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Toseed khan
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Join Date: Mar 2009
Posts: 4

Op Amp Capacitive Load-Deriving Capability

12/11/2009 12:13 AM

Hi,

Can someone please provide a guidance ,what are the parameters of the opamap we have to consider for deriving the 1uf capacitive load.

Is there any mathmatical formula for calculating the capacitance value from the opamp parameters.

I would appreciate your reply!

Thanks & Regards,

Toseed Khan
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bhankiii
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Engineering Fields - Electrical Engineering - New Member

Join Date: Sep 2006
Location: El Lago, Texas, USA
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#1

Re: Opamp Capacitive load deriving capability

12/11/2009 9:10 AM

Q = CV, so

I = (dQ/dT) = c*(dV/dT),

so the amount of current you need the opamp to supply is 1uF times the amount of voltage you need to charge the cap to, divided by the amount of time you have to do it.

For instance, if I need the cap charged to 5V, and I want to do it in less than 1mS, then I = (1uF)*(5/.001) = 5mA.

Check your data sheet for two things - the output current capabilty of the OPAMP, and the slew rate of the output. In this example, the slew rate would need to be greater than 5V/1mS = 5000 v/s.
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Randall
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#2

Re: Op Amp Capacitive Load-Deriving Capability

12/14/2009 3:39 AM

Another way to look at it if you know what sort of frequencies you are working at is to "convert" the capacitive load to an impedance:

Z=1/(2*pi*f*C)

In Bhankiii's example of charging the cap in 1 ms, you could "sort of" regard this as being a 500 Hz signal. The impedance of a 1 µf cap at 500 Hz is about 318Ω.

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Toseed khan
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#4
In reply to #2

Re: Op Amp Capacitive Load-Deriving Capability

12/14/2009 4:05 AM

Thanks Randall!!!

If my output impedance is 100k and frequency is 1KHz then we calculated the capacitive load value from equation Z=1/(2*pi*f*C) it coming around 1.59nF.

If i want improve the opamp capacitive load deriving capability then what can i do?and what parameters of the opamp, we have to be take care while selecting.

Thanks,

Toseed
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Randall
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#6
In reply to #4

Re: Op Amp Capacitive Load-Deriving Capability

12/14/2009 10:03 AM

As bhankiii said earlier the critical parameter is the output current.

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Toseed khan
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#3

Re: Op Amp Capacitive Load-Deriving Capability

12/14/2009 3:54 AM

Thanks bhankiii for reply!

I have seen that the analysis and actual opamp results the output is not stable .In results there is oscillation or ringing.

Again i'm looking for kind of mathematical equation, where i can claim that the selected opamp can derive 1uf capacitive load or more/less cap value considering with parameters of opamp ( e.g Bandwidth ,slew rate and output current etc).

The bottom line is that i have to make justification for selected opamp, which can derive capacitive load.

Thanks,

Toseed
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bhankiii
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#5
In reply to #3

Re: Op Amp Capacitive Load-Deriving Capability

12/14/2009 9:37 AM

Check the gain of your OPAMP. Many are not stable at unity or low gains.

If you're looking for a stability equation, you'll need to determine the transfer function and impedance of your circuit and plot the poles in the complex plane to see if they lie in the stable region. Alas, it's been 30 years since I did that in college. But any good linear signals books will have the procedure in it.
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Anonymous Poster
#7
In reply to #3

Re: Op Amp Capacitive Load-Deriving Capability

02/23/2010 12:29 AM

http://www.analog.com/library/analogDialogue/archives/31-2/appleng.html

You have to watch out for your phase margin of the op amp. I like the above discription.
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