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TMUX646: Switching balanced signals without a DC bias

TonyMotegi
Prodigy 130 points
Part Number: TMUX646

Hi there, 

My application is not MIPI, but the performance of this part looks well suited for my application in terms of crosstalk, insertion loss, bandwidth, etc.

I would like to use this device to switch some balanced signals that I am driving using a 50 Ohm line driver through a BALUN to drive a 100 Ohm diff pair over CAT5 cable. 

I have a concern that the BALUN will lose me any useful DC biasing and therefore the signal will float and potentially clip the rails / cause distortion. Am I right to be concerned here? I don't really want to add a DC biasing resistor as this will introduce other problems to the function of my circuit. 

The attached schematic shows the concept, where pins 1 and 3 of the BALUN will have no DC voltage reference and therefore MIPI switch U5 cannot be guaranteed signals within the recommended operating voltage. Pins 4 and 5 (unbalanced side) will have references further down the signal chain, past U1 and U6, so I am not concerned about this side.

My frequency range is 1MHz to 250MHz. My signal level will be max 10dBm on the 50 Ohm unbalanced side (2V peak to peak).

Any guidance much appreciated.

Thanks

Tony

  • 0
    TI__Mastermind 24440 points

    Hi Tony,

    If the signal voltage into the mux is outside the recommended operating conditions it could lead to the mux being  damaged.

    Just to clarify if the signals you passed through U5 end up being  1MHz to 250MHzMy and (2V peak to peak) then your application should be fine) however we do no know that for sure

    I think the best thing we can do is find out if you could use a different  line driver to help you not lose any DC biasing. This looks to be something the TI RF team might be able to help you on so I will send this over for them to take a look!

    Regards,

    Kameron

  • 0 in reply to Kameron Hill
    Prodigy 130 points

    Hi Kameron, 

    Thanks for your reply. I am actually using a DDS driver which has a single ended output. Reason being, I need to put it through an RF power meter that has a single ended interface. I need to measure return loss or VSWR of the signal going out, which is why I have this constraint. I could look further into biasing the balanced signal using a 1V8 reference and some high value resistors (say 4.7MOhm either side of the 1V8), but I would certainly be interested in any advice from your RF team - thanks for hooking me up. 

  • 0 in reply to TonyMotegi
    Prodigy 130 points

    Hi Kameron, 

    Don't worry - I am using the BALUN the other way round and using the centre tap to bias the network.  Thanks