AD9783-EBZ datasheet(27/36 Pages) AD

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AD9780/AD9781/AD9783

Rev. A | Page 27 of 36

256

512

768

1024

DAC GAIN CODE

Figure 63. IFS vs. DAC Gain Code

DAC TRANSFER FUNCTION

Each DAC output of the AD9780/AD9781/AD9783 drives two

complementary current outputs, IOUTP and IOUTN. IOUTP provides

a near IFS when all bits are high. For example,

DAC CODE = 2N − 1

where N = 12/14/16 bits for AD9780/AD9781/AD9783

(respectively), while IOUTN provides no current.

The current output appearing at IOUTP and IOUTN is a function of

both the input code, and IFS and can be expressed as

IOUTP = (DAC DATA/2N) × IFS

(1)

IOUTN = ((2N − 1) − DAC DATA)/2N × IFS

(2)

where DAC DATA = 0 to 2N − 1 (decimal representation).

The two current outputs typically drive a resistive load directly

or via a transformer. If dc coupling is required, IOUTP and IOUTN

should be connected to matching resistive loads (RLOAD) that are

tied to analog common (AVSS). The single-ended voltage

output appearing at the IOUTP and IOUTN pins is

VOUTP = IOUTP × RLOAD

(3)

VOUTN = IOUTN × RLOAD

(4)

Note that to achieve the maximum output compliance of 1 V at

the nominal 20 mA output current, RLOAD must be set to 50 Ω.

Also note that the full-scale value of VOUTP and VOUTN should

not exceed the specified output compliance range to maintain

specified distortion and linearity performance.

There are two distinct advantages to operating the AD9780/

AD9781/AD9783 differentially. First, differential operation

helps cancel common-mode error sources associated with IOUTP

and IOUTN, such as noise, distortion, and dc offsets. Second, the

differential code-dependent current and subsequent output

voltage (VDIFF) is twice the value of the single-ended voltage

output (VOUTP or VOUTN), providing 2× signal power to the load.

VDIFF = (IOUTP – IOUTN) × RLOAD

(5)

ANALOG MODES OF OPERATION

The AD9780/AD9781/AD9783 use a proprietary quad-switch

architecture that lowers the distortion of the DAC by eliminating a

code-dependent glitch that occurs with conventional dual-switch

architectures. This architecture eliminates the code-dependent

glitches, but creates a constant glitch at a rate of 2 × fDAC. For

communications systems and other applications requiring good

frequency domain performance from the DAC, this is seldom

problematic.

The quad-switch architecture also supports two additional

modes of operation: mix mode and return-to-zero mode. The

waveforms of these two modes are shown in Figure 64. In mix

mode, the output is inverted every other half clock cycle. This

effectively chops the DAC output at the sample rate. This chop-

ping has the effect of frequency shifting the sinc roll-off from dc

to fDAC. Additionally, there is a second subtle effect on the output

spectrum. The shifted spectrum is also shaped by a second sinc

function with a first null at 2 × fDAC. The reason for this shaping

is that the data is not continuously varying at twice the clock

rate, but is simply repeated.

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

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