AD9125 Analog Devices, AD9125 Datasheet - Page 41

• Manufacturer Part Number: AD9125
• Manufacturer: Analog Devices

Specifications of AD9125

• Resolution (bits): 16bit
• Dac Update Rate: 1GSPS
• Dac Settling Time: n/a
• Max Pos Supply (v): +3.47V
• Single-supply: No
• Dac Type: Current Out
• Dac Input Format: Par

PLL Settings
There are three settings for the PLL circuitry that should be
programmed to their nominal values. Table 24 lists the
recommended PLL settings for these parameters.
Table 24. PLL Settings
PLL SPI Control
PLL Loop Bandwidth[2:0]
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 writing 0x80 to Register 0x0A. When this
value is 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
Address
Register
0x0C
0x0C
0x0D
1800
Bits
[7:5]
[4:0]
4
2000
Optimal
Setting
110
10001
1
2200
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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. When in manual mode, the VCO band
is set directly with the value written to the manual VCO band,
(Register 0x0A, Bits[5:0]). To properly select the VCO band,
follow these steps:
1.
2.
3.
4.
5.
Table 25. VCO Control Voltage Range Indications
VCO Control Voltage
1111
1110
1101
1100
1011
1010
1001
1000
0111
0110
0101
0100
0011
0010
0001
0000
Put the device in manual band select mode.
Sweep the VCO band over a range of bands that result in
the PLL being locked.
For each band, verify that the PLL is locked and read the
PLL using the VCO control voltage (Register 0x0E,
Bits[3:0]).
Select the band that results in the control voltage being
closest to the center of the range, that is, 0000 or 1000 (see
Table 25 for more details). The resulting VCO band should
be the optimal setting for the device. Write this band to the
manual VCO band (Register 0x0A, Bits[5:0]) value.
If desired, an indication of where the VCO is within the
operating frequency band can be determined by querying
the VCO control voltage. Table 25 shows how to interpret
the PLL VCO control voltage (Register 0x0E, Bits[3:0]) value.
Indication
Move to a 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 a lower VCO band
AD9125