Bipolar Programmable Gain Amplifier; 10-Bit Bipolar Dac; Programmable Voltage Source With Boosted Output; Edits To Applications - Analog Devices AD5235 Manual

BIPOLAR PROGRAMMABLE GAIN AMPLIFIER

For applications requiring bipolar gain, Figure 49 shows one
implementation. Digital potentiometer U1 sets the adjustment
range; the wiper voltage V
W2
between V and −KV at a given U2 setting. Configure A2 as a
i i
noninverting amplifier that yields a transfer function:
⎛ + ⎞ ⎛
V
R 2 D 2
= × ⎟
O
⎜ ⎜
1
⎝ ⎠ ⎝
V R 1 1024
I
where K is the ratio of R /R
WB1
AD5235
W
U2
A2
B2
A1 B1
Vi
W1
V
DD
V+
AD5235
OP2177
V–
U1
A
V
SS
Figure 49. Bipolar Programmable Gain Amplifier
In the simpler (and much more usual) case where K = 1, V
simplified to
⎛ + ⎞ ⎛
2 D
R 2
= 2 −
⎜
⎜ ⎟
V 1
O
⎝ ⎠
⎝
R 1 1024
Table 21 shows the result of adjusting D2, 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 1024-step
resolution.
Table 21. Result of Bipolar Gain Amplifier
D R1 = ∞, R2 = 0
0 −1
256 −0.5
512 0
768 0.5
1023 0.992

10-BIT BIPOLAR DAC

If the circuit in Figure 49 is changed with the input taken from a
precision reference, U1 is set to midscale, and A2 is configured
as a buffer, a 10-bit bipolar DAC can be realized (Figure 48).
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. On the other hand, this circuit offers a unique
can, therefore, be programmed
⎞
× + −
⎟
1 ( K ) K
⎠
set by U1.
WA1
V
DD
V+
OP2177
V–
R2 C
A2
V
SS
–kVi
R1
⎞
× ⎟
1 V
i
⎠
R1 = R2 R2 = 9 R1
−2 −10
−1 −5
0 0
1 5
1.984 9.92
nonvolatile memory feature that in some cases outweighs any
shortfall in precision.
Without consideration of the wiper resistance, the output of this
circuit is approximately
⎛
=
⎜
V
O ⎝
(4)
Vi
2 U3
6
V V
V
IN OUT
O
+2.5V
5
TRIM
GND
ADR421
U1 = MIDSCALE
PROGRAMMABLE VOLTAGE SOURCE WITH
BOOSTED OUTPUT
For applications that require high current adjustment, such as a
is
O
laser diode driver or tunable laser, a boosted voltage source can
be considered (see Figure 51).
(5)
V
IN
In this circuit, the inverting input of the op amp forces the V
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 . N
1 1
(V − V ) × I
i O
100 mA 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. B | Page 23 of 28
⎞
2 D
− × ⎟
2
1 V
REF
⎠
1024
U2
W2
B2 A2
A1
B1
+2.5V
–2.5V
REF
W1
U1
V+
AD8552
V–
A1
–2.5V
Figure 50. 10-Bit Bipolar DAC
AD5235 2N7002
U2
A
W
V+
B
AD8601
V–
Figure 51. Programmable Booster Voltage Source
power handling must be adequate to dissipate
power. This circuit can source a maximum of
L
AD5235
(6)
+2.5V
V+
AD8552
V
O
V–
A2
–2.5V
REF
U1 = U2 = AD5235
V
OUT
R
BIAS
SIGNAL C
C
I
L
LD
OUT