
Data Sheet
AD5235
Rev. F | Page 25 of 32
BIPOLAR PROGRAMMABLE GAIN AMPLIFIER
implementation. Digital Potentiometer U1 sets the adjustment
range; the wiper voltage (VW2) can, therefore, be programmed
between VI and KVI at a given U2 setting. Configure OP2177 (A2) as a noninverting amplifier that yields a transfer function of
K
D2
R1
R2
V
I
O
)
1
(
1024
1
(4)
where K is the ratio of RWB1/RWA1 set by U1.
V+
V–
OP2177
AD5235
VO
V+
V–
OP2177
AD5235
VI
A1
W1
B1
–KVI
A2
B2
W1
VDD
VSS
R1
R2
VDD
VSS
A1
U2
A2
U1
02
81
6-
0
50
C
Figure 51. Bipolar Programmable Gain Amplifier
In the simpler (and much more usual) case where K = 1, VO is
simplified to
I
O
V
D
R1
R2
V
1
1024
2
1
(5)
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 1024-step resolution.
Table 21. Result of Bipolar Gain Amplifier
D2
R1 = ∞, R2 = 0
R1 = R2
R2 = 9 × R1
0
1
2
10
256
0.5
1
5
512
0
768
0.5
1
5
1023
0.992
1.984
9.92
10-BIT BIPOLAR DAC
If the circuit in
Figure 51 is changed with the input taken from a
precision reference, U1 is set to midscale, and
AD8552 (A2) is
configured as a buffer, a 10-bit bipolar DAC can be realized (as
circuit offers comparable resolution but not the precision because
of the wiper resistance effects. Degradation of the nonlinearity
and temperature coefficient is prominent near the low values
of the adjustment range. Alternatively, this circuit offers a unique
nonvolatile memory feature that, in some cases, outweighs any
shortfalls in precision.
Without consideration of the wiper resistance, the output of this
circuit is approximately
REF
O
V
D
V
1
1024
2
(6)
V+
V–
AD8552
VO
V+
V–
AD8552
–2.5VREF
B2
U1 = U2 = AD5235
A2
A1
B1
W2
A1
W1
U1
U2
+2.5VREF
VIN VOUT
TRIM
5
GND
2
6
U3
ADR421
+2.5V
–2.5V
+2.5V
A2
VI
02
81
6-
05
1
U1 = MIDSCALE
Figure 52. 10-Bit Bipolar DAC
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
AD5235
V+
V–
W
AD8601
VO
A
B
VI
2N7002
RBIAS
SIGNAL CC
LD
IL
02
81
6-
05
2
U2
Figure 53. Programmable Booster 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 N-Ch FET N1
(see Figure 53). N1 power handling must be adequate to dissipate (VI VO) × IL power. This circuit can source a 100 mA maximum
with a 5 V supply.
For precision applications, a voltage reference, such
as ADR421,potentiometer.