AD5260/AD5262
Rev. A | Page 20 of 24
BIPOLAR PROGRAMMABLE GAIN AMPLIFIER
For applications that require bipolar gain,
Figure 62 shows one
implementation. 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
the noninverting mode allows linear gain and attenuation. The
transfer function is
()
+
×
+
=
K
D
R1
R2
V
i
O
1
256
2
1
(5)
where K is the ratio of RWB1/RWA1 set by U1.
–KVi
A1
B1
A2
R1
R2
VDD
VSS
VDD
OP2177
A2
B2
W2
U2
AD5262
U1
AD5262
W1
A1
VO
C1
Vi
02
695
-062
Figure 62. Bipolar Programmable Gain Amplifier
Similar to the previous example, in the simpler and more
common case, where K = 1, with a single digital potentiometer,
AD5260, U1 is replaced by a matched pair of resistors to apply
Vi and Vi at the ends of the digital potentiometer. The relation-
ship becomes
i
O
V
D
R1
R2
V
×
+
=
1
256
2
1
(6)
If R2 is large, a few picofarad compensation capacitors may be
needed to avoid any gain peaking.
Table 11 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 256-step
resolution.
Table 11. 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
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
Vi
A1
VO
W
U1
A
B
CC
IL
5V
SIGNAL LO
N1
R1
10k
P1
RBIAS
U1 = AD5260
A1 = AD8601, AD8605, AD8541
P1 = FDP360P, NDS9430
N1 = FDV301N, 2N7002
026
95-
0
63
Figure 63. Programmable Boosted Voltage Source
In this circuit, the inverting input of the op amp forces VO to be
equal to the wiper voltage set by the digital potentiometer. The
load current is then delivered by the supply via the P-channel
FET, P1. The N-channel FET, N1, simplifies the op amp driving
requirement. A1 must be the rail-to-rail input type. Resistor R1
is needed to prevent P1 from turning off once it is on. The choice
of R1 is a balance between the power loss of this resistor and
the output turn-off time. N1 can be any general-purpose signal
FET. However, P1 is driven in the saturation state, and there-
fore, its power handling must be adequate to dissipate (Vi VO)
× IL power. This circuit can source a maximum of 100 mA at 5 V
supply. Higher current can be achieved with P1 in a larger pack-
age. Note that a single N-channel FET can replace P1, N1, and
R1 altogether. However, the output swing is limited unless sepa-
rate power supplies are used. For a precision application, a
be applied at the input of the digital potentiometer.
PROGRAMMABLE 4 mA-TO-20 mA CURRENT
SOURCE
A programmable 4 mA-to-20 mA current source can be
unique low supply headroom and high current handling
precision reference that can deliver 20 mA at 2.048 V. The load
current is simply the voltage across Terminal B to Terminal W
of the digital potentiometer, divided by RS.
S
REF
L
R
D
V
I
×
=
(7)