Products > Test Equipment
DMM Input Capacitance
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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