AD9747-EBZ datasheet(25/28 Pages) AD

25 / 28 page

AD9741/AD9743/AD9745/AD9746/AD9747

Rev. 0 | Page 25 of 28

–10

–20

–30

–40

0.5

1.5

NORMAL

MIX

Figure 36. Transfer Function for Each Analog Operating Mode

AUXILIARY DACS

Two auxiliary DACs are provided on the AD9741/AD9743/

AD9745/AD9746/AD9747. A functional diagram is shown

in Figure 37. 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 a 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 reduces to 0.8 V to 1.6 V. Either output can be used, but

only one output of the auxiliary 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 37. Auxiliary DAC Functional Diagram

In a single side band 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

configuration 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 outlined in the Power Dissipation

section.

AD9747

AUX

DAC1 OR

DAC2

AD9747

DAC1 OR

DAC2

25Ω TO 50Ω

QUAD MOD

I OR Q INPUTS

QUADRATURE

MODULATOR V+

25Ω TO 50Ω

OPTIONAL

PASSIVE

FILTERING

Figure 38. DAC DC Coupled to Quadrature Modulator with Passive DC Shift

POWER DISSIPATION

Figure 39 shows the power dissipation and current draw of the

AD9741/AD9743/AD9745/AD9746/AD9747. It shows that the

devices have a quiescent power dissipation of about 190 mW.

Most of this comes from the AVDD33 supply. Total power

dissipation increases about 50% as the clock rate is increased

to the maximum clock rate of 250 MHz.

350

310

270

230

190

150

100

150

200

250

fDAC (MHz)

125

175

225

fOUT = DC

fOUT = NYQUIST

Figure 39. AD9747 Power Dissipation vs. fDAC

100

150

200

250

fDAC (MHz)

125

175

225

AD9747

AD9741

Figure 40. DVDD33 Current vs. fDAC

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