TMDS181: Input AC coupling

Hello David-san,

Thank you for your reply.

>If AC-coupling the TMDS181 input main link, then you need to add external 50ohm termination to 3.3V between the AC coupling cap and the TMDS181 on the main link.

According to datasheet of TMDS181, it seems internal termination(50ohm) to 3.3V(VBIAS) is implemented.

Why user need to add external 50ohm termination to 3.3V redundantly ?

(If it is mistake of "you need to add external 50ohm termination to 3.3V between the AC coupling cap and the "source device"(Opposite of TMDS181), I can understand..)

Best Regards,

Hi

I am referring to the case which the source output requires to be AC coupled, you would then need external 50ohm to 3.3V between the AC coupling cap and the TMDS181 to mimic a DC-coupled HDMI source termination.

Thanks

David

David-san,

I understood that you assume following case.

And I understood that user need to add external termination(50 ohm) for TMDS data/clk as shown below from your reply.

Is my understanding correct ?

BR, 

Your block diagram is correct.

Thanks

David

David-san,

OK. So let me re-confirm previous my following question.

Could you please send feedback about above question of highlighted red ?

BR

Sorry, you are right that there is no need for external termination since termination is already inside the TMDS181. So you only need the external AC coupling cap.

If you connect an AC coupled signal to TMDS181 input, the signal will be mounted on 3.3V, depending on the amplitude of the signal Vh=3.3V+A and Vl=3.3V-A, here Vh will go above Vcc.

If you connect an AC coupled signal to DP159 input, the signal will be mounted on 0.7V, depending on the amplitude of the signal Vh=0.7V+A and Vl=0.7V-A., here the signal is within GND and Vcc.

So for a AC coupled signal, we recommend DP159.

Thanks

David

David-san,

Thank you for your reply.

>If you connect an AC coupled signal to TMDS181 input, the signal will be mounted on 3.3V, depending on the amplitude of the signal Vh=3.3V+A and Vl=3.3V-A, here Vh will go above Vcc.

I have question about above sentence.

According to datasheet, following specs are defined as shown below.

max value of VIDpp : 1560mV

max value of VICM : Vcc + 0.1V 

Absolute max value of IN_Dx and IN_CLKx : Vcc + 0.3V

I think that input signal will become out of spec in this case as well even though user use DC coupling.

Here is my calculation.

* VIDpp = VID+ - VID-

* VICM = (VID+ + VID-) / 2

When the max values are applied to above equation, result is as follows.

* 1560mV = VID+ - VID-

* 3.4V = (VID+ + VID-) / 2

So, VID+ is 4.18V and VID- is 2.62V in this case.

Actually, VID- is spec in Vcc-0.75(defined absolute value), however VID+ is spec out Vcc+0.3V.

Hi,

HDMI uses current drive on the source side to develop the low voltage diff signal at the sink side. The termination voltage on the sink side sets the high voltage level of the diff signal. The low voltage is set by the current at the source and the termination resistance at the sink. 

A compliant HDMI source should not drive the TMDS line exceeding the conditions being set in Table 4-23 of the HDMI spec. 

Thanks

David

Your understanding is correct.

Thanks

David