DAC8811 Datasheet, PDF (13/29 Pages) Texas Instruments – 16-Bit, Serial Input Multiplying Digital-to-Analog Converter

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Feature Description (continued)
DAC8811
SLAS411D – NOVEMBER 2004 – REVISED FEBRUARY 2016
0 ” V_OUT ” +2.5V
Figure 21. Positive Voltage Output Circuit
8.3.3 Bipolar 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.
V_OUT
§D
¨© _32,768
-1
·
¸¹
x
V_REF
(2)
External resistance mismatching is the significant error in Figure 22.
10 k:
10 k:
5 k:
-2.5V ” V_OUT ” +2.5V
(-10s G s_OUT G +10V)
Figure 22. Bipolar Output Circuit
8.3.4 Programmable Current Source 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:
R2 R3 / R1
I_L
R3
x V_REF x D
(3)
The value of R3 in the previous equation can be reduced to increase the output current drive of U3. U3 can drive
±20 mA in both directions with voltage compliance limited up to 15 V by the U3 voltage supply. Elimination of the
circuit compensation capacitor C1 in the circuit is not suggested as a result of the change in the output
impedance Z_O, according to Equation 4:
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