Datasheet
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THEORY OF OPERATION
R
R
R
I
OUT
GND
V
REF
2R 2R2R 2R 2R 2R2R 2R2R 2R 2R 2R R
FB
V A/B= V
OUT REF
- ´
D
16384
(1)
V+
V-
DAC8805
I A/B
OUT
V
DD
V
REF
OPA277
V
OUT
U2
V
DD
R
OFS
R
FB
U1
GND
+15V
-15V
DAC8805
SBAS391A – DECEMBER 2006 – REVISED MAY 2007
The DAC8805 is a multiplying, dual-channel, current output, 14-bit DAC. The architecture, illustrated in
Figure 40 , is an R-2R ladder configuration with the three MSBs segmented. Each 2R leg of the ladder is either
switched to GND or to the I
OUT
terminal. The I
OUT
terminal of the DAC is held at a virtual GND potential by the
use of an external I/V converter op amp. The R-2R ladder is connected to an external reference input (V
REF
) that
determines the DAC full-scale output current. The R-2R ladder presents a code-independent load impedance to
the external reference of 5k Ω ± 25%. The external reference voltage can vary in a range of –18V to +18V, thus
providing bipolar I
OUT
current operation. By using an external I/V converter op amp and the R
FB
resistor in the
DAC8805, an output voltage range of –V
REF
to +V
REF
can be generated.
Figure 40. Equivalent R-2R DAC Circuit
The DAC output voltage is determined by V
REF
and the digital data (D) according to Equation 1 :
Each DAC code determines the 2R-leg switch position to either GND or I
OUT
. The external I/V converter op amp
noise gain will also change because the DAC output impedance (as seen looking into the I
OUT
terminal) changes
versus code. Because of this change in noise gain, the external I/V converter op amp must have a sufficiently
low offset voltage such that the amplifier offset is not modulated by the DAC I
OUT
terminal impedance change.
External op amps with large offset voltages can produce INL errors in the transfer function of the DAC8805
because of offset modulation versus DAC code. For best linearity performance of the DAC8805, an op amp
(such as the OPA277 ) is recommended, as shown in Figure 41 . This circuit allows V
REF
to swing from –10V to
+10V.
Figure 41. Voltage Output Configuration
15
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