Datasheet

Data Sheet AD5426/AD5432/AD5443
Rev. G | Page 15 of 24
THEORY OF OPERATION
The AD5426, AD5432, and AD5443 are 8-, 10-, and 12-bit
current output DACs consisting of a standard inverting R-2R
ladder configuration. A simplified diagram for the 8-bit AD5426 is
shown in Figure 40. The matching feedback resistor, R
FB
, has a
value of R. The value of R is typically 10 kΩ (minimum 8 kΩ
and maximum 12 kΩ). If I
OUT
1 and I
OUT
2 are kept at the same
potential, 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 nominally of value R. The DAC
output (I
OUT
) is code-dependent, producing various resistances
and capacitances. External amplifier choice should take into
account the variation in impedance generated by the DAC on
the amplifiers inverting input node.
V
REF
I
OUT
2
DAC DATA LATCHES
AND DRIVERS
I
OUT
1
R
FB
A
2R
S1
2R
S2
2R
S3
2R 2R
S8
R
RRR
03162-041
Figure 40. Simplified Ladder
Access is provided to the V
REF
, R
FB
, I
OUT
1, and I
OUT
2 terminals of
the DAC, making the device extremely versatile and allowing it
to be configured in several different operating modes. For example,
it can be configured to provide a unipolar output, 4-quadrant
multiplication in bipolar or single-supply modes of operation.
Note that a matching switch is used in series with the internal
R
FB
feedback resistor. If users attempt to measure R
FB
, power
must be applied to V
DD
to achieve continuity.
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 41.
When an output amplifier is connected in unipolar mode, the
output voltage is given by
n
REF
OUT
D
VV
2

where
D is the fractional representation of the digital word
loaded to the DAC, and n is the number of bits.
D = 0 to 255 (8-bit AD5426)
= 0 to 1023 (10-bit AD5432)
= 0 to 4095 (12-bit AD5443)
Note that the output voltage polarity is opposite to the V
REF
polarity for dc reference voltages.
These DACs are designed to operate with either negative or
positive reference voltages. The V
DD
power pin is used by only the
internal digital logic to drive the DAC switches’ on and off states.
These DACs are also designed to accommodate ac reference
input signals in the range of −10 V to +10 V.
With a fixed 10 V reference, the circuit shown in Figure 41 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 expected output
voltage for unipolar operation (AD5426, 8-bit device).
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
V
OUT
= 0
TO –V
REF
SCLK SDIN GND
V
REF
SYNC
I
OUT
2
I
OUT
1
R
FB
AGND
AD5426/
AD5432/
AD5443
R1
R2
A1
V
REF
V
DD
V
DD
C1
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.
MICROCONTROLLER
03162-042
A1
Figure 41. Unipolar Operation