Datasheet

AD5450/AD5451/AD5452/AD5453 Data Sheet
Rev. G | Page 16 of 28
GENERAL DESCRIPTION
DAC SECTION
The AD5450/AD5451/AD5452/AD5453 are 8-/10-/12-/14-bit
current output DACs, respectively, consisting of a segmented
(4-bit) inverting R-2R ladder configuration. A simplified
diagram for the 12-bit AD5452 is shown in Figure 43.
2R
S1
2R
S2
2R
S3
2R
S12
2R
DAC DATA LATCHES
AND DRIVERS
R
R
FB
I
OUT
1
V
REF
04587-060
R R R
AGND
Figure 43. AD5452 Simplified Ladder
The feedback resistor, R
FB
, has a value of R. The value of R is
typically 9 kΩ (with a minimum value of 7 kΩ and a maximum
value of 11 kΩ). 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 nominally of value R. The DAC output
(I
OUT
1) is code-dependent, producing various resistances and
capacitances. When choosing the external amplifier, take into
account the variation in impedance generated by the DAC on
the amplifier’s inverting input node.
Access is provided to the V
REF
, R
FB
, and I
OUT
1 terminals of the
DAC, making the device extremely versatile and allowing it to be
configured in several operating modes; for example, it can provide
a unipolar output or can provide 4-quadrant multiplication in
bipolar mode. 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 a 2-quadrant multiplying operation or a unipolar output
voltage swing, as shown in Figure 44. 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 255 (8-bit AD5450).
= 0 to 1023 (10-bit AD5451).
= 0 to 4095 (12-bit AD5452).
= 0 to 16,383 (14-bit AD5453).
n is the number of bits.
Note that the output voltage polarity is opposite to the V
REF
polarity for dc reference voltages.
04587-009
R
FB
I
OUT
1
GND
SCLK
SDIN
V
REF
V
REF
R1
SYNC
AD5450/
AD5451/
AD5452/
AD5453
V
DD
V
DD
AGND
C1
A1
R2
V
OUT
= 0 TO –V
REF
µCONTROLLER
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.
Figure 44. Unipolar Mode Operation
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 on and off states of the
DAC switches.
These DACs are 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 44 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 the digital code and
the expected output voltage for a unipolar operation using the
8-bit AD5450.
Table 5. Unipolar Code Table for the AD5450
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