AD9148BPCZ datasheet(59/73 Pages) AD

59 / 73 page

Preliminary Technical Data

AD9148

Rev. PrA | Page 59 of 73

ANALOG OUTPUTS

TRANSMIT DAC OPERATION

Figure 75 shows a simplified block diagram of one pair of the

transmit path DACs. The DAC core consists of a current source

array, switch core, digital control logic, and full-scale output

current control. The DAC full-scale output current (IOUTFS) is

nominally 20 mA. The output currents from the IOUTx_P and

IOUTx_N 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.

The DAC has a 1.2 V band gap reference with an output impedance

of 5 kΩ. The reference output voltage appears on the VREF pin.

When using the internal reference, the VREF pin should be

decoupled to AVSS with a 0.1 μF capacitor. The internal reference

should only be used for external circuits that draw dc currents

of 2 μA or less. For dynamic loads or static loads greater than

2 μA, the VREF pin should be buffered. If desired, an external

reference (between 1.10 V to 1.30 V) can be applied to the

VREF pin.

A 10 kΩ external resistor, RSET, must be connected from the

RESET pin to AVSS. 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.

The full-scale current can be calculated by

⎟

⎠

⎞

⎜

⎝

⎛

⎟

⎠

⎞

⎜

⎝

⎛

DAC gain

SET

OUTFS

REF

where DAC gain is set individually for the I and Q DACs in

respectively.

For nominal values of VREF (1.2 V), RSET (10 kΩ), and DAC

gain (512), the full-scale current of the DAC is typically 20.16 mA.

The DAC full-scale current can be adjusted from 8.66 mA to

31.66 mA by setting the DAC gain parameter setting as shown

in Figure 74.

1000

DAC GAIN CODE

200

400

600

800

Figure 74. DAC Full-Scale Current vs. DAC Gain Code

Transmit DAC Transfer Function

The output currents from the IOUTx_P and IOUTx_N 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. IOUTx_P provides the maximum output current

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

the DAC outputs are expressed as

OUTFS

OUT

DACCODE

⎥⎦

⎤

⎢⎣

⎡

(1)

IOUT_N = IOUTFS – IOUT_P

(2)

where DACCODE = 0 to 2N − 1.

I DAC

IOUT1_P/IOUT3_P

IOUT1_N/IOUT3_N

Q DAC

IOUT2_N/IOUT4_N

IOUT2_P/IOUT4_P

CURRENT

SCALING

I DAC GAIN

Q DAC GAIN

0.1µF

10kΩ

I120

VREF

RSET

5kΩ

1.2V

Figure 75. Simplified Block Diagram of DAC Core

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