Datasheet
AD5233
Rev. B | Page 24 of 32
BIPOLAR PROGRAMMABLE GAIN AMPLIFIER
There are several ways to achieve bipolar gain. Figure 50 shows
one versatile implementation. Digital potentiometer, U1, sets
the adjustment range; therefore, the wiper voltage, V
W2
, can be
programmed between V
i
and −KV
i
at a given U2 setting.
V+
V–
OP2177
V
O
V+
V–
OP2177
AD5233
AD5233
V
i
A1
W1
B1
–KV
i
A2
B2
W2
V
DD
V
SS
R1
R2
V
DD
V
SS
A1
U2
A2
U1
C
02794-051
Figure 50. Bipolar Programmable Gain Amplifier
Configuring A2 as a noninverting amplifier yields a linear
transfer function:
⎟
⎠
⎞
⎜
⎝
⎛
−+××
⎟
⎠
⎞
⎜
⎝
⎛
+= KK
D2
R1
R2
V
V
i
O
)1(
64
1 (4)
where:
K is the ratio of R
WB
/R
WA
that is set by U1.
D is the decimal equivalent of the input code.
In the simpler (and much more usual) case where K is 1, a
pair of matched resistors can replace U1. Equation 4 can be
simplified to
⎟
⎠
⎞
⎜
⎝
⎛
−×
⎟
⎠
⎞
⎜
⎝
⎛
+= 1
64
2
1
2
D
R1
R2
V
V
i
O
(5)
Table 19 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 64-step resolution.
Table 19. Result of Bipolar Gain Amplifier
D R1 = ∞, R2 = 0 R1 = R2 R2 = 9 × R1
0 −1 −2 −10
16 −0.5 −1 −5
32 0 0 0
48 0.5 1 5
63 0.968 1.937 9.680
PROGRAMMABLE LOW-PASS FILTER
The AD5233 digital potentiometer can be used to construct a
second-order Sallen-Key low-pass filter, as shown in Figure 51.
A
B
V
i
AD8601
+2.5V
V
O
GANGED
TOGETHER
–2.5V
V+
V–
W
R
R2R1
A
B
W
R
C1
C2
U1
0
2794-052
Figure 51. Sallen-Key Low-Pass Filter
The design equations are
2
2
2
O
O
O
i
O
S
Q
S
V
V
ω+
ω
+
ω
=
(6)
C2C1R2R1
O
×××
=ω
1
(7)
C2R2
1
C1R1
Q
×
+
×
=
1
(8)
where:
Q is the Q factor.
V
O
is the resonant frequency.
R1 and R2 are R
WB1
and R
WB2
, respectively.
To achieve maximal flat bandwidth where Q is 0.707, let C1
be twice the size of C2 and let R1 equal R2. Users can first
select convenient values for the capacitors and then gang and
move R1 and R2 together to adjust the −3 dB corner frequency.
Instruction 5, Instruction 7, Instruction 13, and Instruction 15
of the AD5233 make these changes simple to implement.