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Interfacing 3.3V FPGA logic with 5V amplifiers?

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mcdpekala

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I have a series of amplifiers and buses which take a 5V TTL signal. Unfortunately the FPGA I am trying to connect to the things can only give me LVTTL which is 3.3V at max.

Thankfully the threshholds for these two standards are similar so outputwise I don't think it's a problem. However I have a few buses and feed back circuits that also run at TTL as well as some data buses. This is a problem because the FPGA I am using is not 5V tolerent, and I really can't change the amplifiers easily since they interface with devices that expect that 5V TTL logic.

I was hoping someone here might have a good way of dealing with this. Any ideas?
 

Which FPGA are you using? Some FPGAs allow simple techniques to input from 5V logic, such as inserting series resistors to limit the current. Search the FPGA manufacturer's web site for "5V tolerant" suggestions. For example, here's some info on Xilinx Spartan-3 devices:
**broken link removed**
 

Its a Virtex 4, It's not 5V tolerant.
 

This Xilinx Answer Record says the the Virtex-4 can be made 5V tolerant by using series resistors. The FPGA's input clamp diodes can carry up to 10mA.
**broken link removed**

If you don't like that approach, consider the QuickSwitch technique described in this app note:
https://www.xilinx.com/support/documentation/application_notes/xapp646.pdf

TTL outputs usually don't go much higher than about 3.5V.
 

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