Datasheet
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Theory of Operation
R R R R
2R 2R 2R 2R
DB0
(LSB)
DB9DB10DB11
(MSB)
I 1
OUT
R
FB
V
REF
I 2
OUT
R
V
OUT
+ *V
REF
CODE
4096
(1)
OPA277
DAC7811
15V
V+
V-
-15V
GND
I 1
OUT
R
FB
V
REF
V
DD
V
DD
V
OUT
I 2
OUT
DAC7811
SBAS337C – APRIL 2005 – REVISED JULY 2007
The DAC7811 is a single channel, current output, 12-bit digital-to-analog converter (DAC). The architecture,
illustrated in Figure 25 , is an R-2R ladder configuration with the three MSBs segmented. Each 2R leg of the
ladder is either switched to I
OUT
1 or the I
OUT
2 terminal. The I
OUT
1 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 current. The R-2R ladder presents a code independent load
impedance to the external reference of 10k Ω ± 20%. The external reference voltage can vary over a range of
–15V to +15V, thus providing bipolar I
OUT
current operation. By using an external I/V converter and the
DAC7811 R
FB
resistor, output voltage ranges of –V
REF
to V
REF
can be generated.
Figure 25. Equivalent R-2R DAC Circuit
When using an external I/V converter and the DAC7811 R
FB
resistor, the DAC output voltage is given by
Equation 1 :
Each DAC code determines the 2R leg switch position to either GND or I
OUT
. Because the DAC output
impedance as seen looking into the I
OUT
1 terminal changes versus code, the external I/V converter noise gain
will also change. Because of this, 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
1 terminal impedance change. External op amps
with large offset voltages can produce INL errors in the transfer function of the DAC7811 due to offset
modulation versus DAC code.
For best linearity performance of the DAC7811, a low offset voltage op amp (such as the OPA277 ) is
recommended (see Figure 26 ). This circuit allows V
REF
swinging from –10V to +10V.
Figure 26. Voltage Output Configuration
10
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