Datasheet

Data Sheet AD5235
Rev. F | Page 25 of 32
BIPOLAR PROGRAMMABLE GAIN AMPLIFIER
For applications requiring bipolar gain, Figure 51 shows one
implementation. Digital Potentiometer U1 sets the adjustment
range; the wiper voltage (V
W2
) can, therefore, be programmed
between V
I
and −KV
I
at a given U2 setting. Configure OP2177
(A2) as a noninverting amplifier that yields a transfer function of













 KK
D2
R1
R2
V
V
I
O
)1(
1024
1 (4)
where
K is the ratio of R
WB1
/R
WA 1
set by U1.
V+
V–
OP2177
AD5235
V
O
V+
V–
OP2177
AD5235
V
I
A1
W1
B1
–KV
I
A2
B2
W1
V
DD
V
SS
R1
R2
V
DD
V
SS
A1
U2
A2
U1
02816-050
C
Figure 51. Bipolar Programmable Gain Amplifier
In the simpler (and much more usual) case where K = 1, V
O
is
simplified to
I
O
V
D
R1
R2
V 













 1
1024
22
1 (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 R2 = 9 × R1
0 −1 −2 −10
256 −0.5 −1 −5
512 0 0 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
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.
Without consideration of the wiper resistance, the output of this
circuit is approximately
REF
O
V
D
V 






 1
1024
22
(6)
V+
V–
AD8552
V
O
V+
V–
AD8552
–2.5V
REF
B2
U1 = U2 = AD5235
A2
A1
B1
W2
A1
W1
U1
U2
+2.5V
REF
V
IN
V
OUT
TRIM
5
GND
2
6
U3
ADR421
+2.5
V
–2.5V
–2.5V
+2.5V
A2
V
I
02816-051
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
be considered (see Figure 53).
AD5235
V+
V–
W
AD8601
V
O
A
B
V
I
2N7002
R
BIAS
SIGNAL
C
C
LD
I
L
02816-052
U2
Figure 53. Programmable Booster Voltage Source
In this circuit, the inverting input of the op amp forces V
O
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 N
1
(see Figure 53). N
1
power handling must be adequate to dissipate
(V
I
− V
O
) × I
L
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.