AD9154 datasheet(75/124 Pages) AD

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Data Sheet

AD9154

Rev. C | Page 75 of 124

ANALOG OUTPUTS

TRANSMIT DAC OPERATION

Figure 79 shows a simplified block diagram of the transmit path

DAC cores. There are four DAC cores: DAC0 and DAC2 are

designated I DACs; DAC1 and DAC3 are designated Q DACs.

The DAC cores consist of a current switch array, digital control

logic, and full-scale output current control. The DAC full-scale

output current (IOUTFS) is defined in Table 1. The output currents

from the OUTx± pins are complementary, meaning that the sum of

the two currents always equals the full-scale current of the DAC.

OUTx± are current sinks. Current flows into the OUTx± ports.

The digital input code to the DAC determines the differential

current output.

OUT3+

OUT3–

OUT2+

OUT2–

OUT1+

OUT1–

OUT0+

OUT0–

CURRENT

SCALING

I DACS

FULL-SCALE

ADJUST

Q DACS

FULL-SCALE

ADJUST

1.2V

4kΩ

I20

DAC3

DAC2

DAC1

DAC0

Figure 79. Simplified Block Diagram of the DAC Core

A 4 kΩ external resistor, RSET, must be connected from the I120 pin

to ground. This resistor, along with the reference control

amplifier, sets up the correct internal bias currents for each

DAC core.

The full-scale current equation, where the DAC gain is set for

each I DAC core pair and each Q DAC core pair in

Registers 0x040 through Register 0x043 is as follows:

























DAC gain

SET

REF

OUTFS

(5)

Figure 80 is a plot of IOUTFS as a function of DAC_GAIN_Ix and

DAC_GAIN_Qx

DAC GAIN CODE

Figure 80. DAC Full-Scale Current (IOUTFS) vs. DAC Gain Code

Transmit DAC Transfer Function

The output currents drawn by the OUTx+ and OUTx− pins are

complementary, meaning that the sum of the two (positive plus

negative) currents always equals the full-scale current of the DAC,

IOUTFS. The digital input code to a DAC determines the differential

current output. The OUTx+ pins provide the maximum output

current when all bits are high. The output currents vs. DACCODE

for the DAC outputs are expressed as

OUTFS

OUTP

DACCODE









(6)

OUTP

OUTFS

OUTN

−

(7)

where DACCODE = 0 to 2N − 1 and is the digital signal input to

a DAC core consisting of a stream of 16 bit samples.

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