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# How to calculate the value of capacitor based on the drive capability of an IC?

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#### hemant03

##### Junior Member level 1
Hi,
Can anyone tell how to calculate the value of capacitor based on the drive capability of an IC?
For eg. If the drive capability of an op amp is 20-40mA, what capacitor value can be stuck at the output pin of the op amp.

Thanks,
- Hemant

Re: Capacitor value needed

I suggest u may divide open circuit output voltage over maximum output current to obtain output resistance , then

C = 1/(2*pi*fmax * Rout)

### hemant03

Points: 2
Re: Capacitor value needed

What is the purpose of the cap? Are you using it as a filter or is it used as a capacitive coupler to another circuit. Is there a signal going through it? If there is than what is the frequency(or frequencies) of that signal? Just basically, What is the caps function in the circuit, and what are the voltages and frequencies (if its used as a filter include the frequency of noise your trying to filter out) involved.
Also specify what Type of cap you intend to use (electrolytic,monolithic,tantalum, etc.) There are many different types of caps and they will react to your circuit in different ways.

SpectraBEI

Capacitor Value

It depends on the rising and falling time of your application.
assume your application can tolerance T (s) delay, this approximately formula: Vsw=(I*T)/C can get the capacitance for the output load of op amp. ('Vsw' is the output voltage swing, 'I' is the drive capability of op amp, 'T' is the tolerance time).

Ryan

### hemant03

Points: 2
Re: Capacitor value needed

Thanks
- Hemant

The cap is being used for dc applications. In the sense that if the voltage drops due to noise or something; it can provide the necessary oomph to the supporting circuitry.
The reason for asking the cap value was to ensure that I do not overload or unbalance the op amp.

Re: Capacitor value needed

it depends on the rise time you need.... smaller the capacitor then lesser the rise time...

Re: Capacitor Value

Attached is a LF398 Data Sheet (Sample and Hold IC).

Re: Capacitor value needed

hemant03 said:
Thanks
- Hemant

The cap is being used for dc applications. In the sense that if the voltage drops due to noise or something; it can provide the necessary oomph to the supporting circuitry.
The reason for asking the cap value was to ensure that I do not overload or unbalance the op amp.

"If the voltage drops due to noise or something" if this is the case than I would beef up the power supply for this circuit and forget about the cap on the output of the op amp.

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