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| Best Budget 6.5 Digit multimeter |
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| bdunham7:
--- Quote from: Kleinstein on October 11, 2022, 04:07:08 pm ---The video on the Hantek 6 digit meter shows a MAX6325 reference, which is a bit more drifty than the LM399 used in more classic 6 digit DMMs (and the HP3468/3478). This reference alone makes the accuracy specs questionable. I very much doubt they would do an extensive (e.g. 500 hours) burn in to at last reduce the long term drift. In addition to the reference the 10 V range would also need some divider, as the ADC (though I don't know the type) very likely only does some 2.5 or maybe 5 V range. So I would not really count on the accuracy specs - maybe add a k factor of 2 ? --- End quote --- My experience with B- and C brand bench meters is that their specs/performance range from 'typical' (k=1?) to 'aspirational' or perhaps even 'fictional' in this case. Had a 5+ digit GDM-8251 and although it's specs were not great for a 5-digit class instrument, it failed miserably to meet even those. Any number of 4.5 digit meters were significantly better. One characteristic that I've seen is that these meters will use up all of their tolerances--or even more--just in tempco within the 18 to 28C range that they are specified for. The MAX6325 is on par or even inferior to references commonly used in 4.5-digit handhelds, so it seems pretty inadequate in a 6.5-digit model. I doubt they aged and selected them and I'm confident they didn't splurge on things like high quality divider resistor and shunts. It would be interesting to see one (or more) of these actually tested by someone with the instruments to show their actual performance. My guess is that actual performance varies quite a bit between examples. |
| Kleinstein:
The max6325 reference is not that bad for a bandgap reference, though in a plastic case. Not good for 6 digits, but should be OK for 5.5 digits. The SDM3055 uses the slightly inferior max6225. 4.5 digit meter usually use way worse ones. The input divider in the HDM3065 seem to be a Caddock 1776 variant (one of the smaller ones) - so it may not be that bad. However with a usual SD ADC the 10 V range would also have to go through some divider (seperte resistors or maybe part of the same divider). There are a few ADC chips for higher voltage, but these are rare). This makes it odd to have the best accuracy in the 10 V range and how they could reach that accuracy without rather expenside dividers (the 1776 would not be good enough). The 34401 / 34461 and many better 6 digit meters get the 10 V range without a divider before the ADC. So the best accuracy for the 10 V range is natural there. |
| bdunham7:
--- Quote from: Kleinstein on October 12, 2022, 06:53:29 pm --- 4.5 digit meter usually use way worse ones. --- End quote --- Fluke 289 uses LT1019A 20ppm/1000hr, but that is top-of-the-line. I suppose for a modern (less-precise) 5.5 digit model with 120ppm basic accuracy, that might be good enough. --- Quote ---This makes it odd to have the best accuracy in the 10 V range and how they could reach that accuracy without rather expenside dividers (the 1776 would not be good enough). The 34401 / 34461 and many better 6 digit meters get the 10 V range without a divider before the ADC. So the best accuracy for the 10 V range is natural there. --- End quote --- Apparently they've simply copied the HPAK specs verbatim and listed them as their own. I'd give that a 0% confidence interval. |
| 2N3055:
--- Quote from: Kleinstein on October 12, 2022, 06:53:29 pm ---The max6325 reference is not that bad for a bandgap reference, though in a plastic case. Not good for 6 digits, but should be OK for 5.5 digits. The SDM3055 uses the slightly inferior max6225. 4.5 digit meter usually use way worse ones. The input divider in the HDM3065 seem to be a Caddock 1776 variant (one of the smaller ones) - so it may not be that bad. However with a usual SD ADC the 10 V range would also have to go through some divider (seperte resistors or maybe part of the same divider). There are a few ADC chips for higher voltage, but these are rare). This makes it odd to have the best accuracy in the 10 V range and how they could reach that accuracy without rather expenside dividers (the 1776 would not be good enough). The 34401 / 34461 and many better 6 digit meters get the 10 V range without a divider before the ADC. So the best accuracy for the 10 V range is natural there. --- End quote --- MAX6325 are buried zener references. There are even 1ppm MAX tempco versions (MAX6325C). Questions are if they use those though... Biggest problems would be long term drift (30ppm /1000h9 and hysteresis (20ppm) because of the plastic case... DS: The MAX6325/MAX6341/MAX6350 are low-noise, precision voltage references with extremely low, 0.5ppm/°C typical temperature coefficients and excellent, ±0.02% initial accuracy. These devices feature buried-zener technology for lowest noise performance. Load-regulation specifications are guaranteed for source and sink currents up to 15mA One problem is that they seem to have large spread in quality... And price is not low, so LM399 (or new brethren) start to look good for best results, even with additional work and circuitry needed. |
| tautech:
--- Quote from: Kleinstein on October 12, 2022, 06:53:29 pm ---The max6325 reference is not that bad for a bandgap reference, though in a plastic case. Not good for 6 digits, but should be OK for 5.5 digits. The SDM3055 uses the slightly inferior max6225. 4.5 digit meter usually use way worse ones. --- End quote --- :-// Thread title is 'Best Budget 6.5 Digit multimeter' for which SDM3055 does not fit but SDM3065X and its LM399 reference does. |
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