Datasheet

AD5424/AD5433/AD5445 Data Sheet
Rev. D | Page 18 of 28
THEORY OF OPERATION
The AD5424, AD5433, and AD5445 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 AD5424 is
shown in Figure 48. 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 resistance 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.
03160-048
V
REF
RR R
R
2R
S1 S2 S3 S8
2R 2R 2R 2R
DAC DATA LATCHES
AND DRIVERS
R
FB
A
I
OUT
1
I
OUT
2
Figure 48. 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,
to provide a unipolar output, 4-quadrant multiplication in bipolar
mode or in 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 49.
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 resolution of the DAC.
D = 0 to 255 (8-bit AD5424)
= 0 to 1023 (10-bit AD5433)
= 0 to 4095 (12-bit AD5445)
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 only used by 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 49 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 7 shows the relationship between digital code and expected
output voltage for unipolar operation (AD5424, 8-bit device).
Table 7. 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
03160-049
V
REF
V
REF
V
DD
V
DD
R/W
R1
R2
I
OUT
1
I
OUT
2
CS
R
FB
GND
C1
A1
AGND
DATA
INPUTS
V
OUT
=
0 TO –V
REF
R1 AND R2 USED ONLY IF GAIN ADJUSTMENT IS REQUIRED.
C1 PHASE COMPENSATION (1pF TO 2pF) MAY BE REQUIRED
IF A1 IS A HIGH SPEED AMPLIFIER.
NOTES:
1.
2.
AD5424/
AD5433/
AD5445
Figure 49. Unipolar Operation