TMUX646: Amplitude degration in 2 cascaded configuration

Kei Kuwahara

Part Number: TMUX646
Other Parts Discussed in Thread: TS5MP646

Hello team,

My customer is considering to use 2 TMUX646 to switch between 3 cameras for CPU like on the figure below.

He will be using them for MIPI D-PHY HS.

The customer is concerned if the voltage level will decrease after going through 2 TMUX646, and it might not satisfy the Vpp or slew rate reguration of MIPI D-PHY HS (min 140mVpp).

Will it satisfy the MIPI specification in this configuration?

Best Regards,

Kei Kuwahara

over 1 year ago

0 Rami Mooti1 over 1 year ago

TI__Mastermind 28088 points

Hey Kei,

What frequency will they be working with? I'm looking for the per lane frequency, not the overall bandwidth (See [FAQ] How do I Select the Right Analog Switch/Multiplexer Bandwidth Based on the Data Rate of my Protocol?).
MIPI is typically DDR and 4 data lanes so whatever the bandwidth is you should divide by 4 lanes and 2 readings per clock cycle (8 total).

Given that MIPI is typically maxed out at 4.5GHz, the max I would suspect they're working with is ~560MHz per lane.

Knowing this, you certainly will have extra loss when cascading the muxes. How much loss will of course depend on this frequency. I found a similar post for a similar device, the previous version of the TMUX646, the TS5MP646

(+) TS5MP646: (TMUX646 Pin-to-pin upgrade supporting 6-GHz bandwidth available) How many TS5MP646 can be used as serial? What happens when you cascade signal switches - Switches & multiplexers forum - Switches & multiplexers - TI E2E support forums

From the tests done above, it looks like they can expect twice the insertion loss, from ~-0.75dB to ~-1.5dB at 500MHz. If the input is 270mV, my math comes out to about a 45mV drop. This of course doesn't account for extra losses from extended the traces. I would recommend to keep these devices as close as possible to ensure that the trace lengths are also as short as they can possibly be. I'm not as familiar with the thresholds though for what is acceptable for MIPI. Is this drop too much? Or do you know what the bounds are here that we are working with?

Thanks,
Rami