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| DMM Input Capacitance |
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| Alex Nikitin:
Measuring parasitics of multimeters is a very healthy exercise! :-+ The capacitance would also depend on the voltage range setting, when the input divider is connected the capacitance from the "+" input terminal might drop significantly. My measurements with a hand held capacitance meter (1kHz) for the DC voltage mode only: Keithley 2015 Positive input to mains ground - 10V range and below ~100pF, 100V and 1000V ~25pF Negative input to mains ground ~ 180pF Between input terminals ~300pF for 10V range and below, ~20pF for 100V and 1000V ranges HP3456A Positive input to mains ground - 10V range and below ~350pF, 100V and 1000V ~85pF Negative input to mains ground ~ 1nF with the guard open, ~2.2nF with the guard to negative Between input terminals ~600pF for 10V range and below, ~60pF for 100V and 1000V ranges Hioki DM7275-03 Positive input to mains ground - 10V range and below <30pF, 100V and 1000V <10pF Negative input to mains ground <30pF Between input terminals ~1500pF for 10V range and below*, <10pF for 100V and 1000V ranges. Cheers Alex * - possibly something to do with the contact checking feature, even if it is OFF. The cure is simple (if the input C is a problem) - to put a 10K resistor in series with the positive lead and use Hi-Z, on 10V and below the meter has ~20pA input current or ~0.2uV added error with the resistor. |
| KungFuJosh:
--- Quote from: Alex Nikitin on April 23, 2024, 09:55:50 pm ---The capacitance would also depend on the voltage range setting, when the input divider is connected the capacitance from the "+" input terminal might drop significantly. My measurements with a hand held capacitance meter (1kHz) for the DC voltage mode only: --- End quote --- Interesting. I tried that on my SDM3055X-E, and the DCV ranges did have a small impact. 20V to 1000V was 10pF higher than 200mV to 2V. No effect at all for ACV. |
| mawyatt:
--- Quote from: KungFuJosh on April 23, 2024, 10:06:40 pm --- --- Quote from: Alex Nikitin on April 23, 2024, 09:55:50 pm ---The capacitance would also depend on the voltage range setting, when the input divider is connected the capacitance from the "+" input terminal might drop significantly. My measurements with a hand held capacitance meter (1kHz) for the DC voltage mode only: --- End quote --- Interesting. I tried that on my SDM3055X-E, and the DCV ranges did have a small impact. 20V to 1000V was 10pF higher than 200mV to 2V. No effect at all for ACV. --- End quote --- What do you see when switching from DCV to ACV wrt the capacitance between terminals and + or - terminal to ground? Best, |
| KungFuJosh:
--- Quote from: mawyatt on April 24, 2024, 12:05:17 am --- --- Quote from: KungFuJosh on April 23, 2024, 10:06:40 pm ---Interesting. I tried that on my SDM3055X-E, and the DCV ranges did have a small impact. 20V to 1000V was 10pF higher than 200mV to 2V. No effect at all for ACV. --- End quote --- What do you see when switching from DCV to ACV wrt the capacitance between terminals and + or - terminal to ground? --- End quote --- All previous tests were terminal to ground. SDM3055X-E between terminals: Auto (200mV): DCV: 66.27pF ACV: 58.54pF SDM3055X-E Positive Terminal to Ground: Auto (200mV): DCV: 62.61pF ACV: 54.18pF Interesting that there's 4pF less ref to ground vs. terminal to terminal. SDM3055X-E Negative Terminal to Ground: DCV: 559.2pF ACV: 559.3pF |
| ebastler:
--- Quote from: KungFuJosh on April 23, 2024, 06:25:17 pm ---I have a convenient outlet on my bench, so I used a hex key inserted as my ground reference. --- End quote --- I kind of like the idea of having an outlet right in the benchtop for testing. (My main power strip is below the benchtop, hence a bit inconvenient to reach when I want to plug/unplug a DUT often.) In practice, how often do you drop clipped wire ends and stuff into the mains and USB outlets? ;) |
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