AD9518-3A-PCBZ AD [Analog Devices], AD9518-3A-PCBZ Datasheet - Page 27

• Manufacturer Part Number: AD9518-3A-PCBZ
• Description: 6-Output Clock Generator with 6-Output Clock Generator with
• Manufacturer: AD [Analog Devices]

Data Sheet
Charge Pump (CP)
The charge pump is controlled by the PFD. The PFD monitors
the phase and frequency relationship between its two inputs, and
tells the CP to pump up or pump down to charge or discharge the
integrating node (part of the loop filter). The integrated and
filtered CP current is transformed into a voltage that drives the
tuning node of the internal VCO through the LF pin (or the tuning
pin of an external VCO) to move the VCO frequency up or down.
The CP can be set (Register 0x010[6:4]) for high impedance
(allows holdover operation), for normal operation (attempts to
lock the PLL loop), for pump up, or for pump down (test modes).
The CP current is programmable in eight steps from (nominally)
600 μA to 4.8 mA. The exact value of the CP current LSB is set
by the CPRSET resistor, which is nominally 5.1 kΩ. If the value
of the resistor connected to the CP_RSET pin is doubled, the
resulting charge pump current range becomes 300 μA to 2.4 mA.
On-Chip VCO
The
range shown in Table 2. The calibration procedure ensures that
the VCO operating voltage is centered for the desired VCO
frequency. The VCO must be calibrated when the VCO loop
is first set up, as well as any time the nominal VCO frequency
changes. However, once the VCO is calibrated, the VCO has
sufficient operating range to stay locked over temperature and
voltage extremes without needing additional calibration. See the
VCO Calibration section for more information.
The on-chip VCO is powered by an on-chip, low dropout (LDO),
linear voltage regulator. The LDO provides some isolation of
the VCO from variations in the power supply voltage level. The
BYPASS pin should be connected to ground by a 220 nF
capacitor to ensure stability. This LDO employs the same
technology used in the anyCAP® line of regulators from Analog
Devices, Inc., making it insensitive to the type of capacitor used.
Driving an external load from the BYPASS pin is not supported.
Note that the reference input signal must be present and the
VCO divider must not be static during VCO calibration.
PLL External Loop Filter
When using the internal VCO, the external loop filter should
be referenced to the BYPASS pin for optimal noise and spurious
performance. An example of an external loop filter for a PLL
that uses the internal VCO is shown in Figure 32. The third-
order design that is shown in Figure 32 usually offers the best
performance. A loop filter must be calculated for each desired
PLL configuration. The values of the components depend upon the
VCO frequency, the K
desired loop bandwidth, and the desired phase margin. The loop
filter affects the phase noise, loop settling time, and loop stability.
A basic knowledge of PLL theory is helpful for understanding
loop filter design. ADIsimCLK can help with the calculation
of a loop filter according to the application requirements.
AD9518
includes an on-chip VCO that covers the frequency
VCO
, the PFD frequency, the CP current, the
Rev. B | Page 27 of 64
When using an external VCO, the external loop filter should be
referenced to ground. See Figure 33 for an example of an external
loop filter for a PLL using an external VCO. For more information
on suggested loop filters, see the
PLL Reference Inputs
The
allows either a fully differential input or two separate single-ended
inputs. The input frequency range for the reference inputs is
specified in Table 2. Both the differential and the single-ended
inputs are self-biased, allowing for easy ac coupling of input signals.
The differential input and the single-ended inputs share the
two pins, REFIN and REFIN (REF1 and REF2, respectively). The
desired reference input type is selected and controlled by
Register 0x01C (see
When the differential reference input is selected, the self-bias
level of the two sides is offset slightly (~100 mV, see Table 2) to
prevent chattering of the input buffer when the reference is slow
or missing. This increases the voltage swing that is required of the
driver and overcomes the offset. The differential reference input
can be driven by either ac-coupled LVDS or ac-coupled LVPECL
signals.
The single-ended inputs can be driven by either a dc-coupled
CMOS level signal or an ac-coupled sine-wave or square wave.
Each single-ended input can be independently powered down
when not needed to increase isolation and reduce power. Either
a differential or a single-ended reference must be specifically
enabled. All PLL reference inputs are off by default.
The differential reference input is powered down whenever the
PLL is powered down, or when the differential reference input
is not selected. The single-ended buffers power down when the
PLL is powered down, and when their individual power down
registers are set. When the differential mode is selected, the
single-ended inputs are powered down.
Figure 32. Example of External Loop Filter for a PLL Using the Internal VCO
Figure 33. Example of External Loop Filter for a PLL Using an External VCO
AD9518
features a flexible PLL reference input circuit that
AD9518-3
AD9518-3
CHARGE
CHARGE
PUMP
PUMP
VCO
31pF
Table 42
LF
CP
BYPASS
CLK/CLK
CP
and
C
BP
UG-075
Table 44
= 220nF
C1
C1
EXTERNAL
VCO/VCXO
User Guide.
R1
R1
).
C2
C2
R2
R2
C3
C3
AD9518-3