I'm working on a logic analyzer design where a SN74AXC8T245 is used at the input to translate the sampled logic from whatever voltage it's running at to 3.3V logic levels that the analyzer handles. The SN74AXC8T245 is set up to pass A to B and VccB is 3.3V. I set it up so that VccA can be taken from an on-board 100 mA adjustable supply ranging from 0.65V to 3.6V thus dealing with logic levels within that range (or VccA can be supplied from the external circuit - switch-selectable).
I want to provide some protection to the SN74AXC8T245 side A inputs and have included a series resistor which I am thinking could be 10k or higher given the low input current. I could add a 3.6V zener diode as shown in the picture below but doesn't the SN74AXC8T245 have built in clamping diodes?
I've been asked to make the inputs capable of operating with 5V logic rail level inputs; obviously VccA max is 3.6V but I think that it would still translate 0s and 1s from a 5V logic signal as long as the input to the SN74AXC8T245 doesn't get fried which goes back to the previous paragraph.
Any thoughts or suggestions for front end components and values would be appreciated.
Ted
