AD9856SLASHPCB datasheet(33/37 Pages) AD

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AD9856

Rev. C | Page 32 of 36

The second method requires an fMAX of 144 MHz with

externally 2× upsampled input data. The AD9856 is configured

for a CIC interpolation rate of 3 while bypassing the 3rd half-

band filter. The input I/Q sample rate is 12 MHz, which is

further upsampled by a factor of 12 MHz to 144 MHz.

For burst applications with relatively long nonbursting periods,

the sleep bit is useful for saving power. When in sleep mode,

power is reduced to below 6 mW. Consideration must be given

to wake-up time, which generally is in the 400 µs to 750 µs

range. For applications that cannot use the sleep bit due to this

wake-up time, there is an alternate method of reducing power

dissipation when not transmitting. By writing the bypass

REFCLK multiplier bit active, the power is reduced by nearly

the REFCLK multiplier factor. For example, if the external

reference clock is 16 MHz and REFCLK multiplier is set to 10×,

all clocks divide down by a factor of 10 when the REFCLK

multiplier is bypassed. This effectively scales down the power

dissipation by nearly a factor of 10. In this case, both the

REFCLK multiplier function and the DAC, which use relatively

little power, remain fully powered. The REFCLK multiplier

circuit is locked to the 16 MHz external reference clock, but its

output is driving a very small load—thus, there is little power

dissipation. When the REFCLK multiplier is reactivated, the

acquisition time is small. In this power reduction technique,

the larger the REFCLK multiplier factor, the larger the power

savings.

The AD9856 is specified for operation at +3.0 V ±5%. The

thermal impedance of the device in the 48-LQFP plastic

package is 38°C/W. At 200 MHz operation, power dissipation

is 1.5 W. This permits operation over the industrial temperature

range without exceeding the maximum junction temperature of

150°C. To realize this quoted thermal impedance, all power and

ground pins must be soldered down to a multilayer PCB with

power and ground copper planes directly available at the

package pins.

Under worst-case conditions, that is, with power supplies at

2.85 V and ambient temperatures of 85°C, device operation

at 200 MHz is guaranteed for single-tone mode only. For

modulation mode at 200 MHz, 85°C operation, the minimum

power supply voltage is 3.0 V.

AD9856 EVALUATION BOARD

An evaluation board is available to facilitate bench and system

analysis of AD9856 quadrature digital upconverter. The

AD9856 printed circuit board (PCB) contains the AD9856

device and Windows® software that the device to be controlled

via the printer port of a PC. The DAC output is provided on a

jack for spectral analysis. The AD9856/PCB provides a single-

ended 65 MHz, 5 Ω, elliptical low-pass filter on the output of

the DAC.

The user can also implement the AD8320/AD8321 program-

mable cable driver amplifier on the AD9856/PCB evaluation

board. The AD8320/AD8321 gain is programmed through the

AD9856 via the menu driven control software.

SUPPORT

Applications assistance is available for the AD9856 and the

AD9856/PCB evaluation board. Please call 1-800-ANALOGD

or visit www.analog.com.

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