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Sensors, Actuators, and Robotics » Cap charging measurment

January 05, 2011
by Jalex
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Is the A/D fast enough to measure the charging time on a 5meg/30pf RC? I want to gate out a position on the ramp and convert it to a voltage to be displayed after a simple calculation. I can do some of this with op-amps but I just wondered if I can eliminate them all together. I will need a trigger voltage to start the charge and one to "0" it after it been read. Also on line two of the display I will want a frequency read out by gating in a section of a pulse train. Is there a nerdkit project that is close enough to this to start me in the right direction?

January 11, 2011
by Yourself
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Well, the fundamental time scale in an RC circuit is the product of R and C. In this case:

510^6 Ohm * 3010^-12 Farad = 150*10^-6 seconds = 150 microseconds

This is the length of time it takes for the voltage across the capacitor to reach ~63% (1-1/e) of its maximum voltage. According to the data sheet the ADC takes 13 clock cycles to perform a conversion (25 cycles on first conversion). This is in terms of the ADC clock. I haven't done enough with the ADC yet to say exactly how long that is.

January 11, 2011
by mrobbins
(NerdKits Staff)

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Hi Jalex,

You may want to take a look at our capacitive proximity sensor project, in which we used the analog comparator to time the charge/discharge of a small RC time constant. I think that's going to be fairly similar to what you want.


January 12, 2011
by Jalex
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Thanks Mike That's pretty close to what I had in mind and I will be trying it but could I get a fairly accurate measurement of time using Say a 1M resistor and a 40pf capacitor? I am looking for numbers like 0-180.

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