| General > General Technical Chat |
| How much accuracy do YOU really need? |
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| TomKatt:
--- Quote from: shapirus on February 09, 2023, 01:45:07 pm --- --- Quote from: tggzzz on February 09, 2023, 12:57:45 pm ---I'll expand "repeatability" to "repeatability over time". --- End quote --- Plus resolution, to observe slowly changing values. --- End quote --- One reason I keep a few old analog lab meters kicking around. The lcd bargraphs have their place, but are no substitute for a moving coil. These used to be dirt cheap on eBay when schools started upgrading to 'modern' equipment. They're probably not 'accurate', but they are 'repeatable over time' 8) |
| madires:
accuracy <-> precision <-> resolution ... and what you really need depends on the work you're doing. For tinkering with Arduinos any inexpensive DMM might be sufficient, while for calibrating an 8.5 digit bench DMM in a metrology lab you would need something more suitable. |
| TimFox:
--- Quote from: tom66 on February 09, 2023, 12:11:51 pm ---Even a modern high performance scope probably has around a dB of gain error, which is very roughly around +/-2% voltage accuracy. I have found, at least on cheaper scopes, the attenuation ranges are far less consistent than a multimeter. For instance my Rigol DS1074Z jumps up about 2-3% in voltage measurement when the attenuator relay clicks in around 200mV/div. Also you have to be aware most scopes are only 8-bit and so quantisation error can dominate any other error, especially on scopes where the lowest range (1mV/div or so) is usually a doubling of the last range rather than a true analog gain. Since an oscilloscope is usually a tool to measure time-voltage signals the voltage accuracy is less critical than the time accuracy. Time accuracy on an oscilloscope is sacrosanct, you cannot measure things like baud rate accurately if it is out by more than a few percent. Thankfully nearly every scope at least uses a basic TCXO for the master clock which obtains around +/-10~20ppm accuracy. If you are measuring more precisely than that, I guess you can use the 10MHz ref input on many scopes. --- End quote --- A 1 dB gain error translates into approximately 12% voltage error: 20 x log10(1.12) = 0.98 dB. |
| tom66:
--- Quote from: TimFox on February 09, 2023, 05:18:16 pm ---A 1 dB gain error translates into approximately 12% voltage error: 20 x log10(1.12) = 0.98 dB. --- End quote --- D'oh. I've probably slipped a decimal point a place somewhere in my memory, I didn't actually do the maths, but recall some Rigol datasheet that specified both. Thanks for the correction :) |
| Someone:
--- Quote from: tom66 on February 09, 2023, 06:03:21 pm --- --- Quote from: TimFox on February 09, 2023, 05:18:16 pm ---A 1 dB gain error translates into approximately 12% voltage error: 20 x log10(1.12) = 0.98 dB. --- End quote --- D'oh. I've probably slipped a decimal point a place somewhere in my memory, I didn't actually do the maths, but recall some Rigol datasheet that specified both. Thanks for the correction :) --- End quote --- The DC accuracy is often in the 2% ballpark, with AC accuracy worse. So everyone is correct! |
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