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AD9578 reference input

Question asked by cosmolittle on Jun 8, 2016
Latest reply on Jun 23, 2016 by SteveBeccue

I have repeated this question as a new post, rather than as a comment in my previous post.


Referring to the development board, I want to modify the XO4,XO3 inputs for an  external differential clock. I have added the balun to the board, however I find the section in the data sheet about the reference/crystal inputs very confusing. Presumably for an external input I should disable the xtal oscillator amplifier. This is the section from the data sheet:



The crystal oscillator amplifier is automatically enabled when

either the PLLx or REFOUT bit in Register 2 uses the crystal

oscillator for either Reference 1 or Reference 2. Otherwise, the

crystal oscillator amplifier is disabled if neither the PLLx nor

REFOUT bit selects that input. However, this setting can be

overridden with the enable XTAL1 bit in Register 3 and enable

XTAL2 bit in Register 10. Setting these bits forces the

corresponding crystal oscillator on.

These bits are useful to allow a crystal to power up and stabilize

before it is needed. However, these bits are usually set to 0

under normal operation.



In order to select an external reference on XO4 and XO3 for both PLL, I should set REFSEL1 enable, REFSEL1,REFSEL2 enable, and REFSEL2  to 1. However this appears to leave the xtal oscillator amplifier on, however the XTAL2 bit is set.

What effect does this have on the input characteristics of XO4 and XO3?


If possible, could you please supply an equivalent circuit of the reference inputs.


The development board biases both inputs (XO3 and XO4) at 0.9V, however the specifications for the reference inputs in the data sheet states that these may be AC coupled for differential inputs, with no mention of the need for DC biasing.


best regards


Cosmo Little