Electronics > Projects, Designs, and Technical Stuff
active low-pass filter from Valhalla 2701C DC standard
David Hess:
--- Quote from: SilverSolder on February 26, 2020, 08:26:33 pm ---
--- Quote from: Kleinstein on February 26, 2020, 08:11:15 pm ---The simple version with just 1 OP has not that many advantages over a simple active filter. One still has the leakage from the 2 capacitors.
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Do film capacitors have leakage issues, generally?
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In high impedance and high temperature applications, film capacitor leakage can become a factor. Mylar (polyester) capacitors have higher leakage than the other types for example.
Film capacitor leakage is also a factor in precision applications.
SilverSolder:
--- Quote from: David Hess on February 28, 2020, 09:10:43 pm ---
--- Quote from: SilverSolder on February 26, 2020, 08:26:33 pm ---
--- Quote from: Kleinstein on February 26, 2020, 08:11:15 pm ---The simple version with just 1 OP has not that many advantages over a simple active filter. One still has the leakage from the 2 capacitors.
[...]
--- End quote ---
Do film capacitors have leakage issues, generally?
--- End quote ---
In high impedance and high temperature applications, film capacitor leakage can become a factor. Mylar (polyester) capacitors have higher leakage than the other types for example.
Film capacitor leakage is also a factor in precision applications.
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What would you expect from a good film cap e.g. 1uF with 10V on it... more than 1pA?
Kleinstein:
1 pA is not much current. So I would not be surprised to see more than 1 pA of leakage at 10 V. Even a reed relay may show more leakage.
It may be possible to get capacitors that good, but I would take it for granted.
Measuring the leakage of a 1 µF capacitor at 10 V, there are 2 additional problems: one is separating dielectric absorption from real leakage and to keep the voltage really constant. 1 pA with 1µF would give a rate of change of 1 µV/second, so a really slow drifting voltage. It would also need a stable temperature. With a TC of some 100 ppm/K for the capacitance, a changing temperature would result in about 1 mV/K of temperature effect.
SilverSolder:
--- Quote from: Kleinstein on February 29, 2020, 08:29:49 am ---1 pA is not much current. So I would not be surprised to see more than 1 pA of leakage at 10 V. Even a reed relay may show more leakage.
It may be possible to get capacitors that good, but I would take it for granted.
Measuring the leakage of a 1 µF capacitor at 10 V, there are 2 additional problems: one is separating dielectric absorption from real leakage and to keep the voltage really constant. 1 pA with 1µF would give a rate of change of 1 µV/second, so a really slow drifting voltage. It would also need a stable temperature. With a TC of some 100 ppm/K for the capacitance, a changing temperature would result in about 1 mV/K of temperature effect.
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Maybe we could try soaking the capacitor at 10V for a really long time, then disconnect the supply and measure the capacitor voltage as the first point... - then, let it sit for a long time to leak down on its own... finally, we come back later and measure its voltage as the second point, and calculate the leakage current based on the difference between the two measurements? (in a temperature stable environment)
Kleinstein:
--- Quote from: SilverSolder on March 01, 2020, 01:28:30 am ---Maybe we could try soaking the capacitor at 10V for a really long time, then disconnect the supply and measure the capacitor voltage as the first point... - then, let it sit for a long time to leak down on its own... finally, we come back later and measure its voltage as the second point, and calculate the leakage current based on the difference between the two measurements? (in a temperature stable environment)
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This is a reasonable method to get the real capacitor leakage. The difficult (but not impossible) part here is the switch to connect the voltmeter. It should be low leakage, probably switching the voltmeter between the cap and a 10 V source.
With enough time (e.g. 1l hour) the temperature is no longer that critical: 1 pA would still be 3.6 mV/hour and this equivalent to some 3.5 K in temperature change. Chances are one would do a few more than just 2 readings from the capacitor. So more like soak to 10 V for over night, measure for a few seconds second, wait 1 minute, measure, wait 1 hour , measure, wait 1 hour, measure.
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