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- 22nd January 2008, 07:35 #1

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## capacitor rise time

how to get the value of the capacitor across the output of a driver? Please see the ckt attach.

please let me know if you need some more information in order to answer my question.

Thank you very much!

- 22nd January 2008, 11:31 #2

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## rise time of capacitor

Why Do you want to use a capacitor?

- 22nd January 2008, 11:31

- 22nd January 2008, 22:33 #3

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## rise time capacitor

myql,

Your question is not clear to me.

.

Are you trying to measure the value of the capacitor by measuring the rise time?

.

Are you trying to determine the value of capacitor required to get a specified rise time?

Regards,

Kral

- 23rd January 2008, 04:57 #4

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## rise time capacitance

kral,

Yes, I am trying to determine the value of the capacitor required to get a specified rise time.

could you help me on this matter?

Thank you very much!

Regards,

myql

- 23rd January 2008, 06:20 #5

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## capacitor rise time calculation

try this method.

t= k. R.c

t = time

k = constant

R = Resistence

c = capasitance

Im not sure with the value of k (may be 0,6)

93_1201065577.jpg][/url]

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- 23rd January 2008, 06:20

- 23rd January 2008, 08:44 #6

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## 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

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- 23rd January 2008, 08:44

- 23rd January 2008, 19:50 #7

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## how to measure rise time and fall time

myql

There are 2 commonly used definitions of rise time:

.

1) Time required for output to undergo 63% (X.63) of its total swing (the exact number is [1-(1/e)]

For this definition, C = t/R

.

2) Time required for the output to go from 10% of its total swing to 90% of its total swing. This is the more commonly used definition for digital systems. For this definition, C =.455t/R.

.

I’m assuming that you know the source resistance R. Let me know if you want the derivations.

.

Regards,

Kral

- 23rd January 2008, 20:24 #8

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## capacitor rise time formula

rise time =.35/F_H(f=freuq.)

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- 23rd January 2008, 20:40 #9

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## measure output rise time

myql,

a-tek7 is correct. This results in C =2.85t/R. Ignore my post regarding the 10-90 equation.

Regards,

Kral

- 23rd January 2008, 22:17 #10

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## 33 capacitor value

low rise time means high bandwidth. A parallel cap means

filtering high frequencies, sort of low past filter, limiting

bandwidth. So a cap in parallel will lower the rise time.

Unless what you are looking for is to limit the rise time to a

certain value the question does not make sense.

- 24th January 2008, 02:34 #11

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## trying to measure capacitor values

Kral,

can you send me the derivation?

Thank you very much!

Regards,

myql

- 24th January 2008, 08:35 #12

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## rising time of the capacitor

The requirement of the controlled rise time or fall time is crutial. There are devices, in fact a large number of analog devices, which shows capacitive load to the drivers. Then any sharp change in the voltage levels in the input to it can give rise to a large spike of curent depending on the output resistance of the driver. A smaller output resistance makes the driver wider in bandwidth of course but there are chances that the large spike f current can even exced the allowable input current limit of the capacitive load. In that case the device can get damaged. Especially sensitive devices like CCD, APS all have these sort of problems. The rise time spec is very important for the driver.

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- 24th January 2008, 08:35

- 24th January 2008, 13:37 #13

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## rise time and fall time

F_H=1/(2ΠRC)& T_R=2.2RC.

RC=RC so T_R/2.2=1/(2ΠF_H) so T_R=(2.2/2Π)/F_H=.35/F_H

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