
AD5280/AD5282
Rev. C | Page 21 of
28
8-BIT BIPOLAR DAC
0
29
-05
8
VIN
VOUT
U1
A1
+5VREF
U2
VO
A2
U2 –
W
–15V
–5VREF
–15V
+
–
OP2177
+
–
OP2177
B
RR
A
V1
TRIM
ADR425
GND
AD5280
+15V
Figure 58. 8-Bit Bipolar DAC
Figure 58 shows a low cost, 8-bit, bipolar DAC. It offers the same
number of adjustable steps but not the precision of conventional
DACs. The linearity and temperature coefficients, especially at
low value codes, are skewed by the effects of the digital potenti-
ometer wiper resistance. The output of this circuit is
REF
O
V
×
=
1
256
D
2
(6)
BIPOLAR PROGRAMMABLE GAIN AMPLIFIER
02
92
9-
05
9
U2
U1
A2
V1
A1
B1
W2
A2
W1
VDD
A1
VDD
VS8
B2
V
VS8
O
R2
R1
C1
–kVI
V+
V–
+
–
OP2177
V+
V–
+
–
OP2177
AD5282
Figure 59. Bipolar Programmable Gain Amplifier
For applications that require bipolar gain,
Figure 59 shows one
implementation similar to the previous circuit. The digital
potentiometer, U1, sets the adjustment range. The wiper voltage
at W2 can therefore be programmed between Vi and –KVi at a
given U2 setting. Configuring A2 in noninverting mode allows
linear gain and attenuation. The transfer function is
()
+
×
+
=
K
R1
V
i
O
1
256
1
D2
R2
V
(7)
where K is the ratio of RWB1/RWA1 set by U1.
As in the previous example, in the simpler and more common
case where K = 1, a single digital AD5280 potentiometer is
used. U1 is replaced by a matched pair of resistors to apply
Vi and Vi at the ends of the digital potentiometer. The
relationship becomes
i
O
V
D2
R1
R2
V
×
+
=
1
256
2
1
(7)
If R2 is large, a compensation capacitor having a few pF may be
needed to avoid any gain peaking.
Table 7 shows the result of adjusting D, with A2 configured as a
unity gain, a gain of 2, and a gain of 10. The result is a bipolar
amplifier with linearly programmable gain and a 256-step
resolution.
Table 7. Result of Bipolar Gain Amplifier
D
R1 = ∞, R2 = 0
R1 = R2
R2 = 9R1
0
1
2
10
64
0.5
1
5
128
0
192
0.5
1
5
255
0.968
1.937
9.680
PROGRAMMABLE VOLTAGE SOURCE WITH
BOOSTED OUTPUT
For applications that require high current adjustments, such as a
laser diode driver or tunable laser, a boosted voltage source can
02
92
9-
06
0
CC
VI
5V
U1 = AD5280
A1 = AD8501, AD8605, AD8541
N1 = FDV301N, 2N7002
U1
RBIAS
IL
N1
VO
V+
V–
+
–
A1
A
B
W
SIGNAL
LD
Figure 60. Programmable Booster Voltage Source
In this circuit, the inverting input of the op amp forces the
VBIAS to be equal to the wiper voltage set by the digital potenti-
ometer. The load current is then delivered by the supply via the
N-channel FET N1. The N1 power handling must be adequate
to dissipate (Vi – VO) × IL power. This circuit can source a
maximum of 100 mA with a 5 V supply. A1 needs to be a rail-
to-rail input type. For precision applications, a voltage reference
input of the digital potentiometer.