AD5439 Analog Devices, AD5439 Datasheet - Page 12

• Manufacturer Part Number: AD5439
• Description: (AD5424 - AD5547) High Bandwidth Multiplying DACs
• Manufacturer: Analog Devices

AD5547/AD5557
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 –V
or 0 V to +V
to +18 V references. In addition to the precision conversion R
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 R
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 R
can swing from –V
The reference voltage inputs exhibit a constant input resistance
of 5 kΩ ± 20%. The impedance of I
dependent. External amplifier choice should take into account
OFS
FB
and R
and 4-quadrant resistor R
REF
), and bipolar (±V
FB
connected as shown in Figure 2, the output
REF
to +V
RCOM
V
REF
R1
REF
Figure 19. 16-Bit AD5547 Equivalent R-2R DAC Circuit with Digital Section, One Channel Shown
5kΩ
5kΩ
.
R2
R1
LDAC
WR
REF
OUT
) output ranges from –18 V
OFS
, the DAC output, is code
80kΩ
15
have values of 10 kΩ.
2R
LDAC
WR
D15 D14
RA
40kΩ
80kΩ
RB
80kΩ
R
2R
8
2R
40kΩ
80kΩ
80kΩ
R
R
80kΩ
2R
2R
4
2R
40kΩ
80kΩ
80kΩ
R
R
ADDRESS DECODER
2R
2R
INPUT REGISTER
DAC REGISTER
REF
Rev. 0 | Page 12 of 20
40kΩ
80kΩ
80kΩ
FB
R
2R
R
2R
40kΩ
80kΩ
80kΩ
R
2R
R
2R
40kΩ
80kΩ
80kΩ
R
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
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 V
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.
2R
2R
80kΩ
40kΩ
R
2R
DD
4-BIT R2R
power is used primarily by the internal logic to drive
40kΩ
80kΩ
D0
R
2R
RS
RS
80kΩ
2R
80kΩ
2R
RS
10kΩ
4 MSB
15 SEGMENTS
8-BIT R2R
10kΩ
REF
ROFS
RFB
IOUT
AGND
and R
FB
should match.