Easy to say, but there are question of measuring real low capacitances. For instance, for tubes:
length (l)=120millimeter =0.12meter
outer conductor diameter (b)=28millimeter =0.028meter
inner conductor diameter (a)=19millimeter =0.019meter
This wonderful cylindrical capacitor calculator shows only 17pF for dry one!
Conversions:
permittivity (ε)=8.85picofarad/meter =8.85E-12farad/meter (air)
capacitance (C)=17.20 picofarad
But inside every Cortex chip ( for instance: STM32F050F4P6) there are a powerful ADC, with near 1μs sample time, 7pF input capacitance, and 12bit resolution. And it can be synchronized with other peripheral (e.g. timers) by dedicated hardware link, configured with other peripheral registers. This feature called event chaining.
Let's calculate time constant for dry sensor with 91kΩ resistor (assume this enough for low interference) RC Time Constant Calculator says 1.547μs for 17pF 91kΩ. Seems like ADC can catch it! Notice, we need to sample fist half of curve, because this part less nonlinear.
All this give us a hope, that for this task we don't need any additional active components like FET or op-amp. Just resistor and diode!
length (l)=120millimeter =0.12meter
outer conductor diameter (b)=28millimeter =0.028meter
inner conductor diameter (a)=19millimeter =0.019meter
Conversions:
permittivity (ε)=8.85picofarad/meter =8.85E-12farad/meter (air)
capacitance (C)=17.20 picofarad
But inside every Cortex chip ( for instance: STM32F050F4P6) there are a powerful ADC, with near 1μs sample time, 7pF input capacitance, and 12bit resolution. And it can be synchronized with other peripheral (e.g. timers) by dedicated hardware link, configured with other peripheral registers. This feature called event chaining.
Let's calculate time constant for dry sensor with 91kΩ resistor (assume this enough for low interference) RC Time Constant Calculator says 1.547μs for 17pF 91kΩ. Seems like ADC can catch it! Notice, we need to sample fist half of curve, because this part less nonlinear.
All this give us a hope, that for this task we don't need any additional active components like FET or op-amp. Just resistor and diode!
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