I want to probe 3.3V signal @ 33MHz. Which probe is better, with 1MOhm or 100kOhm resistance and why?
Answer:
Besides resistance, a real probe loads the circuit capacitively as well. If both probes have a similar geometrical configuration, they will also have similar capacitance, typically about 2 pf for a 10:1 probe. This makes the RC time constant for the 100 kOhm probe about 200 nsec, far longer than the 30 nsec period of the waveform. Both probes will provide a largely capacitive impedance to the circuit of about 2400 ohms at the 33 MHz fundamental frequency and proportionately lower impedance at the higher overtone frequencies.
DEPENDS ON HOW SENSITIVE YOUR METER IS BUT PROBABILY THE 100K RESISTOR TO GET A SIGNAL WORTH VIEWING.
The impedance of the probe is very important,the higher the probe impedance, the less affect it will have on the impedances in the curcuit. At 33 MHz that is an important consideration.
Of course the 1 M ohm. Because it has lesser effect on the circuit in which you are measuring with. The ideal probe is the one which having no effect on the circuit in which it measure. Having infinity input impedance.
Actually, the better probe is the one with the lower capacitance specification. The capacitance will have a bigger impact on "distorting" your signal than the 100k resistance will. A capacitance of 5pF has an impedance < 1k ohm at 33MHz, so this will dominate the response.
Xc = 1/(2 pi() * C * Freq)
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