AD5546/AD5556

Data Sheet

Rev. D | Page 10 of 20

CIRCUIT OPERATION

DIGITAL-TO-ANALOG (DAC) CONVERTER SECTION

The AD5546/AD5556 are 16-/14-bit multiplying, current out-

put, and parallel input DACs. The devices operate from a single

commonly found in current output DACs, there are three addi-

tional precision resistors for four-quadrant bipolar applications.

The AD5546/AD5556 consist of two groups of precision R-2R

ladders, which make up the 12/10 LSBs, respectively. Further-

more, the four MSBs are decoded into 15 segments of resistor

value 2R. Figure 17 shows the architecture of the 16-bit AD5546.

Each of the 16 segments in the R-2R ladder carries an equally

weighted current of one-sixteenth of full scale. The feedback

kΩ. Each four-quadrant resistor, R1 and R2, equals 5 kΩ. In

four-quadrant operation, R1, R2, and an external op amp work

together to invert the reference voltage and apply it to the REF

The reference voltage inputs exhibit a constant input resistance

dent. External amplifier choice should take into account the

variation of the AD5546/AD5556 output impedance. The

feedback resistance in parallel with the DAC ladder resistance

dominates output voltage noise. To maintain good analog

performance, it is recommended to bypass the power supply

with a 0.01 μF to 0.1 μF ceramic or chip capacitor in parallel

with a 1 μF tantalum capacitor. Also, to minimize gain error,

Every code change of the DAC corresponds to a step function;

gain peaking at each output step may occur if the op amp has

limited GBP and excessive parasitic capacitance present at the

op amp inverting node. A compensation capacitor, therefore,

may be needed between the I-V op amp inverting and output

nodes to smooth the step transition. Such a compensation

capacitor should be found empirically, but a 20 pF capacitor is

generally adequate for the compensation.

drive the DAC switches. Note that the output precision

degrades if the operating voltage falls below the specified

voltage. The user should also avoid using switching regulators

because device power supply rejection degrades at higher

frequencies.

2R

80kΩ

R

40kΩ

2R

80kΩ

2R

80kΩ

2R

80kΩ

2R

80kΩ

2R

80kΩ

R

40kΩ

2R

80kΩ

R

2R

80kΩ

R

2R

80kΩ

R

2R

80kΩ

R

2R

80kΩ

2R

80kΩ

R

40kΩ

R2

5kΩ

R1

5kΩ

REF

2R

80kΩ

R

40kΩ

2R

80kΩ

R

40kΩ

2R

80kΩ

R

40kΩ

2R

80kΩ

R

40kΩ

2R

80kΩ

R

40kΩ

2R

80kΩ

R1

ADDRESS DECODER

DAC REGISTER

INPUT REGISTER

LDAC

WR

RS

RS

4 MSB

15 SEGMENTS

8-BIT R–2R

4-BIT R–2R

16

8

4

LDAC

WR

D15 D14

D0

RS

10kΩ

10kΩ

GND

RA

RB

Figure 17. 16-Bit AD5546 Equivalent R-2R DAC Circuit with Digital Section