> It's not clear C-coupling is what's going on here (the wave shape looks > pretty sharp for what I understand of the circuit/layout). > Notably though, C-coupling would remove any DC bias, but David's screen shot > indicates a DC bias on the line. > > Is this line currently connected to the FPGA, or is it just the wire and R? > Perhaps the bias is coming from the FPGA, with C-coupling of the wave via the > wire. > Or perhaps it's all crosstalk from within the FPGA, 'visible' because of the > high load R.
Yes, the wire is connected to the FPGA at one end. That FPGA I/O pin is *supposed* to be configured for high-Z but that's the only place I can see the DC bias coming from. > If the wire and FPGA pin are connected, separate them (reduce the wire > circuit to just the wire and R to GND): see whether the DC bias and/or the > square wave disappear. Because of the way it's built, I'm not seeing a reasonable way to disconnected the FPGA while leaving the rest intact. However, I did move the signal to another wire in the ribbon cable and the problem is basically gone. This lets me move on with other issues but am still a bit puzzled with this one.
