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AD5546

Current Output, Parallel Input, 16-/14-Bit Multiplying DACs with 4-Quadrant Resistors

Analog Devices 

AD5546/AD5556

CIRCUIT OPERATION

D/A CONVERTER SECTION

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

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

2.7 V to 5.5 V supply, and provide both unipolar 0 V to –VREF, or

0 V to +VREF, and bipolar ±VREF output ranges from a –18 V to

+18 V reference. In addition to the precision conversion RFB

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

tional precision resistors for 4-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 19 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 resistor, RFB, and 4-quadrant resistor, ROFS, have values

of 10 kΩ. Each 4-quadrant resistor, R1 and R2, equals 5 kΩ. In

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

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

input. With ROFS and RFB connected as shown in Figure 2, the

output can swing from –VREF to +VREF.

The reference voltage inputs exhibit a constant input resistance

of 5 kΩ ±20%. The DAC output, IOUT, impedance is code depen-

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 tantulum capacitor. Also, to minimize gain error,

PCB metal traces between VREF and RFB should match.

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.

The VDD power is used primarily by the internal logic and to

drive the DAC switches. Note that the output precision degrades

if the operating voltage falls below the specified voltage. Users

should also avoid using switching regulators because device

power supply rejection degrades at higher frequencies.

REF

R2

5kΩ

RCOM

R1

5kΩ

R1

2R 2R 2R

80kΩ 80kΩ 80kΩ

2R

80kΩ

R

R

R

R

R

R

R

R

40kΩ 40kΩ 40kΩ 40kΩ 40kΩ 40kΩ 40kΩ 40kΩ

4 MSB

15 SEGMENTS

2R 2R 2R 2R 2R 2R 2R 2R 2R

80kΩ 80kΩ 80kΩ 80kΩ 80kΩ 80kΩ 80kΩ 80kΩ 80kΩ

8-BIT R2R

RA

R

R

R

R

RB

2R 2R 2R 2R 2R

80kΩ 80kΩ 80kΩ 80kΩ 80kΩ

4-BIT R2R

16

8

4

ADDRESS DECODER

ROFS

RFB

10kΩ 10kΩ

IOUT

GND

LDAC

LDAC

DAC REGISTER

RS

RS

WR

WR

INPUT REGISTER

RS

D15 D14

D0

Figure 19. 16-Bit AD5546 Equivalent R-2R DAC Circuit with Digital Section

Rev. 0 | Page 11 of 16

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