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Bipolar Output Circuit
V
OUT
+
ǒ
D
16, 384
* 1
Ǔ
V
REF
(2)
R
FB
OPA277
U4
−2.5 V 3 V
OUT
3 +2.5 V
V
OUT
10 kW10 kW
+2.5 V
V
REF
V
DD
V
DD
GND
DAC8801
OPA277
U2
I
OUT
C1
C2
5 kW
−
+
−
+
(+10 V)
(−10 V 3 V
OUT
3 +10 V)
Programmable Current Source Circuit
I
L
+
(
R2 ) R3
)
ń R1
R3
V
REF
D
(3)
V
DD
V
DD
V
REF
V
REF
DAC8801
U1
R
FB
OPA277
U2
I
OUT
GND
150 kW
R14
15 kW
R24
C1
10 pF
OPA277
U2
15 kW
R2
50 W
R34
50 W
R3
V
OUT
150 kW
R1
I
L
LOAD
−
+
+
−
DAC8801
SLAS403B – NOVEMBER 2004 – REVISED FEBRUARY 2007
APPLICATION INFORMATION (continued)
The DAC8801, 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.
Figure 22. Bipolar Output Circuit
A DAC8801 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. A application of this circuit includes a 4-mA to 20-mA current transmitter with up to a 500- Ω load.
With a matched resistor network, the load current of the circuit is shown in Equation 3 :
Figure 23. Programmable Bidirectional Current Source Circuit
11
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