Datasheet

AD5280/AD5282
Rev. C | Page 21 of 28
8-BIT BIPOLAR DAC
02929-058
V
IN
V
OUT
U
1
A
1
+5V
REF
U
2
V
O
A
2
U
2
–
W
–15V
–5V
REF
–15V
+
–
OP2177
+
–
OP2177
B
RR
A
V
1
TRIM
ADR425
GND
AD5280
+15V
+15
V
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
VV ×
⎟
⎠
⎜
⎝
−= 1
256
D
⎞⎛
2
(6)
BIPOLAR PROGRAMMABLE GAIN AMPLIFIER
02929-059
U
2
U
1
A
2
V
1
A
1
B
1
W
2
A
2
A
2
W
1
V
DD
A
1
V
DD
V
S8
B
2
V
V
S8
O
R2
R1
C1
–kVI
V+
V–
+
–
OP2177
V+
V–
+
–
OP2177
AD5282
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, U
1
, sets the adjustment range. The wiper voltage
at W
2
can therefore be programmed between V
i
and –KV
i
at a
given U
2
setting. Configuring A
2
in noninverting mode allows
linear gain and attenuation. The transfer function is
()
⎟
⎠
⎜
⎝
−+××
⎟
⎠
⎜
⎝
+= KK
R1V
i
O
1
256
1
⎞⎛⎞⎛
D2R2
V
(7)
where K is the ratio of R
WB1
/R
WA 1
set by U
1
.
As in the previous example, in the simpler and more common
case where K = 1, a single digital AD5280 potentiometer is
used. U
1
is replaced by a matched pair of resistors to apply
V
i
and −V
i
at the ends of the digital potentiometer. The
relationship becomes
iO
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.
Tabl e 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 0 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
be considered (see Figure 60).
02929-060
C
C
V
I
5V
U
1
= AD5280
A
1
= AD8501, AD8605, AD8541
N
1
= FDV301N, 2N7002
U
1
R
BIAS
I
L
N
1
V
O
V+
V–
+
–
A1
A
B
W
SIGNAL
L
D
Figure 60. Programmable Booster Voltage Source
In this circuit, the inverting input of the op amp forces the
V
BIAS
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 (V
i
– V
O
) × I
L
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
such as ADR423, ADR292, or AD1584 can be applied at the
input of the digital potentiometer.