AD9783 datasheet(29/32 Pages) AD

29 / 32 page

Data Sheet

AD9780/AD9781/AD9783

Rev. C | Page 29 of 32

In return-to-zero mode, the output is set to midscale every

other half clock cycle. The output is similar to the DAC output

in normal mode except that the output pulses are half the width

and half the area. Because the output pulses have half the width,

the sinc function is scaled in frequency by two and has a first

null at 2 × fDAC. Because the area of the pulses is half that of the

pulses in normal mode, the output power is half the normal

mode output power.

D10

INPUT DATA

DAC CLK

QUAD-SWITCH

DAC OUTPUT

(

fS MIX MODE)

QUAD-SWITCH

DAC OUTPUT

(RETURN-TO-

ZERO MODE)

Figure 64. Mix Mode and Return-to-Zero Mode DAC Waveforms

The functions that shape the output spectrums for the three

modes of operation, normal mode, mix mode, and return-to-

zero mode, are shown in Figure 65. Switching between the

analog modes reshapes the sinc roll-off inherent at the DAC

output. This ability to change modes in the AD9780/AD9781/

AD9783 makes the parts suitable for direct IF applications. The

user can place a carrier anywhere in the first three Nyquist

zones depending on the operating mode selected. The perfor-

mance and maximum amplitude in all three Nyquist zones is

impacted by this sinc roll-off depending on where the carrier is

placed, as shown in Figure 65.

–10

–20

–30

–40

0.5

1.5

1.0

2.0

(

fS)

NORMAL

MIX

RETURN-TO-ZERO

Figure 65. Transfer Function for Each Analog Operating Mode

Auxiliary DACs

Two auxiliary DACs are provided on the AD9780/AD9781/

AD9783. A functional diagram is shown in Figure 66. The

auxiliary DACs are current output devices with two output

pins, AUXP and AUXN. The active pin can be programmed to

either source or sink current. When either sinking or sourcing,

the full-scale current magnitude is 2 mA. The available compliance

range at the auxiliary DAC outputs depends on whether the output

is configured to sink or source current. When sourcing current,

the compliance voltage is 0 V to 1.6 V, but when sinking current,

the output compliance voltage is reduced to 0.8 V to 1.6 V. Either

output can be used, but only one output of the AUX DAC (P or

N) is active at any time. The inactive pin is always in a high

impedance state (>100 kΩ).

VBIAS

AUXP

AUXN

SINK

SOURCE

POSITIVE

NEGATIVE

0mA

2mA

0mA

2mA

Figure 66. Auxiliary DAC Functional Diagram

In a single sideband transmitter application, the combination of

the input referred dc offset voltage of the quadrature modulator

and the DAC output offset voltage can result in local oscillator

(LO) feedthrough at the modulator output, which degrades system

performance. The auxiliary DACs can be used to remove the dc

offset and the resulting LO feedthrough. The circuit configura-

tion for using the auxiliary DACs for performing dc offset

correction depends on the details of the DAC and modulator

interface. An example of a dc-coupled configuration with low-

pass filtering is shown in Figure 67.

AD9783

AUX

DAC1 OR

DAC2

AD9783

DAC1 OR

DAC2

25Ω TO 50Ω

QUAD MOD

I OR Q INPUTS

QUADRATURE

MODULATOR V+

25Ω TO 50Ω

OPTIONAL

PASSIVE

FILTERING

Figure 67. DAC DC-Coupled to Quadrature Modulator with a Passive DC Shift

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