheet refers to 'Effective internal shunt value' (see first post).QuoteThanks - the penny has just dropped - (.15 + .1) must be the TC of the VHP203 used to autocalibrate the 'shunts'. That seems quite bold given that whilst Vishay claim '± 0.05 ppm/°C (0 °C to 60 °C)' they only guarantee ± 2 ppm/°C (-55 tp +125C). I wonder if they individually qualify them? OTOH, the addded uncertainty due to the VHP203 TC is pretty small compared to even the enhanced accuracy specs.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: February 19, 2020, 02:16:06 pm
content: Quote from: splin on February 19, 2020, 01:48:40 pmThis is unofficial information. Just a guess.1. In these ranges, with shorted inputs, a huge error of the order of 3000 ppm is observed. What is characteristic of electrometers using a transimpedance amplifier.2. The voltage drop in these ranges is much less than in other ranges.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 19, 2020, 02:40:54 pm
content: There are a few other DMMs that use a TIA instead of the classical shunt + amplifier form. I know some of the Prema meters and the DMM6510. This works well for small currents and with modern OPs may not be more expensive. In principle it does not change that much - mainly using larger resistors in the feedback and a little different way of amplification. The low burden voltage is convenient as it helps against leakage through the protection.For the precision resistors the TC tends to be smaller for the reduced temperature range, than the full -55 - +125 C range. So 0.15 ppm/K may be ambiguous but not impossible. So the main part is that the TC is rather small compared to other errors. Even if the TC of the resistor would be a little larger there is still a rather larger other terms in the specs.I would be more worried about increasing leakage at higher temperature, that could effect the low current ranges.