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Is it OK to connect an AC coupled SoC into the ADV7619, even though the input may be over 3.3V?

Question asked by fpga77 on Dec 2, 2015
Latest reply on Dec 7, 2015 by PaulS

Note, I originally posted this question as a reply to this discussion on ADV7619 AC coupling.

https://ez.analog.com/message/145899#145899

 

Here it is again as a new question.

 

On my schematic I have a ADV7619 receiving an HDMI signal from an embedded SoC (System on Chip). The SoC's output signals are internally AC coupled with a Vswing measured at about 500mV on its evaluation board when connected to a Dell monitor. The HDMI levels were roughly ~3.3 +/- 250mV = 3.05 to 3.55V, (DC coupled scope signal). scope_111.png attached. The AC coupled scope signal, scope_108.png attached, shows more detail including some transitions with higher Vswing overshoot.


Some questions:

Q1 If I connect such a signal to the ADV7619, will that be OK for all HDMI rates up to 4K30 ? The connection is on-board across 3"( 7cm) of FR4 ?

Q2 The signal exceeds AVCC of 3.3v but does that matter, or are there clamping diodes on the TMDS inputs that would clip a 3.6V signal?

 

Q3 If I use the AC coupling schematic from page 3 of your AC coupling solution.ppt file (attached in the email thread https://ez.analog.com/message/145899#145899) are the following correct?

3a. The mid level at the ADV7619 TMDS inputs will still be the termination voltage of 3.3V (for the dc balanced, ac coupled signal)

3b. The single ended Vswing seen at the receiver will be half that present on the HDMI input connection on the left, because the external 50ohms and internal ADV7619 50ohms are in series and act as a potential divider.

3c My V swing of 500mV will be halved to 250mV but that's still over the level the ADV7619 can receive.

 

Thank you

 

Munro

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