AD9136-EBZ datasheet(75/117 Pages) AD

75 / 117 page

Data Sheet

AD9135/AD9136

Rev. C | Page 75 of 117

ANALOG OUTPUTS

TRANSMIT DAC OPERATION

Figure 83 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.

Figure 83. 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 either 0 or 1, corresponding to DAC0 or

DAC1) 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 0x041 and Register 0x044

to Register 0x045 as shown in Table 74.

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 nominally 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, Register 0x041,

and Register 0x044, and Register 0x045. Analog output full-scale

current vs. gain DAC code is shown in Figure 84.

Table 74. DAC Full-Scale Current Registers

Address

Value

Description

0x040[1:0]

DACFSC_0[9:8]

DAC0 MSB gain code

0x041[7:0]

DACFSC_0[7:0]

DAC0 LSB gain code

0x044[1:0]

DACFSC_1[9:8]

DAC1 MSB gain code

0x045[7:0]

DACFSC_1[7:0]

DAC1 LSB gain code

Figure 84. DAC Full-Scale Current (IOUTFS) vs. Gain DAC 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)

IOUTN = IOUTFS − IOUTP

(6)

where DACCODEBIN is the 11-/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

–

(7)

IOUTN = IOUTFS − IOUTP

(8)

where DACCODETWOS is the 11-/16-bit input to the DAC in twos

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

Powering Down Unused DACs

Power down any unused DAC outputs to avoid burning excess

power. The DAC power downs are located in Register 0x011.

Write a 1 to each bit to power down the appropriate DACs.

gap and DAC master bias, respectively.

OUT1+

OUT1–

OUT0+

OUT0–

CURRENT

SCALING

DAC1

FULL-SCALE

ADJUST

DAC0

FULL-SCALE

ADJUST

1.2V

4kΩ

I120

DAC1

DAC0

1024

896

768

640

512

GAIN DAC CODE

384

256

128

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