AD6650SLASHPCB datasheet(27/44 Pages) AD

27 / 44 page

AD6650

Rev. A | Page 27 of 44

Another option is to ac-couple a differential ECL/PECL signal

to the encode input pins as shown in Figure 37. A device that

offers excellent jitter performance is the MC100EL16 (or a

device from the same family) from Motorola.

CLK

AD6650

0.1µF

ECL/

PECL

Figure 37. Differential ECL for Encode

DRIVING THE ANALOG INPUTS

As with most new high speed, high dynamic range devices, the

analog input to the AD6650 is differential. Differential inputs

allow much improvement in performance on-chip because signals

are processed through attenuation and gain stages. Most of the

improvement is a result of differential analog stages that have

high rejection of even-order harmonics. Differential inputs are

also beneficial at the PCB level. First, differential inputs have

high common-mode rejection of stray signals, such as ground

and power noise, and good rejection of common-mode signals,

such as local oscillator feedthrough.

The AD6650 analog input voltage range is offset from ground

by 1.3 V. The resistor network on the input properly biases the

followers for maximum linearity and range. Therefore, the

analog source driving the AD6650 should be ac-coupled to the

input pins. The input resistance for the AD6650 is 200 Ω, and

the input voltage range is 2 V p-p differential. This equates to

4 dBm full-scale input power. The recommended method for

driving the analog input of the AD6650 is to use an RF balun.

J101

C101

47000pF

R101

68.9Ω

C102

47000pF

C103

47000pF

AIN/BIN

T101

T102

AIN/BIN

Figure 38. Balun-Coupled Analog Input Circuit

EXTERNAL REFERENCE

The reference should be connected as shown in Figure 39 to

achieve the results specified in this data sheet.

AIN/BIN

VREF

REFGND

SELECT

LOGIC

REF

AMP

CHANNEL A/

CHANNEL B

ADC

CORE

REFT

REFB

0.5V

0.1µF

10µF

RINT

Figure 39. Reference Connection

POWER SUPPLIES

Care should be taken when selecting a power source. Linear

supplies are strongly recommended. Switching supplies tend to

have radiated components that may be received by the AD6650.

Each of the power supply pins should be decoupled as closely to

the package as possible using 0.1 μF chip capacitors.

The AD6650 is susceptible to low frequency power supply

interference as shown in Figure 40. This low frequency energy is

translated into spurious tones in the output signal. Analog

power supply ripple couples into the LO through the VCO. This

can be observed from the ripple frequency vs

. sideband spur

level plot (

see Figure 40). Note that this plot has the spur level

referenced to 1 mV rms supply ripple and is referred to the LO.

Thus, this plot shows a transfer function rather than an absolute

value. The spurious level can be extrapolated to any supply

ripple level from this data using Equation 22.

))

(

log(

SupRipple

Lev

Spur

Lev

Spur

(22)

where:

Spur_Lev is the output spurious level relative to the LO.

Spur_Lev1 mV is the 1 mV referred level from Figure 40 and

Figure 41.

SupRipple (mV) is the RMS ripple on the AVDD power supply.

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