AD5444 Analog Devices, AD5444 Datasheet - Page 15

• Manufacturer Part Number: AD5444
• Description: 12-Bit High Bandwidth Multiplying DAC with Serial Interface
• Manufacturer: Analog Devices

Specifications of AD5444

• Resolution (bits): 12bit
• Dac Update Rate: 2.7MSPS
• Dac Settling Time: 40ns
• Max Pos Supply (v): +5.5V
• Single-supply: Yes
• Dac Type: Current Out
• Dac Input Format: Ser,SPI

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.
The feedback resistor (R
typically 9 kΩ (7 kΩ minimum, 11 kΩ maximum). If I
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
nally of value R. The DAC output (I
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
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
when measuring R
V
REF
FB
. Power must be applied to V
2R
S1
R
DAC DATA LATCHES
FB
AND DRIVERS
2R
S2
.
Figure 37. Simplified Ladder
R
FB
) has a value of R. The value of R is
2R
S3
REF
, R
REF
FB
R
is always constant and nomi-
, and both I
OUT
2R
V
S12
REF
1) is code-dependent,
DD
to achieve continuity
NOTES
1. R1 AND R2 USED ONLY IF GAIN ADJUSTMENT IS REQUIRED.
2. C1 PHASE COMPENSATION (1pF TO 2pF) MAY BE REQUIRED,
2R
R1
IF A1 IS A HIGH SPEED AMPLIFIER.
OUT
R
MICROCONTROLLER
terminals of
V
SYNC
REF
R
I
I
OUT
OUT
OUT
SCLK
FB
AD5444/
1
2
Figure 38. Unipolar Operation
AD5446
V
V
1 is
DD
DD
SDIN
Rev. C | Page 15 of 28
R
FB
I
I
OUT
OUT
1
2
R2
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
where:
D is the fractional representation of the digital word loaded to
the DAC:
n is the number of bits.
Note that the output voltage polarity is opposite to the V
polarity for dc reference voltages.
This DAC is designed to operate with either negative or positive
reference voltages. The V
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 fixed
+10 V reference, the circuit shown in Figure 38 provides a
unipolar 0 V to −10 V output voltage swing. When V
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
1111 1111 1111
1000 0000 0000
0000 0000 0001
0000 0000 0000
AGND
C1
D = 0 to 4095 (12-bit AD5444)
D = 0 to 16383 (14-bit AD5446)
V
A1
OUT
=
V
OUT
−
2
D
= 0V TO –V
n
×
V
REF
REF
DD
Analog Output (V)
−V
−V
−V
−V
power pin is used by the internal
REF
REF
REF
REF
(4095/4096)
(2048/4096) = −V
(1/4096)
(0/4096) = 0
AD5444/AD5446
REF
/2
IN
is an
REF