ZHCSHN9A February 2018 – April 2018 LMK05028
PRODUCTION DATA.
The XO input is the reference clock for the fractional-N APLLs. The combination of XO and APLL determines the jitter and phase noise performance of the output clocks. For optimal performance, the XO frequency should be at least 48 MHz and have a non-integer frequency relationship with the VCO frequencies so the APLLs operate in fractional mode. When the TCXO input is not used by either PLL channel, the XO input determines the output frequency accuracy and stability in free-run or holdover modes.
The XO input buffer has programmable input on-chip termination and AC-coupled input biasing configurations as shown in Figure 24.
The buffered XO path also drives the input monitoring blocks as well as output muxes, allowing buffered copies of the XO input on OUT0 and/or OUT1.
Table 4 lists the typical XO input buffer configurations for common clock interface types.
XO_DIFF_TYPE | INPUT TYPES | INTERNAL SWITCH SETTINGS | |
---|---|---|---|
INTERNAL TERM. (S1, S2)(1) | INTERNAL BIAS (S3)(2) | ||
0h | LVDS, CML, LVPECL, LVCMOS
(DC-coupled) |
OFF | OFF |
1h | LVDS, CML, LVPECL
(AC-coupled) |
OFF | ON (1.3 V) |
3h | LVDS, CML, LVPECL
(AC-coupled, internal 100-Ω) |
100 Ω | ON (1.3 V) |
4h | HCSL
(DC-coupled, internal 50-Ω) |
50 Ω | OFF |