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:
CRYSTAL OSCILLATOR AMPLIFIER ENABLE
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.