Looking at the schematics, trying to learn.
Please correct me if I'm wrong. Am I correct to think that the purpose of this set is to:
- Decouple/buffer (not sure of the correct term here, but those two should convey the idea-hopefully?) the inputs (the 200R resistor)
- Slightly load the inputs so that their levels aren't floating (the 10k resistors)
- Shift The inputs levels to a specific voltage (using the SN74AXC8T245)
- Convert the single-ended inputs to a differential (LVDS) signal (DS90LV047A), which is what the scope inputs expect
And the main difficulty is that this is a rather high-speed "product" so length matching and impedance of the power sources becomes critical?
Right?
I am just clueless about the note "Vref is connected to GND at remote end" (on the connector towards the scope). Why is the Vref connected to ground? Why on the remote end and not locally?
1. There are 200 Ohm resistors in series as it's good practice to consider what happens if an over current or over voltage situation occurs
2. Yes, the 10k resistors are there to provide a load on the inputs. The level translator datasheet says that the inputs should not be left floating but I find it works just fine with no 10k load resistors
3. Yes, on the probe PCB, one of the key functions is to perform voltage level translation
4. Yes, the scope needs to see LVDS differential signals
Other comments.
- Trace length matching isn't super critical at 200 MHz but I have matched within a few mm for differential pairs. I have tested at 200 MHz clock rate and it works.
- Vref is a ground that has a confusing label. I originally fed Vref from the scope, through the connector PCB, through the ribbon cable, and to the header on the probe PCB but then didn't use it so I made it an extra ground feed from the connector PCB; I'm just pointing out that the pin marked Vref is really a ground connection. The Vref that comes from the scope is the one that is supposed to set the logic level but
it doesn't do anything, the voltage level translators take care of that based on the VccA voltage, it's detailed in the datasheets but you don't need to worry about it, just set the VccA for the system you're probing.