If an application requires the DAC to have zero gain error, the
circuit may be used, see Figure 6. Resistors R2 and R4 induce
a positive gain error greater than worst-case initial negative
gain error. Trim resistors R1 and R3 provide a variable
negative gain error and have sufficient trim range to correct
for the worst-case initial positive gain error plus the error
produced by R2 and R4.
DATA INPUT
↓ LSB
1111 1111 1111
1000 0000 0001
1000 0000 0000
0111 1111 1111
0000 0000 0000
ANALOG OUTPUT
MSB ↓
+VREF (2047/2048)
+VREF (1/2048)
0 Volts
–VREF (1/2048)
–VREF (2048/2048)
TABLE III. Bipolar Output Code.
+5V
R1
VDD
20kΩ
VREF A
R2
20kΩ
–
+
CD
1µF
VOUT A
A2
R3
+
10kΩ
RFB A
IOUT A
C1
10pF
–
A1
DAC A
AGND A
+
DAC7802 has a single analog common, AGND.
A1–A4, OPA602 or 1/2 OPA2107.
DAC780X
RFB B
IOUT B
C2
10pF
–
A3
+
DAC B
AGND B
R4
20kΩ
R5
10kΩ
R6
20kΩ
–
DGND
VOUT B
A4
+
VREF B
FIGURE 5. Bipolar Configuration.
DAC1 and DAC2 can be updated in parallel with a single word
to set the center frequency of the filter. DAC 4, which makes
use of the uncommitted op amp in UAF42, sets the Q of the
filter. DAC3 sets the gain of the filter transfer function without
changing the Q of the filter. The reverse is also true.
APPLICATIONS
12-BIT PLUS SIGN DACS
For a bipolar DAC with 13 bits of resolution, two solutions are
possible. The addition of a precision difference amplifier and
a high speed JFET switch provides a 12-bit plus sign voltage-
output DAC, see Figure 7. When the switch selects the op
amp output, the difference amplifier serves as a noninverting
output buffer. If the analog ground side of the switch is
selected, the output of the difference amplifier is inverted.
The center frequency is determined by fC = 1/2πRC where R is
the ladder resistance of the DAC (typical value, 10kΩ) and C
the internal capacitor value (1000pF) of the UAF42. External
capacitors can be added to lower the center frequency of the
filter. But the highest center frequency for this circuit will be
about 16kHz because the effective series resistance of the
DAC cannot be less than 10kΩ.
Another option, see Figure 8, also produces a 12-bit plus sign
output without the additional switch and digital control line.
Note that the ladder resistance of the DAC may vary from
device to device. Thus, for best tracking, DAC2 and DAC3
should be in the same package. Some calibration may be
necessary from one filter to another.
DIGITALLY PROGRAMMABLE ACTIVE FILTER
See Figure 9 for the DAC780x in a digitally programmable
active filter application. The design is based on the state-
variable filter, Texas Instruments UAF42, an active filter topol-
ogy that offers stable and repeatable filter characteristics.
DAC7800, 7801, 7802
10
SBAS005A
www.ti.com
TI [ TEXAS INSTRUMENTS ]
