AD5452 datasheet(12/20 Pages) AD

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AD5547/AD5557

Data Sheet

Rev. D | Page 12 of 20

CIRCUIT OPERATION

DAC SECTION

The AD5547/AD5557 are 16-/14-bit, multiplying, current-

output, 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 –18 V

to +18 V references. 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 AD5547/AD5557 consist of two groups of precision R-2R

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

the 4 MSBs are decoded into 15 segments of resistor value 2R.

Figure 18 shows the architecture of the 16-bit AD5547. Each of

the 16 segments and 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 VREF 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 impedance of IOUT, the DAC output, is code

dependent. External amplifier choice should take into account

the variation of the AD5547/AD5557 output impedance. The

feedback resistance in parallel with the DAC ladder resistance

dominates output voltage noise. To maintain good analog

performance, it is recommended that the power supply is

bypassed 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, 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

inverting node of the op amp. A compensation capacitor, therefore,

may be needed between the I-to-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 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.

80kΩ

40kΩ

80kΩ

40kΩ

80kΩ

40kΩ

5kΩ

VREF

80kΩ

40kΩ

80kΩ

40kΩ

80kΩ

40kΩ

80kΩ

40kΩ

80kΩ

40kΩ

80kΩ

RCOM

ADDRESS DECODER

DAC REGISTER

INPUT REGISTER

LDAC

4 MSB

15 SEGMENTS

8-BIT R2R

4-BIT R2R

LDAC

D15

D14

10kΩ 10kΩ

ROFS

RFB

IOUT

AGND

Figure 18. 16-Bit AD5547 Equivalent R-2R DAC Circuit with Digital Section, One Channel Shown

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