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Positive Voltage Output Circuit
GND
-2.5V
+2.5VReference
OPA277
GND
I 1
OUT
R
FB
0 V<
OUT
+2.5V<
V
OUT
V
REF
V
DD
V
IN
V
DD
V
OUT
I 2
OUT
C
1
OPA277
DAC7821
Bipolar Output Section
V
OUT
+
ǒ
D
0.5 2
N
*1
Ǔ
V
REF
(2)
+2.5V
(+10V)
U4
OPA277
10kW10kW
5kW
- £2.5V £ +2.5VV
OUT
( 10V- £ £ +10V)V
OUT
U2
GND
I 1
OUT
R
FB
V
REF
V
DD
V
DD
V
OUT
I 2
OUT
C
1
OPA277
DAC7821
C
2
DAC7821
SBAS365B – OCTOBER 2005 – REVISED JULY 2007
As Figure 28 illustrates, in order to generate a positive voltage output, a negative reference is input to the
DAC7821. This design is suggested instead of using an inverting amp to invert the output because of possible
resistor tolerance errors. For a negative reference, V
OUT
and GND of the reference are level-shifted to a virtual
ground and a –2.5V input to the DAC7821 with an op amp.
Figure 28. Positive Voltage Output Circuit
The DAC7821, as a 2-quadrant multiplying DAC, can be used to generate a unipolar output. The polarity of the
full-scale output I
OUT
is the inverse of the input reference voltage at V
REF
.
Some applications require full 4-quadrant multiplying capabilities or bipolar output swing. As shown in Figure 29 ,
external op amp U4 is added as a summing amp and has a gain of 2X that widens the output span to 5V. A
4-quadrant multiplying circuit is implemented by using a 2.5V offset of the reference voltage to bias U4.
According to the circuit transfer equation given in Equation 2 , input data (D) from code 0 to full-scale produces
output voltages of V
OUT
= –2.5V to V
OUT
= +2.5V.
External resistance mismatching is the significant error in Figure 29 .
Figure 29. Bipolar Output Circuit
14
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