I am trying to design an RC differentiator that will detect the edges of a square wave.
The square wave is 1khz (1ms period) with 5v amplitude. I need to select the values of R and C.
To have a good differentiator, I've read that the impedance of the capacitor must be at least 10 times larger than the resistance of the resistor at the operating frequency, and that the time constant RC must be smaller than the period of the input signal.
Zc = -j/wC = -j/(2*Pi*f*C)
and therefore -j/(2*Pi*f*C) > 10*R
plugging f = 1000 hz and solving for C
C must then be < 1/(20000*R*Pi)
We also need RC << 1ms.
Setting R = 100 ohms,
C < 0.159 uF
I choose C = 50nF.
So RC = 5 us which is << 1ms.
Here is what I get (no load):
Questions:
1. are my assumptions and constraints for a good RC differentiator corrrect? I read that on the internet.
2. The differentiated output is from 0.3 to 0.4V, which is too small to drive a diode (0.6V forward drop). What can I do to make the differentiated signal larger? Obviously I need to play with R and C but I don't know which one and how. Id rather understand why and how rather than do it blindly.
Thank you
The square wave is 1khz (1ms period) with 5v amplitude. I need to select the values of R and C.
To have a good differentiator, I've read that the impedance of the capacitor must be at least 10 times larger than the resistance of the resistor at the operating frequency, and that the time constant RC must be smaller than the period of the input signal.
Zc = -j/wC = -j/(2*Pi*f*C)
and therefore -j/(2*Pi*f*C) > 10*R
plugging f = 1000 hz and solving for C
C must then be < 1/(20000*R*Pi)
We also need RC << 1ms.
Setting R = 100 ohms,
C < 0.159 uF
I choose C = 50nF.
So RC = 5 us which is << 1ms.
Here is what I get (no load):
Questions:
1. are my assumptions and constraints for a good RC differentiator corrrect? I read that on the internet.
2. The differentiated output is from 0.3 to 0.4V, which is too small to drive a diode (0.6V forward drop). What can I do to make the differentiated signal larger? Obviously I need to play with R and C but I don't know which one and how. Id rather understand why and how rather than do it blindly.
Thank you