DP83848 Load capacitance range at 25MHz

masahiko shimizu

In Table 7-3 of datasheet, Load capacitance is showed as "Min:25pF, Max:40pF".
But there is the following sentence in 7.3.2 Crystal.
"A 25 MHz, parallel, 20 pF load crystal resonator should be used if a crystal source is desired."

Could you let me know Load capacitance range at 25MHz, please?

Best regards,
Shimizu

over 10 years ago

0 Patrick OFarrell over 10 years ago

TI__Mastermind 31600 points

I have moved your post to the Ethernet forum. We will respond shortly with guidance on your question.

Patrick

0 Rob Rodrigues over 10 years ago

TI__Mastermind 19780 points

Hi Shimizu,

The range in table 7-3 is correct based on the the parallel load cap configuration shown in Figure 7-3 of the datasheet.

Cl1 and Cl2 appear in series to the crystal(or closely so), therefor the series equivalent of Cl1 + Cl2 have to equal the load capacitance required by the crystal.

A 20pF load crystal will require approximately Cl1 = CL2 = (Load * 2 - 7pf) which is why our DS recommends starting with 33pF caps.

The range in the table 25pF to 40pF allows for adjustment of load capacitance to meet the +/-50ppm spec.

Best Regards,

0 masahiko shimizu over 10 years ago in reply to Rob Rodrigues

Guru 11235 points

Hi Rob,

Thank you for your support.
I understood.

Thanks,
Shimizu

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DP83848 Load capacitance range at 25MHz