Datasheet

Data Sheet AD5444/AD5446
Rev. E | Page 15 of 28
GENERAL DESCRIPTION
DAC SECTION
The AD5444/AD5446 are 12-bit and 14-bit current output
DACs consisting of segmented (4 bits), inverting R– 2R ladder
configurations. A simplified diagram for the 12-bit AD5444
is shown in Figure 37.
2R
S1
2R
S2
2R
S3
2R
S12
2R
DAC DATA LATCHES
AND DRIVERS
R
R
FB
I
OUT
1
I
OUT
2
V
REF
0
4464-029
RR R
Figure 37. Simplified Ladder
The feedback resistor (R
FB
) has a value of R. The value of R is
typically 9 kΩ (7 kΩ minimum, 11 kΩ maximum). If I
OUT
1 is
kept at the same potential as GND, a constant current flows in
each ladder leg, regardless of digital input code. Therefore, the
input resistance presented at V
REF
is always constant and nomi-
nally of value R. The DAC output (I
OUT
1) is code-dependent,
producing various resistances and capacitances. The external
amplifier choice should take into account the variation in
impedance generated by the DAC on the amplifiers inverting
input node.
Access is provided to the V
REF
, R
FB
, and both I
OUT
terminals of
the DAC, making the device extremely versatile and allowing it
to be configured in several different operating modes. For
example, the device provides unipolar output mode, 4-quadrant
multiplication in bipolar mode, and single-supply mode of
operation. Note that a matching switch is used in series with the
internal R
FB
. Power must be applied to V
DD
to achieve continuity
when measuring R
FB
.
CIRCUIT OPERATION
Unipolar Mode
Using a single op amp, the AD5444/AD5446 can easily be
configured to provide 2-quadrant multiplying operation or
a unipolar output voltage swing, as shown in Figure 38.
When an output amplifier is connected in unipolar mode, the
output voltage is given by
REF
n
OUT
V
D
V 
2
where:
D is the fractional representation of the digital word loaded to
the DAC:
D = 0 to 4095 (12-bit AD5444)
D = 0 to 16383 (14-bit AD5446)
n is the number of bits.
Note that the output voltage polarity is opposite to the V
REF
polarity for dc reference voltages.
This DAC is designed to operate with either negative or positive
reference voltages. The V
DD
power pin is used by the internal
digital logic only to drive the on and off states of the DAC
switches. The DAC is also designed to accommodate ac refer-
ence input signals in the range of −10 V to +10 V. With a xed
+10 V reference, the circuit shown in Figure 38 provides a
unipolar 0 V to −10 V output voltage swing. When V
IN
is an
ac signal, the circuit performs 2-quadrant multiplication.
Table 5 shows the relationship between digital code and
expected output voltage for unipolar operation.
Table 5. Unipolar Code
Digital Input Analog Output (V)
1111 1111 1111 −V
REF
(4095/4096)
1000 0000 0000 −V
REF
(2048/4096) = −V
REF
/2
0000 0000 0001 −V
REF
(1/4096)
0000 0000 0000 −V
REF
(0/4096) = 0
AD5444/
AD5446
04588-030
NOTES
1. R1 AND R2 USED ONLY IF GAIN ADJUSTMENT IS REQUIRED.
2. C1 PHASE COMPENSATION (1pF TO 2pF) MAY BE REQUIRED,
IF A1 IS A HIGH SPEED AMPLIFIER.
I
OUT
1
I
OUT
2
V
REF
V
DD
C1
A1
V
OUT
= 0V TO –V
REF
AGND
R2
V
DD
V
REF
SDIN
SCLKSYNC
MICROCONTROLLER
R
FB
R1
Figure 38. Unipolar Operation