Datasheet

AD5263
Rev. 0 | Page 21 of 28
Similar to the previous example, in the simpler (and much more
usual) case where K = 1, a single channel is used and U1 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
8-BIT BIPOLAR DAC
Figure 59 shows a low cost, 8-bit, bipolar DAC. It offers the
same number of adjustable steps but not the precision as
compared to conventional DACs. The linearity and temperature
coefficient, especially at low values codes, are skewed by the
effects of the digital potentiometer wiper resistance. The output
of this circuit is
I
(7)
O
V
D2
R1
R2
V ×






−
×
×






+= 1
256
2
1
REF
O
V
D
V ×






−= 1
256
2
(5)
If
R2 is large, a compensation capacitor of a few pF may be
needed to avoid any gain peaking.
Table 10 shows the result of adjusting D, with A2 configured
with unity gain, gain of 2, and gain of 10. The result is a bipolar
amplifier with linearly programmable gain and 256-step
resolution.
03142-0-059
A
B
A1
V–
V+
W
U1
VIN
GND
V
O
OP2177
1
VOUT
+15V
ADR425
V–
V+
OP2177
+15V
TRIM
–15V
+5VREF
–5VREF
A2
V
I
AD5263
Table 10. Result of Bipolar Gain Amplifier
D
R1 = ∞, R2 = 0
R1 = R2 R2 = 9 × R1
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
Figure 59. 8-Bit Bipolar DAC
BIPOLAR PROGRAMMABLE GAIN AMPLIFIER
For applications that require high current adjustment, such as a
laser diode driver or tunable laser, a boosted voltage source can
be considered. See Figure 61.
For applications requiring bipolar gain, Figure 60 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 V
I
and –KV
I
at a
given U2 setting. Configuring A2 in the noninverting mode
allows linear gain and attenuation. The transfer function is
()






−+××






+=
KK
D2
R1
R2
V
V
I
O
1
256
1 (6)
03142-0-061
+V
W
SIGNAL
C
C
R
BIAS
LD
V
IN
A
B
V
OUT
U1
AD5263
U3 2N7002
AD8601
U2
–V
I
L
where
K is the ratio of R
WB1
/R
WA 1
set by U1.
Figure 61. Programmable Booster Voltage Source
03142-0-060
OP2177
A1
AD5263
W1
V
O
OP2177
A2
A2
B2
V+
V
_
V+
V
_
V
SS
V
DD
U2
–KV
I
B1
A1
C1
V
I
R2
R1
AD5263
U1
W2
V
DD
V
SS
In this circuit, the inverting input of the op amp forces the V
OUT
to be equal to the wiper voltage set by the digital potentiometer.
The load current is then delivered by the supply via the N-
channel FET N1. N1 power handling must be adequate to
dissipate power equal to (V
IN
– V
OUT
) × I
L
. This circuit can
source a maximum of 100 mA with a 5 V supply. For precision
applications, a voltage reference such as ADR421, ADR03, or
ADR370 can be applied at the A terminal of the digital
potentiometer.
Figure 60. Bipolar Programmable Gain Amplifier