How to protect high-bandwidth low-voltage lines from transients?

Question

I've been using bidirectional TVS diodes for protecting a 2 Mbit single-ended 5V serial bus. Unfortunately, the TVS-es I've been using have too high capacitance, and distort the shape of the bus so serial decode doesn't work. Looking at data sheets, pretty much all of the low-voltage TVS-es have about 3 nF of capacitance at 5V stand-off / 6.5V break-down, or thereabouts. A representative data sheet is here: http://www.littelfuse.com/~/media/Files/Littelfuse/Technical%20Resources/Documents/Data%20Sheets/Littelfuse_TVS%20Diode_SA.pdf

3 nF is enough to distort the output waveform when the inputs connected to that capacitance are all high impedance, and the output is not super low impedance (an AVR microcontroller with a 70 Ohm current limiting resistor.) Quick calculator check says corner freqency of that RC circuit is about 750 kHz, so it sounds plausible that this is my problem.

So, what can I use to protect this bus? Sub-questions: - Is there some particular part/series of TVS that performs substantially better? - Should I re-buffer the signal? (Unfortunately, I'm out of board space, and it'd be a pain to make more.) - Would a small-signal Zener be "good enough" or not fast enough for transient protection? - Or maybe Zeners have high capacitance, too?

Answer 1

You just have to find the right part. For example, this NUP2114 TVS diode module has an advertised 0.8pF typical capacitance, and 1pF max. This module was designed for USB so it has 2 bi-directional data lines and 1 uni-directional line for the Vbus line, but you can find stand-alone bidirectional parts (or put 2 unidirectional parts in reverse bias) which meet your capacitance allowance.

Browsing quickly on Digikey brought up this ESD9B5.0ST5G single bi-directional TVS diode which has a typical 15pF capacitance.