BIPOLAR PROGRAMMABLE GAIN AMPLIFIER
For applications requiring bipolar gain, Figure 51 shows one
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
VO = ⎜⎛1 + R2 ⎟⎞ × ⎜⎛ D2 ×(1 + K) − K ⎟⎞
(4)
VI ⎝ R1 ⎠ ⎝ 1024
⎠
where K is the ratio of RWB1/RWA1 set by U1.
VDD
AD5235
W1
U2
A2 B2
VI
A1 B1
W1 VDD
V+
OP2177
V–
R2
A2 VSS
–KVI
R1
VO
C
AD5235
U1
V+
OP2177
V–
A1 VSS
Figure 51. Bipolar Programmable Gain Amplifier
In the simpler (and much more usual) case where K = 1, VO is
simplified to
VO
= ⎜⎝⎛1+
R2
R1
⎟⎠⎞
⎜⎝⎛
2D2
1024
−1⎟⎠⎞ ×VI
(5)
Table 21 shows the result of adjusting D2, with OP2177 (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 1024-step resolution.
Table 21. Result of Bipolar Gain Amplifier
D2
R1 = ∞, R2 = 0
R1 = R2
0
−1
−2
256
−0.5
−1
512
0
0
768
0.5
1
1023
0.992
1.984
R2 = 9 × R1
−10
−5
0
5
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
shown in Figure 52). Compared to the conventional DAC, this
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.
AD5235
Without consideration of the wiper resistance, the output of this
circuit is approximately
VO
=
⎜⎝⎛
2D2
1024
− 1⎟⎠⎞ ×VREF
(6)
+2.5V
U2
VI
2 U3
6
VIN VOUT
+2.5VREF
5
TRIM
GND
ADR421
W2
B2 A2
A1 B1
+2.5V
W1
U1
V+
AD8552
V–
V+
AD8552
V–
VO
A2
–2.5V
–2.5VREF
U1 = U2 = AD5235
U1 = MIDSCALE
A1
–2.5V
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
be considered (see Figure 53).
VI
AD5235
2N7002
A
W
B
U2
V+
AD8601
V–
VO
SIGNAL CC
RBIAS
IL
LD
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,
ADR03, or ADR370, can be applied at Terminal A of the digital
potentiometer.
Rev. E | Page 25 of 32