AD9122 Analog Devices, AD9122 Datasheet - Page 45

• Manufacturer Part Number: AD9122
• Manufacturer: Analog Devices

Specifications of AD9122

• Resolution (bits): 16bit
• Dac Update Rate: 1.23GSPS
• Dac Settling Time: n/a
• Max Pos Supply (v): +3.47V
• Single-supply: No
• Dac Type: Current Out
• Dac Input Format: LVDS,Par

PLL SETTINGS
Three settings for the PLL circuitry should be programmed to
their nominal values. The PLL values shown in Table 23 are the
recommended settings for these parameters.
Table 23. PLL Settings
PLL Control Register
PLL Loop Bandwidth[1:0]
PLL Charge Pump Current[4:0]
PLL Cross-Control Enable
CONFIGURING THE VCO TUNING BAND
The PLL VCO has a valid operating range from approximately
1.0 GHz to 2.1 GHz covered in 63 overlapping frequency bands.
For any desired VCO output frequency, there may be several
valid PLL band select values. The frequency bands of a typical
device are shown in Figure 65. Device-to-device variations and
operating temperature affect the actual band frequency range.
Therefore, it is required that the optimal PLL band select value
be determined for each individual device.
Automatic VCO Band Select
The device has an automatic VCO band select feature on chip.
Using the automatic VCO band select feature is a simple and
reliable method of configuring the VCO frequency band. This
feature is enabled by starting the PLL in manual mode, then
placing the PLL in auto band select mode. This is done by
setting Register 0x0A to a value of 0xCF, then to a value of
0xA0. When these values are written, the device executes an
automated routine that determines the optimal VCO band
setting for the device. The setting selected by the device ensures
that the PLL remains locked over the full −40°C to +85°C
operating temperature range of the device without further
adjustment. (The PLL remains locked over the full temperature
range even if the temperature during initialization is at one of
the temperature extremes.)
Figure 65. PLL Lock Range over Temperature for a Typical Device
12
16
20
24
28
32
36
40
44
48
52
56
60
1000
0
4
8
1200
1400
VCO FREQUENCY (MHz)
1600
Register
Address
0x0C
0x0C
0x0D
1800
Bits
[7:6]
[4:0]
4
2000
Optimal
Setting
11
10001
1
2200
Rev. B | Page 45 of 60
Manual VCO Band Select
The device also has a manual band select mode (PLL manual
enable, Register 0x0A, Bit 6 = 1) that allows the user to select
the VCO tuning band. In manual mode, the VCO band is set
directly with the value written to the manual VCO band bits
(Register 0x0A, Bits[5:0]). To properly select the VCO band,
follow these steps:
1.
2.
3.
4.
5.
Table 24. VCO Control Voltage Range Indications
VCO Control Voltage
(Register 0x0E, Bits[3:0])
1111
1110
1101
1100
1011
1010
1001
1000
0111
0110
0101
0100
0011
0010
0001
0000
Put the device in manual band select mode by setting
Register 0x0A, Bit 6 = 1.
Sweep the VCO band over a range of bands that results in
the PLL being locked.
For each band, verify that the PLL is locked and read the PLL
using the VCO control voltage bits (Register 0x0E, Bits[3:0]).
Select the band that results in the control voltage being
closest to the center of the range, that is, 1001 or 1000 (see
Table 24). The resulting VCO band should be the optimal
setting for the device. Write this value to the manual VCO
band bits (Register 0x0A, Bits[5:0]).
If desired, an indication of where the VCO is within the
operating frequency band can be determined by querying the
VCO control voltage. Table 24 shows how to interpret the
PLL VCO control voltage value (Register 0x0E, Bits[3:0]).
Indication
Move to higher VCO band
VCO is operating in the higher end
of the frequency band
VCO is operating within an optimal
region of the frequency band
VCO is operating in the lower end
of the frequency band
Move to lower VCO band
AD9122