---
+ Post New Thread
Results 1 to 13 of 13
  1. #1
    Member level 1
    Points: 1,155, Level: 7

    Join Date
    Nov 2007
    Posts
    33
    Helped
    1 / 1
    Points
    1,155
    Level
    7

    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!

  2. #2
    Banned
    Points: 2,735, Level: 12

    Join Date
    Feb 2006
    Location
    Bandar Abbas
    Posts
    161
    Helped
    14 / 14
    Points
    2,735
    Level
    12

    rise time of capacitor

    Why Do you want to use a capacitor?



    •   Alt22nd January 2008, 10:31

      advertising

        
       

  3. #3
    Advanced Member level 4
    Points: 11,332, Level: 25

    Join Date
    Mar 2005
    Location
    USA
    Posts
    1,323
    Helped
    272 / 272
    Points
    11,332
    Level
    25

    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



  4. #4
    Member level 1
    Points: 1,155, Level: 7

    Join Date
    Nov 2007
    Posts
    33
    Helped
    1 / 1
    Points
    1,155
    Level
    7

    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



  5. #5
    Member level 1
    Points: 1,144, Level: 7

    Join Date
    Jan 2008
    Posts
    38
    Helped
    4 / 4
    Points
    1,144
    Level
    7

    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]


    1 members found this post helpful.

  6. #6
    Full Member level 4
    Points: 2,684, Level: 12

    Join Date
    Jan 2008
    Location
    Bangalore,India
    Posts
    207
    Helped
    35 / 35
    Points
    2,684
    Level
    12

    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


    1 members found this post helpful.

    •   Alt23rd January 2008, 07:44

      advertising

        
       

  7. #7
    Advanced Member level 4
    Points: 11,332, Level: 25

    Join Date
    Mar 2005
    Location
    USA
    Posts
    1,323
    Helped
    272 / 272
    Points
    11,332
    Level
    25

    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.
    .
    Im assuming that you know the source resistance R. Let me know if you want the derivations.
    .
    Regards,
    Kral



  8. #8
    Banned
    Points: 2,735, Level: 12

    Join Date
    Feb 2006
    Location
    Bandar Abbas
    Posts
    161
    Helped
    14 / 14
    Points
    2,735
    Level
    12

    capacitor rise time formula

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


    1 members found this post helpful.

  9. #9
    Advanced Member level 4
    Points: 11,332, Level: 25

    Join Date
    Mar 2005
    Location
    USA
    Posts
    1,323
    Helped
    272 / 272
    Points
    11,332
    Level
    25

    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



  10. #10
    Advanced Member level 1
    Points: 4,907, Level: 16

    Join Date
    Jul 2005
    Posts
    452
    Helped
    78 / 78
    Points
    4,907
    Level
    16

    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.



  11. #11
    Member level 1
    Points: 1,155, Level: 7

    Join Date
    Nov 2007
    Posts
    33
    Helped
    1 / 1
    Points
    1,155
    Level
    7

    trying to measure capacitor values

    Kral,

    can you send me the derivation?

    Thank you very much!


    Regards,
    myql



  12. #12
    Full Member level 4
    Points: 2,684, Level: 12

    Join Date
    Jan 2008
    Location
    Bangalore,India
    Posts
    207
    Helped
    35 / 35
    Points
    2,684
    Level
    12

    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.


    1 members found this post helpful.

  13. #13
    Banned
    Points: 2,735, Level: 12

    Join Date
    Feb 2006
    Location
    Bandar Abbas
    Posts
    161
    Helped
    14 / 14
    Points
    2,735
    Level
    12

    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


    1 members found this post helpful.

+ Post New Thread
Please login