DAC8801_16 Datasheet, PDF (11/22 Pages) Texas Instruments – 14-Bit, Serial Input Multiplying Digital-to-Analog Converter

DAC8801
www.ti.com
SLAS403B – NOVEMBER 2004 – REVISED FEBRUARY 2007
APPLICATION INFORMATION (continued)
Bipolar Output Circuit
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.
ǒ Ǔ V_OUT +
D
16, 384
*
1
V_REF
(2)
10 kW
10 kW
+2.5 V
(+10 V)
V_DD
V_DD
R_FB
V_{REF DAC8801}
GND
5 kW
C2
−
C1
+
U4
V_OUT
OPA277
−
I_OUT
+ _U2
OPA277
−2.5 V 3 V_OUT 3 +2.5 V
(−10 V 3 V_OUT 3 +10 V)
Figure 22. Bipolar Output Circuit
Programmable Current Source 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:
(R2 ) R3) ń R1
I_L +
R3
V_REF D
(3)
R24
15 kW
C1
10 pF
V_REF
V_DD
V_DD R_FB
V_REF
U1
DAC8801
GND
U2
OPA277
I_OUT
−
+
R14
150 kW
R1
150 kW
U2
OPA277
−
+
R2
15 kW
R34
50 W
V_OUT
R3
50 W
I_L
LOAD
Figure 23. Programmable Bidirectional Current Source Circuit
Submit Documentation Feedback
11