I have ~100 cards that have 10 FPGA pins connected to a JST SH connector. I would like to connect some of these pins to a oscilloscope for latency measurements.
What I have done so far is using half of the pins as signal, and the second half as reference level 0.
I have prepared a schematics of the connections here:
In the top half of the PDF file, I show two example connections of my setup. The first one looks very good even using a much longer cable. It does not seem to have impedance discontinuities. However, it is also running 4 times slower (40 MHz). The second is connected from the FPGA to the scope thorugh the JST SH connector and my adapter box.
In the bottom half of the PDF file, I have a schematics of the bad setup. As there is no ground connected to the connector, I assign the logic level 0 and disable OCT for the pins used as reference.
The Signal is driven by the same type of buffer but with 50 Ohm series termination enabled (without calibration).
I have tried to to drive GND instead of logic level 0 to the connector pins used as reference. Not much success with it.
The scope is terminated to 50 Ohm for all inputs.
Given that the board cannot be changed, what is the best way to permanently connect FPGA pins through a header connector to a oscilloscope ? I would not like to use a probe. I can open the design box and change the connections.
Can I know probe you using is FET based one ? Couple of years back I saw very good note from NXP about probing ..Sorry not remebering now which application notes.
if you able to maintain the 50 ohm on the measuring end , then defintely you will measure the perfect waveform.