DAC8806 Datasheet, PDF (12/20 Pages) Texas Instruments – 14-Bit, Parallel Input Multiplying Digital-to-Analog Converter

DAC8806
SBAS385A – APRIL 2006 – REVISED JUNE 2006
TYPICAL CHARACTERISTICS (continued)
At T_A = +25°C, unless otherwise noted.
UNIPOLAR MULTIPLYING MODE THD
vs FREQUENCY
-45
Code 3FFFh
-55 ^{VREF = 6VRMS}
V_DD = +5V
-65 ^{One OPA627}
C₁ = 20pF
-75
500kHz Filter
80kHz Filter
30kHz Filter
-85
-95
-105
-115
10
100
1000
Frequency (Hz)
Figure 37.
10k 20k 30k
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THEORY OF OPERATION
The DAC8806 is a multiplying, single-channel current output, 14-bit DAC. The architecture, illustrated in
Figure 38, is an R-2R ladder configuration with the three MSBs segmented. Each 2R leg of the ladder is either
switched to GND or to the I_OUT terminal. The I_OUT terminal of the DAC is held at a virtual GND potential by the
use of an external I/V converter op amp. The R-2R ladder is connected to an external reference input (V_REF) that
determines the DAC full-scale current. The R-2R ladder presents a code independent load impedance to the
external reference of 6kΩ ± 25%. The external reference voltage can vary in a range of –10V to +10V, thus
providing bipolar I_OUT current operation. By using an external I/V converter op amp and the R_FB resistor in the
DAC8806, an output voltage range of –V_REF to +V_REF can be generated.
V_REF
R
R
R
R_FB
2R 2R 2R 2R 2R 2R 2R
2R
2R 2R 2R
2R
R_FB
I_OUT
GND
Figure 38. Equivalent R-2R DAC Circuit
The DAC output voltage is determined by V_REF and the digital data (D) according to Equation 1:
V_OUT + −V_REF
D
16384
(1)
Each DAC code determines the 2R-leg switch position to either GND or I_OUT. The external I/V converter op amp
noise gain will also change because the DAC output impedance (as seen looking into the I_OUT terminal) changes
versus code. Because of this, the external I/V converter op amp must have a sufficiently low offset voltage such
that the amplifier offset is not modulated by the DAC I_OUT terminal impedance change. External op amps with
large offset voltages can produce INL errors in the transfer function of the DAC8806 because of offset
modulation versus DAC code. For best linearity performance of the DAC8806, an op amp (OPA277) is
recommended; see Figure 39. This circuit allows V_REF to swing from –10V to +10V.
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