AD9144BCPZ datasheet(79/125 Pages) AD

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Data Sheet

AD9144

Rev. B | Page 79 of 125

ANALOG OUTPUTS

TRANSMIT DAC OPERATION

Figure 80 shows a simplified block diagram of the transmit path

DACs. The DAC core consists of a current source array, a switch

core, digital control logic, and full-scale output current control.

The DAC full-scale output current (IOUTFS) is nominally 20.48 mA.

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. The digital input code to the DAC

determines the effective differential current delivered to the load.

OUT3+

OUT3–

OUT2+

OUT2–

OUT1+

OUT1–

OUT0+

OUT0–

CURRENT

SCALING

Q DACs

FULL-SCALE

ADJUST

I DACs

FULL-SCALE

ADJUST

1.2V

4kΩ

I120

DAC3

DAC2

DAC1

DAC0

Figure 80. Simplified Block Diagram of DAC Core

The DAC has a 1.2 V band gap reference. A 4 kΩ external

resistor, RSET, must be connected from the I120 pin to the

ground plane. This resistor, along with the reference control

amplifier, sets up the correct internal bias currents for the DAC.

Because the full-scale current is inversely proportional to this

resistor, the tolerance of RSET is reflected in the full-scale output

amplitude.

DACFSC_x (where x is a number from 0 to 3 that corresponds

to DAC0 through DAC3) is a 10-bit twos complement value

that controls the full-scale current of each of the four DAC outputs.

These values are stored in Register 0x040 to Register 0x047, as

shown in Table 84.

The typical full-scale current for each DAC is given by:

IOUTFS = 20.45 + (DACFSC_x × 6.55 mA)/2(10− 1)

For nominal values of VREF (1.2 V), RSET (4 kΩ), and DACFSC_x

(0, which is midscale in twos complement), the full-scale current

of the DAC is typically 20.48 mA. The DAC full-scale current

can be adjusted from 13.9 mA to 27.0 mA, by programming the

appropriate DACFSC_x values in Register 0x040 to Register 0x047.

Analog output full-scale current vs. DAC gain code is plotted in

Figure 81.

Table 84. DAC Full-Scale Current Registers

Address

Value

Description

0x040[1:0]

DACFSC_0[9:8]

Dual A I DAC MSB gain code

0x041[7:0]

DACFSC_0[7:0]

Dual A I DAC LSB gain code

0x042[1:0]

DACFSC_1[9:8]

Dual A Q DAC MSB gain code

0x043[7:0]

DACFSC_1[7:0]

Dual A Q DAC LSB gain code

0x044[1:0]

DACFSC_2[9:8]

Dual B I DAC MSB gain code

0x045[7:0]

DACFSC_2[7:0]

Dual B I DAC LSB gain code

0x046[1:0]

DACFSC_3[9:8]

Dual B Q DAC MSB gain code

0x047[7:0]

DACFSC_3[7:0]

Dual B Q DAC LSB gain code

–512

512

384

256

128

GAIN DAC CODE

–128

–256

–384

Figure 81. DAC Full-Scale Current (IOUTFS) vs. DAC Gain Code

Transmit DAC Transfer Function

The output currents from the OUTx+ and OUTx− pins are

complementary, meaning that the sum of the positive and negative

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

digital input code to the DAC determines the effective differential

current delivered to the load. OUTx± provides the maximum

output current when all bits are high for binary data. The

output currents vs. DACCODE for the DAC outputs using

binary format are expressed as

OUTFS

BIN

OUTP

DACCODE

−

(5)

OUTP

OUTFS

OUTN

−

(6)

where DACCODEBIN is the 16-bit input to the DAC in unsigned

binary. DACCODEBIN has a range of 0 to 2N − 1.

If the data format is twos complement, the output currents are

expressed as

OUTFS

TWOS

OUTP

DACCODE

−

2 1

(7)

OUTP

OUTFS

OUTN

−

(8)

where DACCODETWOS is the 16-bit input to the DAC in twos

complement. DACCODETWOS has a range of −2N− 1 to 2N − 1 − 1.

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