AD5547 Просмотр технического описания (PDF) - Analog Devices

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AD5547

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

Analog Devices 

AD5547/AD5557

Data Sheet

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.

VREF

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

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

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

RA

R

R

R

R

RB

2R 2R 2R 2R 2R

80kΩ 80kΩ 80kΩ 80kΩ 80kΩ

4-BIT R2R

15

8

4

ADDRESS DECODER

4 MSB

15 SEGMENTS

8-BIT R2R

ROFS

RFB

10kΩ 10kΩ

IOUT

AGND

LDAC

LDAC

DAC REGISTER

RS

RS

WR

WR

INPUT REGISTER

RS

D15 D14

D0

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

Rev. D | Page 12 of 20

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