AD9785-EBZ datasheet(49/64 Pages) AD

49 / 64 page

AD9785/AD9787/AD9788

Rev. 0 | Page 49 of 64

Configuring the PLL Band Select Value

The PLL VCO has a valid operating range from approximately

1.0 GHz to 2.0 GHz covered in 63 overlapping frequency bands

as shown in Table 35. For any desired VCO output frequency,

there are multiple valid PLL band select values. Note that the

data shown in Table 35 is for a typical device. Device-to-device

variations can shift the actual VCO output frequency range by

30 MHz to 40 MHz. Also, the VCO output frequency varies as

a function of temperature. Therefore, it is required that the

optimal PLL band select value be determined for each

individual device at the particular operating temperature.

The device has an automatic PLL band select feature on chip.

When enabled, the device determines the optimal PLL band

setting for the device at the given temperature. This setting holds

for a ±60°C temperature swing in ambient temperature. If the

device operates in an environment that experiences a larger

temperature swing, an offset should be applied to the automat-

ically selected PLL band. The following procedure outlines a

method for setting the PLL band select value for a device

operating at a particular temperature that holds for a change in

ambient temperature over the total −40°C to +85°C operating

range of the device without further user intervention. (Note that

REFCLK must be applied to the device during this procedure.)

Configuring PLL Band Select with Temperature Sensing

The values of N1 (Register 0x04, Bits [14:13]) and N2

(Register 0x04, Bits [12:11]) should be programmed along

with the PLL settings shown in Table 34.

Set the PLL Band Select [5:0] value (Register 0x04,

Bits [7:2]) to 63 to enable PLL auto mode.

Wait for the PLL_LOCK pin or the PLL lock indicator

(Register 0x09, Bit 3) to go high. This should occur

within 5 ms.

Read back the 6-bit PLL band select value (Register 0x04,

Bits [7:2]).

Based on the temperature when the PLL auto mode is

enabled, set the PLL band indicated in Table 36 or Table 37

by rewriting the readback values into the PLL Band Select

[5:0] parameter (Register 0x04, Bits [7:2]).

Table 36. Setting Optimal PLL Band for Lower Range

(0 to 31) Bands

System Start-Up Temperature

Set PLL Band to

−40°C to −10°C

Readback Band + 2

−10°C to +15°C

Readback Band + 1

15°C to 55°C

Readback Band

55°C to 85°C

Readback Band − 1

Table 37. Setting Optimal PLL Band for Higher Range

(32 to 62) Bands

System Start-Up Temperature

Set PLL Band to

−40°C to −30°C

Readback Band + 3

−30°C to −10°C

Readback Band + 2

−10°C to +15°C

Readback Band + 1

15°C to 55°C

Readback Band

55°C to 85°C

Readback Band − 1

Known Temperature Calibration with Memory

The procedure in the Configuring PLL Band Select with

Temperature Sensing section requires temperature sensing

upon start-up or reset of the device to choose the optimal PLL

band select value to hold over the entire operating temperature

range. If temperature sensing is not available in the system,

another option is to use the automatic PLL band select to

determine the optimal setting for the device when the device is

in a factory environment where the temperature is known. The

optimal band can then be stored in nonvolatile memory.

Whenever the system is powered up or restarted, the optimal

value can be loaded back into the device.

AD9785-EBZ Datasheet(PDF) 49 Page - Analog Devices

AD9785-EBZ Datasheet(HTML) 49 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64

Datasheet Download

Link URL