AD5545 datasheet(15/25 Pages) AD

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AD5405

Rev. B | Page 14 of 24

GENERAL DESCRIPTION

DAC SECTION

The AD5405 is a 12-bit, dual-channel, current-output DAC

consisting of a standard inverting R-2R ladder configuration.

Figure 31 shows a simplified diagram for a single channel of the

AD5405. The feedback resistor RFBA has a value of 2R. The

value of R is typically 10 kΩ (with a minimum of 8 kΩ and a

maximum of 13 kΩ). If IOUT1A and IOUT2A are kept at the same

potential, a constant current flows into each ladder leg,

regardless of digital input code. Therefore, the input resistance

presented at VREFA is always constant.

VREFA

IOUT 2A

DAC DATA LATCHES

AND DRIVERS

S12

IOUT1A

RFB A

Figure 31. Simplified Ladder Configuration

Access is provided to the VREF, RFB, IOUT1, and IOUT2 terminals of

the DAC, making the device extremely versatile and allowing it

to be configured for several operating modes, such as unipolar

output, bipolar output, or single-supply mode.

CIRCUIT OPERATION

Unipolar Mode

Using a single op amp, this DAC can easily be configured to

provide 2-quadrant multiplying operation or a unipolar output

voltage swing, as shown in Figure 32.

RFB

AD5405

12-Bit DAC A

R1A

RFBA

VDD

VREFA

VOUT = 0V TO –VIN

IOUT2A

IOUT1A

AGND

GND

AGND

R2A

R2_3A

R3A

AGND

NOTES

1. SIMILAR CONFIGURATION FOR DAC B.

2. C1 PHASE COMPENSATION (1pF TO 2pF) MAY BE REQUIRED

IF A1 IS A HIGH SPEED AMPLIFIER.

Figure 32. Unipolar Operation

When an output amplifier is connected in unipolar mode, the

output voltage is given by

REF

OUT

−

where:

is the fractional representation, in the range of 0 to 4,095, of

the digital word loaded to the DAC.

is the resolution of the DAC.

With a fixed 10 V reference, the circuit shown in Figure 32 gives

a unipolar 0 V to −10 V output voltage swing. When VIN is an ac

signal, the circuit performs 2-quadrant multiplication.

Table 5 shows the relationship between digital code and the

expected output voltage for unipolar operation.

Table 5. Unipolar Code

Digital Input

Analog Output (V)

1111 1111 1111

−VREF (4,095/4,096)

1000 0000 0000

−VREF (2,048/4,096) = −VREF/2

0000 0000 0001

−VREF (1/4,096)

0000 0000 0000

−VREF (0/4,096) = 0

Bipolar Operation

In some applications, it may be necessary to generate full

4-quadrant multiplying operation or a bipolar output swing.

This can be easily accomplished by using another external

amplifier, as shown in Figure 33.

RFB

AD5405

12-Bit DAC A

R1A

RFBA

VDD

VREFA

R2A

R2_3A

R3A

IOUT2A

IOUT1A

AGND

GND

AGND

VIN

VOUT =–VIN TO +VIN

NOTES

1. SIMILAR CONFIGURATION FOR DAC B.

2. C1 PHASE COMPENSATION (1pF TO 2pF) MAY BE REQUIRED

IF A1 IS A HIGH SPEED AMPLIFIER.

Figure 33. Bipolar Operation (4-Quadrant Multiplication)

When in bipolar mode, the output voltage is given by

REF

OUT

−

− )

(

where:

is the fractional representation, in the range of 0 to 4,095, of

the digital word loaded to the DAC.

is the number of bits.

When VIN is an ac signal, the circuit performs 4-quadrant

multiplication.

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