TS3USB221: AC coupling to inputs/outputs

Graeme Parker

Part Number: TS3USB221

Hello

This device is being used as an LVDS clock selector. The design I'm creating is based on an existing one. The existing design AC couples both the inputs and outputs. Is this a suitable or sensible thing to do using this device?

My thoughts were to AC couple the output as that is what is suggested by the downstream component, but DC couple the inputs.

over 5 years ago

0 Malik Barton57 over 5 years ago

TI__Guru 64326 points

Hi Graeme,

This is sensible, AC coupling the inputs in your design will protect against common voltages that violate the TS3USB221 datasheet. These common mode voltages would be generated by the clock sources.

0 Graeme Parker over 5 years ago in reply to Malik Barton57

Expert 1265 points

Thanks.

So the TS3USB221 is happy without an applied DC level on its inputs.

0 Malik Barton57 over 5 years ago in reply to Graeme Parker

TI__Guru 64326 points

Hi Graeme,

Not exactly, I always recommend that a weak pull down be placed on inputs to bias the fully AC coupled passive switch.

0 Graeme Parker over 5 years ago in reply to Malik Barton57

Expert 1265 points

What value would you put on "weak"?

Thanks.

0 Malik Barton57 over 5 years ago in reply to Graeme Parker

TI__Guru 64326 points

About ~330k ohm to GND is what I recommend for typical USB use cases.

0 Graeme Parker over 5 years ago in reply to Malik Barton57

Expert 1265 points

Thanks again.

I notice that your evaluation module does not have coupling caps.

If we can guarantee that the common mode voltage won't be violated, then we should also dc couple? And not need the 330k pull-down.

0 Malik Barton57 over 5 years ago in reply to Graeme Parker

TI__Guru 64326 points

This is correct, AC coupling for passive muxes typical is only needed to prevent common mode voltage violation.

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TS3USB221: AC coupling to inputs/outputs