Datasheet
AD5263
Rev. 0 | Page 23 of 28
PROGRAMMABLE LOW-PASS FILTER PROGRAMMABLE OSCILLATOR
In A/D conversion applications, it is common to include an
antialiasing filter to band-limit the sampling signal. Dual-
channel digital potentiometers can be used to construct a
second order Sallen-Key low-pass filter (see Figure 64). The
design equations are
In a classic Wien-bridge oscillator (Figure 65), the Wien
network (R, R
′, C, C′) provides positive feedback, while R1 and
R2 provide negative feedback. At the resonant frequency,
f
O
, the
overall phase shift is zero, and the positive feedback causes the
circuit to oscillate.
2
2
2
O
O
O
I
O
S
Q
S
V
V
ω
ω
ω
++
=
(11)
With R = R
′, C = C′, and R2 = R2A||(R2B + R
DIODE
), the
oscillation frequency is
C2C1R2R1
O
×××
=
1
ω
(12)
R
C
O
1
=
ω
, or
R
C
f
O
π
2
1
=
(14)
where
R
is equal to R
WA
, such that
C
2
R
2
C
1
R
1
Q
×
+
×
=
11
(13)
AB
R
D
R
256
−
=
256
(15)
Users can first select some convenient values for the capacitors.
To achieve maximally flat bandwidth where Q = 0.707, let C1 be
twice the size of C2, and let R1 = R2. As a result, the user can
adjust R1 and R2 to the same settings to achieve the desired
bandwidth.
At resonance, setting
2=
R
1
R2
(16)
balances the bridge. In practice,
R2
/
R1
should be set slightly
greater than 2 to ensure that the oscillation can start. On the
other hand, the alternating turn-on of the diodes D1 and D2
ensures that
R2
/
R1
is momentarily less than 2, thereby
stabilizing the oscillation.
03142-0-064
AD8601
V
O
–2.5V
R1
C1
W
U1
V+
V–
AB
W
AB
R2
C
C2
C
RR
+2.5V
V
I
ADJUSTED TO
SAME SETTING
Once the frequency is set, the oscillation amplitude can be
tuned by
R2B
, since
DD
O
VR2BIV
+×=
3
2
(17)
Figure 64. Sallen-Key Low-Pass Filter
V
O
,
I
D
, and
V
D
are interdependent variables. With proper
selection of
R2B
, an equilibrium will be reached such that
V
O
converges.
R2B
can be in series with a discrete resistor to
increase the amplitude, but the total resistance should not be so
large that it saturates the output.
03142-0-065
+2.5V
10kΩ
R2A
U1
–2.5V
D1
VN
OP1177
1kΩ
R1
W
V+
V–
D2
10kΩ
2.1kΩ
R2B
A
B
W
V
O
R'
A
B
2.2nF
C
VP
10kΩ
W
R
A
B
2.2nF
R1 = R1' = R2B = AD5263
D1 = D2 = 1N4148
FREQUENCY
ADJUSTMENT
AMPLITUDE
ADJUSTMENT
Figure 65. Programmable Oscillator with Amplitude Control