We're using a DS90UB954 deserializer with a 20-ft coax cable in an application that requires 4 kV of isolation and 60601 compliance. Our current solution, which works fine, takes the MIPI data into an FPGA which converts it to a wide 20-bit parallel bus that goes into a stack of digital isolators. While this works, we're making architectural changes that mean we need to keep everything MIPI between the deserializer and the processor.
There are two places we could put the isolation barrier: at the high-speed serial input, or at the MIPI output.
Let's start with the MIPI side. So, I've never seen a MIPI isolator solution. LVDS isolators don't work, since MIPI isn't purely LVDS; there are high-voltage single-ended parts of the protocol. I'm not sure you could use transformers to isolate the MIPI bus since there's quite a bit of a DC component to the signal — does anyone have thoughts about this?
Moving onto serial input isolation, the inputs to the DS90UB954 are capacitively-coupled, so we thought about just using those capacitors as an isolation barrier, but they're rated at 0.1 uF, which would cause way too much leakage current with a mains-applied 60601 test (we need to say in the 100-1000 pF range). I tried reducing the capacitance but the deserializer drops out.
So that brings me to transformer-based isolation. We'd like to try something like this:
I ran the numbers on the back of a napkin and I think high-quality 2.5 Gbps Ethernet magnetics will have low enough return loss to pass a signal across them (while it's passable, the numbers aren't great, and I could be wrong on my math, though). I'm just not sure which input configuration to use on the DS90UB954. Do we connect it as shown above? Should we tie the CT pin to a capacitor connecting to GND to establish a reference voltage? Or some other DC operating point?
I assume we have to change the configuration register to put it into STP mode instead of coax mode? (I'm not exactly sure what that register change does....)
Or we could keep the chip in single-ended mode and do something like this:
There's not a lot of information in the datasheet about what the inputs look like in the DS90UB954, so I'm not sure the correct configuration. Or maybe the idea won't work at all for some other reason I'm not thinking of?
We spun a little breakout board that has several resistor/capacitor options that will allow us to evaluate different configurations:
Unfortunately, so far I've been unable to get it working, and there's a lot of variables to change, so I'd love some guidance!
