SN74LS38: Input capacitance(Cin) of Logic IC below

Hung Ching Hsu

Expert 3086 points

Part Number: SN74LS38

Dear,

I would like to ask if anyone knows the Input capacitance(Cin) of Logic IC below.

I have looked up the datasheet of both ICs, but I couldn't find it out.

Or can i get Cin by calculating the values shown in the datasheets?

①SN74LS38NS
②SN74LS645-1NSR

I look forward to your response.

Best Regards,

over 5 years ago

0 Clemens Ladisch over 5 years ago

Guru 316600 points

Bipolar logic devices do not specify C_i because their inputs source current when driven low, and that current is so large that any small capacitance is of no consequence.

What do you need to the C_i value for?

0 Dylan Hubbard over 5 years ago

TI__Mastermind 45725 points

Hey Jo,

Unfortunately, if its not spec'd in the DS then it will be tough to ascertain this info. My suggestion would be to estimate it at 5 pF, it should be close to this value.

0 Hung Ching Hsu over 5 years ago in reply to Clemens Ladisch

TI__Expert 3086 points

Hi Clemens

Thanks for your kindly reply. I understood it.

So only FET specify Ci because of the different characteristic, am i right?

Someone asked why can i find Ci of SN74LVC16373ADGGR(which is a MOSFET), but can't find Ci for those two.

0 Hung Ching Hsu over 5 years ago in reply to Dylan Hubbard

TI__Expert 3086 points

Hi Dylan

Thanks for your kindly response and advice.

So any small capacitance such as 5pF would be alright.

I got it!

0 Clemens Ladisch over 5 years ago in reply to Hung Ching Hsu

Guru 316600 points

Yes; a MOSFET gate behaves like a small capacitor, so C_i would be the most useful property to describe it.

0 Hung Ching Hsu over 5 years ago in reply to Clemens Ladisch

TI__Expert 3086 points

Thanks for your kindly response.

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SN74LS38: Input capacitance(Cin) of Logic IC below