LMH1982 Datasheet, PDF (11/28 Pages) National Semiconductor (TI) – Multi-Rate Video Clock Generator with Genlock

able level depends primarily on the charge leakage on the
loop filter. A typical VCXO has an input impedance of several
tens of kΩ, which will be the dominant leakage path seen by
the loop filter. As the leakage current discharges the residual
control voltage to GND, the output frequencies of the VCXO
and LMH1982 will drift accordingly. If a longer time constant
is required, a precision op amp with low input bias current and
rail-to-rail input and output (e.g. LMP7701) can be used to
buffer the control voltage. The buffer will isolate the relatively
low input impedance of the VCXO and reduce the charge
leakage on the loop filter during Holdover.
The output frequency accuracy will degrade as the VCXO ac-
curacy drifts with the decaying control voltage. Moreover,
because the H_ERROR setting (register 00h) affects the ref-
erence error threshold for LOR indication, a higher setting for
H_ERROR may result in reduced output accuracy upon LOR
indication compared to when H_ERROR = 0. For more infor-
mation on programming H_ERROR, see section 6.1.1 Pro-
gramming the Loss of Reference (LOR) Threshold.
30052411
FIGURE 2. Optional Buffer to Isolate VCXO Input
Impedance from the Loop Filter
3.3 Recognized Video Timing Formats And Standards
Table 2 lists the supported video timing standard formats and
various timing parameter values that can be used to program
the device registers related to the input reference and output
formats.
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