ADS54J40: about LVPECL input amplitude

user5983069

Part Number: ADS54J40
Other Parts Discussed in Thread: LMK04828

Hello

I desghined like a below figure.
Received clock Amplitude between PN pin is about 900mV(frequency is 737.28MHz)
LMK04828 outputs LVPECL 1600mV when I removed both ac-coupled capacitor.
Would you please teach me why amplitude reduced?

Thank you

over 2 years ago

0 jim s over 2 years ago

TI__Guru* 91947 points

User,

You must use AC coupling for the clock. Why are you removing them? The clock inputs are self biased to 1.15V and if you use DC coupling, if the DC common mode voltage is not correct, the amplitude will be effected.

Regards,

Jim

0 user5983069 over 2 years ago in reply to jim s

Intellectual 320 points

Jim

Thank you for replying.

I just wanted to confirm that LMK04828 output is correct(1,6V).

So I use AC coupling capacitor ,but amplitude is 0.9V.

Added in figure below.

Takeshi

0 jim s over 2 years ago in reply to user5983069

TI__Guru* 91947 points

Takeshi,

The LMK has two options for LVPECL, 1600mV and 2000mV output swing. I would suggest using 2000mV with AC coupling.

Regards,

Jim

0 user5983069 over 2 years ago in reply to jim s

Intellectual 320 points

Jim

Thank you for replying.

I know LMK has other option. I just want to know why amplitude is reduced.

+1 jim s over 2 years ago in reply to user5983069

TI__Guru* 91947 points

Takeshi,

The drop is due to the 400 Ohm single ended internal resistance from CLKP and CLKM to common mode and the 100 Ohm termination resistor. The minimum spec on the input swing is 0.8V so you are within spec.

Regards,

Jim

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ADS54J40: about LVPECL input amplitude