Datasheet

Data Sheet AD5429/AD5439/AD5449
Rev. E | Page 15 of 28
THEORY OF OPERATION
DIGITAL-TO-ANALOG CONVERTER
The AD5429/AD5439/AD5449 are 8-, 10-, and 12-bit, dual-
channel, current output DACs consisting of a standard inverting
R-2R ladder configuration. Figure 37 shows a simplified diagram
for a single channel of the AD5449. The feedback resistor, R
FB
A,
has a value of R. The value of R is typically 10 kΩ (with a
minimum of 8 kΩ and a maximum of 12 kΩ). If I
OUT
1A and
I
OUT
2A are kept at the same potential, a constant current flows
into each ladder leg, regardless of digital input code. Therefore,
the input resistance presented at V
REF
A is always constant.
2R
S1
2R
S2
2R
S3
2R
S12
2R
DAC DATA LATCHES
AND DRIVERS
R
R
FB
A
I
OUT
1A
I
OUT
2A
V
REF
A
04464-006
RR R
Figure 37. Simplified Ladder
Access is provided to the V
REF
x, R
FB
x, I
OUT
1x, and I
OUT
2x termi-
nals of the DACs, making the devices extremely versatile and
allowing them to be configured in several operating modes, such
as unipolar mode, bipolar output mode, or single-supply mode.
CIRCUIT OPERATION
Unipolar Mode
Using a single op amp, these devices 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
n
REF
OUT
DVV 2/
where:
D is the fractional representation of the digital word loaded to
the DAC.
D = 0 to 255 (AD5429)
= 0 to 1023 (AD5439)
= 0 to 4095 (AD5449)
n is the number of bits.
With a fixed 10 V reference, the circuit shown in Figure 38 gives
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 the
expected output voltage for unipolar operation using the 8-bit
AD5429 DAC.
Table 5. Unipolar Code Table
Digital Input Analog Output (V)
1111 1111 −V
REF
(255/256)
1000 0000 −V
REF
(128/256) = −V
REF
/2
0000 0001 −V
REF
(1/256)
0000 0000 −V
REF
(0/256) = 0
AD5429/
AD5439/
AD5449
04464-007
NOTES
1. R1 AND R2 USED ONLY IF GAIN ADJUSTMENT IS REQUIRED.
2. C1 PHASE COMPENSATION (1pF TO 2pF) MAY BE REQUIRED
3. DAC B OMITTED FOR CLARITY.
I
OUT
1A
I
OUT
2A
V
REF
x
V
DD
C1
A1
V
OUT
= 0V TO –V
REF
AGND
R2
V
DD
V
REF
SDIN GND
SCLKSYNC
R
FB
A
R1
IF A1 IS A HIGH SPEED AMPLIFIER.
MICROCONTROLLER
Figure 38. Unipolar Operation