SN74AXC8T245: Need for termination resistors on input to a logic analyzer front end

Ted Mawson 3.0

Part Number: SN74AXC8T245

I'm using this IC as the input for a logic analyzer that can adapt to different logic levels by changing Vcca.

I've included a 200 Ohm series input resistor (R0) and there's a Zener diode protecting the input against over voltage after that resistor (see picture). But the question arises as to whether I should terminate the signal by a load resistor across the Zener; if a square wave signal comes in, won't it bounce back from the A8 input?

I've built and tested this and it works fine without the resistor but I'm curious what TI would recommend - my tests have gone up to 50 MHz (from a square wave generator) which (I think) is the highest I would possibly need.

What do you guys think, should I include the extra load resistor or not?

Ted

over 5 years ago

0 Chad Crosby over 5 years ago

TI__Expert 6680 points

Hi Ted,

If you've already tested the circuit, and can see that it works as intended, I would advise not changing anything.

If you were to add a resistor in parallel with the zener, the only thing that would change is the voltage seen by the A8 pin would be smaller (from the voltage divider equation).

Thanks!

Chad Crosby

0 Ted Mawson 3.0 over 5 years ago in reply to Chad Crosby

Expert 1795 points

Thanks Chad,

I'll leave it as is.

Ted

Logic

Logic forum

SN74AXC8T245: Need for termination resistors on input to a logic analyzer front end