capacitor fall time
Hi,
You can do one thing. Suppose you put a known capacitor C_kn_1 at the output. The driver may have some capacitance between its o/p terminal and ground.Lets say it is C_out. Then the total capacitance at the o/p is C_out + C_kn_1. Measure the rise time t_r_1. The rise time will be
t_r_1 = R_out * (C_kn_1 + C_out)
Similarly put another known resistor C_kn_2 and the rise time can be measured. This will be
t_r_2 = R_out * (C_kn_2 + C_out)
Subtracting these two you get
t_r_1 ~ t_r_2 = R_out (C_kn_1 ~ C_kn_2)
where only unknown term is R_out. You can find it out and from the expression of either t_r_1 or t_r_2 you can find out C_out.
Therefore for any predetermined rise time t_r the capacitance required is found from the equation
t_r = R_out* (C_out + C_unkn)
or C_unkn = t_r / R_out.
I don't know how accurately you can find it out with this. the accuracy of course depends on a lot other things, like, when you load something to the driver the load will also have some input cvapacitance which adds to the total capacitance to reduce the rise time further.
where R_out is the output resistance of the driver. Then, knowing the output resistance