AD9775EB datasheet(20/48 Pages) AD

20 / 48 page

REV. 0

AD9775

–20–

OFFSET REGISTER 1 ADJUSTED

DAC1, DAC2 – Offset Register Codes

–1024

–768

–80

1024

–512

–256

256

512

768

–70

–60

–50

–40

–30

–20

–10

OFFSET REGISTER 2

ADJUSTED, WITH OFFSET

TO OPTIMIZED VALUE

Figure 9. Offset Adjust Control, Effect on LO

Suppression

1R/2R MODE

In the 2R mode, the reference current for each channel is set

independently by the FSADJ resistor on that channel. The AD9775

can be programmed to derive its reference current from a single

resistor on Pin 60 by placing the part in the 1R mode. The trans-

fer functions in Equation 1 are valid for the 2R mode. In the

1R mode, the current developed in the single FSADJ resistor is

split equally between the two channels. The result is that in the

1R mode, a scale factor of one-half must be applied to the for-

mulas in Equation 1. The full-scale DAC current in the 1R mode

can still be set to as high as 20 mA by using the internal 1.2 V

reference and a 950

Ω resistor, instead of the 1.9 kΩ resistor

typically used in the 2R mode.

CLOCK INPUT CONFIGURATIONS

The clock inputs to the AD9775 can be driven differentially or

single-ended. The internal clock circuitry has supply and ground

(CLKVDD, CLKGND) separate from the other supplies on the

chip to minimize jitter from internal noise sources.

Figure 10 shows the AD9775 driven from a single-ended clock

source. The CLK+/CLK–

Pins form a differential input (CLKIN),

so that the statically terminated input must be dc-biased to the

midswing voltage level of the clock driven input.

RSERIES

VTHRESHOLD

AD9775

CLK+

CLKVDD

CLK–

CLKGND

0.1

µF

Figure 10. Single-Ended Clock Driving Clock Inputs

A configuration for differentially driving the clock inputs is given

in Figure 11. DC-blocking capacitors can be used to couple a

clock driver output whose voltage swings exceed CLKVDD or

CLKGND. If the driver voltage swings are within the supply

range of the AD9775, the dc-blocking capacitors and bias resistors

are not necessary.

AD9775

CLK+

CLKVDD

CLK–

CLKGND

0.1 F

ECL/PECL

0.1 F

Figure 11. Differential Clock Driving Clock Inputs

A transformer, such as the T1-1T from Mini-Circuits, can also

be used to convert a single-ended clock to differential. This

method is used on the AD9775 evaluation board so that an exter-

nal sine wave with no dc offset can be used as a differential clock.

PECL/ECL drivers require varying termination networks, the

details of which are left out of Figures 10 and 11 but can be found

in application notes such as AND8020/D from On Semiconductor.

These networks depend on the assumed transmission line imped-

ance and power supply voltage of the clock driver. Optimum

performance of the AD9775 is achieved when the driver is placed

very close to the AD9775 clock inputs, thereby negating any

transmission line effects such as reflections due to mismatch.

The quality of the clock and data input signals is important in

achieving optimum performance. The external clock driver cir-

cuitry should provide the AD9775 with a low jitter clock input

that meets the min/max logic levels while providing fast edges.

Although fast clock edges help minimize any jitter that will manifest

itself as phase noise on a reconstructed waveform, the high gain

bandwidth product of the AD9775’s differential comparator can

tolerate sine wave inputs as low as 0.5 V p-p, with minimal

degradation of the output noise floor.

PROGRAMMABLE PLL

CLKIN can function either as an input data rate clock (PLL

enabled) or as a DAC data rate clock (PLL disabled) according

to the state of Address 02h, Bit 7 in the SPI port register. The

internal operation of the AD9775 clock circuitry in these two

modes is illustrated in Figures 12 and 13.

The PLL clock multiplier and distribution circuitry produce the

necessary internal synchronized 1

×, 2×, 4×, and 8× clocks for

the rising edge triggered latches, interpolation filters, modula-

tors, and DACs. This circuitry consists of a phase detector,

charge pump, voltage controlled oscillator (VCO), prescaler,

clock distribution, and SPI port control. The charge pump and

VCO are powered from PLLVDD while the differential clock

input buffer, phase detector, prescaler, and clock distribution

are powered from CLKVDD. PLL lock status is indicated by

the logic signal at the PLL_LOCK Pin, as well as by the status of

Bit 1, Register 00h. To ensure optimum phase noise performance

from the PLL clock multiplier and distribution, PLLVDD and

CLKVDD should originate from the same clean analog supply.

The speed of the VCO with the PLL enabled also has an effect

on phase noise. Optimal phase noise with respect to VCO speed

is achieved by running the VCO in the range of 450 MHz to

550 MHz. The VCO speed is a function of the input data rate,

the interpolation rate, and the VCO prescaler, according to the

following function:

VCO Speed MHz

Input Data Rate MHz

InterpolationRate

escaler

()

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