DAC3171EVM: Data converters forum

joao presa

Part Number: DAC3171EVM
Other Parts Discussed in Thread: DAC3171, CDCE62005

Hi,

The schematics of the DAC3171EVM show (on the top left corner following the signals DAC_DACCLKC,DAC_DACCLK,DACCLKC,DACCLKC) a circuit like this:

(this image was adapted from TI's application report "AC-Coupling Between Differential LVPECL, LVDS, HSTL, and CML" )

But according to the data sheet the input clock DACCLKP/N is supposed to be LVPECL(self biased). What am I missing? Is this the correct way to couple the clock signal?

Thank you.

over 3 years ago

0 jim s over 3 years ago

TI__Guru* 91947 points

Joao,

The circuit shown in the DAC3171EVM should be used if you are using the DAC3171 with a LVPECL clock source. This is a LVPECL to LVPECL interface. The circuit shown above from the application report is a LVPECL to LVDS interface.

Regards,

Jim

0 joao presa over 3 years ago in reply to jim s

Prodigy 170 points

Thank you Jim,

But my problem is that the circuit shown in the DAC3171EVM schematics is identical to the one shown above. That's why I don't understand. The input of the dac clk should be lvpecl.

Regards,

Joao

0 jim s over 3 years ago in reply to joao presa

TI__Guru* 91947 points

Joao,

If you look closely at the schematic, the two pull-up resistors have 'DNI" labeled by them. This means "Do not install". The circuits are not the same when these are not installed. The board has options for different types of termination to be used.

Regards,

Jim

0 joao presa over 3 years ago in reply to jim s

Prodigy 170 points

Thanks for the reply,

I'm refering to the resistors R66 and R67 (150 ohms) , those don't say DNI. Sorry for not uploading the image from the schematic, I don't know if I can upload it.

Regards,

Joao

0 joao presa over 3 years ago in reply to jim s

Prodigy 170 points

Thanks for the reply,

I'm refering to the resistors R66 and R67 (150 ohms) , those don't say DNI. Sorry for not uploading the image from the schematic, I don't know if I can upload it.

Regards,

Joao

0 jim s over 3 years ago in reply to joao presa

TI__Guru* 91947 points

Joao,

These are required for LVPECL. This is what we use on almost all of our EVM's when driving a LVPECL output. See page 16 of the CDCE62005 data sheet.

Regards,

Jim

0 joao presa over 3 years ago in reply to jim s

Prodigy 170 points

Thank you,

I get it now. I was making assumptions. My problem actually appeared from comparing the DAC3171 to CDCE62005 since it has internal terminations. I assumed that the lvpecl interface would be similar (even if external). (Which from a differential signal point of view it is.)

Regards,

Joao