I am designing a board with 50 and 75 ohm tracks on the same layer.My board receives SDI video via RG179 coax. The board has an SMA connector that receives the said input. The track is 75Ω track.

This is given to an LMH0344 chip that converts it into differential pair. The said chip is an adaptive filter chip which accepts either differential pair or 50Ω signal. So I am using an impedance matching pad - Link.

The differential signal is sent to an LMH0346 reclocker chip. The output of which is sent to an LMH0307 chip. The output is split RG179 video which means the 2 signals are 75Ω.

Now, the same layer has 75,50 and differential 100Ω. I am using a 4 layer stack-up Top->Gnd layer->Vcc layer->Bottom layer. Any tips in layout and routing of these signals ?

  • \$\begingroup\$ Beware of CM noise causing data errors at GHz even with balanced differential to an external monitor with local ground noise issues. I suggest you use a miniature SMD Balun with y cap for Tx,Tx_ . My i8 Asus MOBO has HDMI issues with best coax on HDMI 4x with CM noise. FYI only. Video is usually 75 Ohms . Then add gnd lines between strip line or microstrip on same layer at gap= thickness for reducing crosstalk \$\endgroup\$ Feb 10 '18 at 16:20
  • \$\begingroup\$ Board-man, have you done with this Q and A now? If you have you should sign it off. If not then please raise a comment for clarification. \$\endgroup\$
    – Andy aka
    Jan 22 at 8:39

The said chip is an adaptive filter chip which accepts either differential pair or 50Ω signal.

Get rid of the unnecessary impedance matching pad to convert 75 to 50 ohm: -

enter image description here

The front page of the data sheet says this: -

Equalizes up to 120 Meters of Belden 1694A

And Belden 1694A cable is already 75 ohm.

Belden 1694A is an excellent, low loss 75 ohm coaxial cable, with superb bandwidth, low capacitance and double shielding (one foil screen, one tinned copper braid).

Taken from here.

This is my only tip in the absense of your schematic.


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