Datasheet

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TotalHarmonicDistortion(dB)
45
55
65
75
85
95
105
115
-
-
-
-
-
-
-
-
10 100 1000 10k 20k 30k
Frequency(Hz)
500kHzFilter
80kHzFilter
30kHzFilter
Code3FFFh
V =6V
REF RMS
1
DD
V =+5V
OneOPA627
C =20pF
THEORY OF OPERATION
R R
2R 2R2R2R2R2R2R2R 2R R
FB
2R2R
R
2R
V
REF
I
OUT
GND
R
FB
V
OUT
+ −V
REF
D
16384
(1)
DAC8806
SBAS385B – APRIL 2006 – REVISED FEBRUARY 2008
TYPICAL CHARACTERISTICS (continued)
At T
A
= +25 ° C, unless otherwise noted.
UNIPOLAR MULTIPLYING MODE THD
vs FREQUENCY
Figure 37.
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 6 k Ω ± 25%. The external reference voltage can vary in a range of – 10 V to +10 V, 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.
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 :
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 – 10 V to +10 V.
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