Digital inputs bandwidth

[bobtidey]

@jpa

If the external diodes have high capacitance, they ruin the digital input bandwidth. Because the digital inputs otherwise go directly to FPGA, they should be good for up to 72MSps without the external diode.

The TPD2EUSB30 pack I have ordered is designed for protecting 6GB/s USB 3 connections and has 0.7pf capacitance to ground. It should have negligible impact on achieved bandwidth.