Datasheet

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GND
OPA277
−2.5V
GND
0 3 V
OUT
3 +2.5V
V
OUT
I
OUT
OPA277
C1
R
FB
DAC8811
V
DD
V
DD
+2.5V Reference
V
IN
V
OUT
V
REF
−
+
−
+
Bipolar Output Circuit
V
OUT
+
ǒ
D
32, 768
*1
Ǔ
V
REF
(2)
R
FB
OPA277
U4
−2.5V 3 V
OUT
3 +2.5V
(−10V 3 V
OUT
3 +10V)
V
OUT
10 kW10 kW
+2.5V
(+10V)
V
REF
V
DD
V
DD
GND
DAC8811
OPA277
U2
I
OUT
C1
C2
5 kW
−
+
−
+
Programmable Current Source Circuit
I
L
+
(
R2)R3
)
ń R1
R3
V
REF
D
(3)
DAC8811
SLAS411B – NOVEMBER 2004 – REVISED FEBRUARY 2007
APPLICATION INFORMATION (continued)
Figure 21. Positive Voltage Output Circuit
The DAC8811, 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 22 ,
external op amp U4 is added as a summing amp and has a gain of 2X that widens the output span to 5 V. A
4-quadrant multiplying circuit is implemented by using a 2.5-V 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
= -2.5 V to V
OUT
= +2.5 V.
External resistance mismatching is the significant error in Figure 22 .
Figure 22. Bipolar Output Circuit
A DAC8811 can be integrated into the circuit in Figure 23 to implement an improved Howland current pump for
precise voltage to current conversions. Bidirectional current flow and high voltage compliance are two features of
the circuit. With a matched resistor network, the load current of the circuit is shown by Equation 3 :
10
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