AD5547 datasheet(12/20 Pages) AD

12 / 20 page

AD5547/AD5557

Rev. 0 | Page 12 of 20

CIRCUIT OPERATION

D/A CONVERTER 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. Further-

more, the 4 MSBs are decoded into 15 segments of resistor value

2R. Figure 19 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

op amp’s 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 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.

80k

Ω

40k

Ω

80k

Ω

80k

Ω

80k

Ω

80k

Ω

80k

Ω

40k

Ω

80k

Ω

80k

Ω

80k

Ω

80k

Ω

80k

Ω

80k

Ω

40k

Ω

VREF

80k

Ω

40k

Ω

80k

Ω

40k

Ω

80k

Ω

40k

Ω

80k

Ω

40k

Ω

80k

Ω

40k

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80k

Ω

RCOM

ADDRESS DECODER

DAC REGISTER

INPUT REGISTER

LDAC

4 MSB

15 SEGMENTS

8-BIT R2R

4-BIT R2R

LDAC

D15

D14

10k

Ω

10k

Ω

ROFS

RFB

IOUT

AGND

Figure 19. 16-Bit AD5547 Equivalent R-2R DAC Circuit with Digital Section, One Channel Shown

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