Datasheet

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THEORY OF OPERATION
R R R R
2R 2R 2R 2R 2R
DB0
(LSB)
DB9DB10DB11
(MSB)
I 1
OUT
R
FB
V
REF
I 2
OUT
V
OUT
+ * V
REF
CODE
4096
(1)
OPA277
DAC7821
15V
V+
V-
-15V
GND
I 1
OUT
R
FB
V
REF
V
DD
V
DD
V
OUT
I 2
OUT
DAC7821
SBAS365B – OCTOBER 2005 – REVISED JULY 2007
The DAC7821 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
DAC7821 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 DAC7821 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 DAC7821 as a result of offset
modulation versus DAC code.
For best linearity performance of the DAC7821, an op amp with a low input offset voltage (OPA277 ) is
recommended (see Figure 26 ). This circuit allows V
REF
swinging from –10V to +10V.
Figure 26. Voltage Output Configuration
11
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