SN74AHC245: what is the input impedance of SN74AHC245QPWR

Yangbo

Part Number: SN74AHC245

What is the input impedance of SN74AHC245QPWR? When I put an RC filter(100ohm+1nF) at the input. The high level and low level of input signal are 5V and 0V. But i can only see 4.1V after the resistor. So i am wondering if it is ok to add an RC filter at the input pin.

Regards,

Yangbo

over 5 years ago

0 Dylan Hubbard over 5 years ago

TI__Mastermind 42600 points

Hi Yango,

The input is high impedance. There is an input leakage spec which shows the max amount of current that will flow in or out of the device. I don't recommend using a filter at the input for another reason. That reason being, it will slow down the rise and fall times at the input which would most likely violate the input transition spec.

0 Yangbo over 5 years ago in reply to Dylan Hubbard

TI__Intellectual 1690 points

Hi Dylan,

Thank you for your reply.

I still don't understand the connection between RC filter and leakage specs you mentioned as well as input transition spec. can you give an example or explain more detail?

Regards,

Yangbo

0 Dylan Hubbard over 5 years ago in reply to Yangbo

TI__Mastermind 42600 points

Hi Yangbo,

Here are the specs I am talking about from the datasheet:

Input Transition rate

Input Leakage

0 Yangbo over 5 years ago in reply to Dylan Hubbard

TI__Intellectual 1690 points

Hi Dylan,

Thank, but it still can't explain the voltage drop on the 100 ohm resistor if the input impedance is high.

i know these two spes, what i want to learn from you is how the RC filter effect them. what will happen if i add a RC filter?

Regards,

Yangbo

0 Dylan Hubbard over 5 years ago in reply to Yangbo

TI__Mastermind 42600 points

Hi Yangbo,

Could I get a schematic and scope measurements to better help me debug this? The RC filter will slow down the transition rate.

0 Dylan Hubbard over 5 years ago in reply to Dylan Hubbard

TI__Mastermind 42600 points

Hi Yangbo,

I haven’t heard back from you, I’m assuming you were able to resolve your issue. If not, just post a reply below (or create a new thread if the thread has locked due to time-out)

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SN74AHC245: what is the input impedance of SN74AHC245QPWR