Datasheet

AD5263
Rev. A | Page 24 of 28
8-BIT BIPOLAR DAC
Figure 60 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
REF
O
V
D
V ×
⎟
⎠
⎞
⎜
⎝
⎛
−= 1
256
2
(5)
03142-059
AB
A1
W
U1
VIN
GND
V
O
1
VOUT
ADR425
+15V
TRIM
+5VREF
–5VREF
A2
V
I
AD5263
OP2177
V–
V+
+15
V
–15V
OP2177
V–
V+
Figure 60. 8-Bit Bipolar DAC
BIPOLAR PROGRAMMABLE GAIN AMPLIFIER
For applications requiring bipolar gain, Figure 61 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)
where K is the ratio of R
WB1
/R
WA 1
set by U1.
03142-060
A1
AD5263
W1
V
O
A2
A2
B2
V
SS
V
DD
U2
–KV
I
B1
A1
C1
V
I
R2
R1
AD5263
U1
W2
V
DD
V
SS
OP2177
V–
V+
OP2177
V–
V+
Figure 61. Bipolar Programmable Gain Amplifier
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
I
O
V
D2
R1
R2
V ×
⎟
⎠
⎞
⎜
⎝
⎛
−
×
×
⎟
⎠
⎞
⎜
⎝
⎛
+= 1
256
2
1 (7)
If R2 is large, a compensation capacitor of a few pF may be
needed to avoid any gain peaking.
Table 8 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.
Table 8. 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
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 62.
03142-061
+V
W
SIGNAL
C
C
R
BIAS
LD
V
IN
A
B
V
OUT
U1
AD5263
U3 2N7002
U2
–V
I
L
AD8601
Figure 62. Programmable Booster Voltage Source
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.