I am struggling with an input protection design for my FX2LP logic analyzer (3.3 V microcontroller with 5 V tolerant pins).
I wanted to achieve the following constraints for the 8 input pins of the logic analyzer:
- high impedance (> 50k)
- high speed (16 MHz, 62.5 ns signal period)
- 12 V tolerant (continuous)
- threshold voltage of 2 V (already matches the FX2LP specification)
- ESD protected (2k V)
- 3.3V or 5V supply
So far I have been looking at the following ideas:
- Buffer IC
- ESD Protection Diode + Zener Diode (for continous 12v over-voltage protection)
- Using an TVS Diode array for continuous over-voltage and ESD protection
1) Buffer IC
The CD4050BPW fulfills all input voltage requirements, but seems too slow (propagation delay of 70 ns and rise time of 80 ns with 5 V input) It was suggested here 2.
2) ESD Protection Diode + Zener Diode (for continous 12v over-voltage protection)
A regular zener diode however seems to cause too much capacitive load, reducing the maximum signal frequency 3.
3) Using an TVS Diode array for continuous over-voltage and ESD protection
I was looking into the SLVU2.8-4, and starting a simulation in spice based on the data mentioned here 4. By doing so I realized: The high impedance (added via a resistor before the TVS) causes a low input current which results in slower diode charge times.
To give a better picture, here is a screenshot of the simulation run with 3 different resistors (500, 5k and 50k): Good signal quality seemed only possible with the lowest resistor (500 Ohm) The SLVU2.8-4 already has a "extremely low" Junction Capacitance of 5 pF. So realizing a high impedance, ESD-protected and continuous 12 V tolerant input with just a TVS Diode array seems difficult to me. (Of course, it could also be that my results are wrong?).
I am still quite new to electrical engineering and I might be taking a completely wrong approach. So if you have some ideas how to achieve this (maybe Digital Isolator ICs or a faster buffer IC?), I would be very grateful to hear about it!
Thanks a lot!