AD9789BBCZ datasheet(55/76 Pages) AD

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AD9789

Rev. A | Page 55 of 76

INPUT DATA

DACCLK

4-SWITCH

DAC OUTPUT

(

fS MIX MODE)

4-SWITCH

DAC OUTPUT

(RETURN-TO-

ZERO MODE)

D10

–D10

–D1

–D6

–D5

–D2

–D3

–D4

–D9

–D8

–D7

Figure 105. Mix Mode and RZ Mode DAC Waveforms

Switching between analog modes reshapes the sinc roll-off

inherent at the DAC output. The performance and maximum

amplitude in all three Nyquist zones is affected by this sinc

roll-off depending on where the carrier is placed, as shown in

Figure 106.

–10

–5

–15

–20

–25

–30

–35

0.5

1.0

1.5

2.0

2.5

3.0

FREQUENCY (Hz)

FIRST

NYQUIST ZONE

SECOND

NYQUIST ZONE

THIRD

NYQUIST ZONE

NORMAL

MODE

RZ MODE

MIX MODE

Figure 106. Sinc Roll-Off for Each Analog Operating Mode (fS = 2 × DACCLK)

The RZ mode, with its lower but flat response, can be quite

useful for quick checks of system frequency response.

ANALOG CONTROL REGISTERS

The AD9789 includes registers for optimizing its analog

performance. These registers include noise reduction in the

output current mirror and output current mirror headroom

adjustments.

Mirror Roll-Off Frequency Control

Using the MSEL[1:0] bits (Register 0x36[1:0]), the user can

adjust the noise contribution of the internal current mirror to

optimize the 1/f noise. Figure 107 shows MSEL vs. the 1/f noise

with 20 mA full-scale current into a 50 Ω resistor.

FREQUENCY (kHz)

–110

–115

–120

–125

–130

–140

–135

110

100

MSEL = 11

MSEL = 10

MSEL = 01

MSEL = 00

Figure 107. 1/f Noise with Respect to MSEL Bits

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