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 –V
REF
or 0 V to +V
REF
) and bipolar (±V
REF
) output ranges from –18 V
to +18 V references. In addition to the precision conversion R
FB
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 R
FB
and 4-quadrant resistor R
OFS
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 V
REF
input. With R
OFS
and R
FB
connected as shown in Figure 2, the output can swing from
−V
REF
to +V
REF
.
The reference voltage inputs exhibit a constant input resistance
of 5 kΩ ± 20%. The impedance of I
OUT
, 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 V
REF
and R
FB
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 V
DD
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.
04452-011
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
V
REF
2R
80kΩ
R
40kΩ
2R
80kΩ
R
40kΩ
2R
80kΩ
R
40kΩ
2R
80kΩ
R
40kΩ
2R
80kΩ
R
40kΩ
2R
80kΩ
RCOM
R1
ADDRESS DECODER
DAC REGISTER
INPUT REGISTER
LDAC
WR
RS
RS
4 MSB
15 SEGMENTS
8-BIT R2R
4-BIT R2R
15 8 4
LDAC
WR
D15 D14 D0
RS
10kΩ 10kΩ
ROFS
RFB
IOUT
AGND
RA
RB
Figure 18. 16-Bit AD5547 Equivalent R-2R DAC Circuit with Digital Section, One Channel Shown
