Datasheet
AD5280/AD5282
Rev. C | Page 23 of 28
PROGRAMMABLE LOW-PASS FILTER
At resonance, setting the following balances the bridge:
In analog-to-digital 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 63). The
design equations are
2
2
OO
V ω
=
2
O
O
i
S
Q
S
V
ω+
ω
+
(11)
R1R2C1C2
O
1
=ω (12)
R2C2R1C1
Q
11
+= (13)
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, R1 and R2 can be
adjusted to the same settings to achieve the desirable bandwidth.
V
I
U
1
V
O
R1
A
R
W
B
R2
A
R
W
B
C2 C
C1
C
+2.5V
V+
V–
–2.5V
AD8601
ADJUSTED TO
SAME SETTING
02929-063
Figure 63. Sallen Key Low-Pass Filter
PROGRAMMABLE OSCILLATOR
In a classic Wien-bridge oscillator (Figure 64), 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 0, and the positive feedback causes the
circuit to oscillate. With R = R', C = C', and R2 = R2
A
//(R2
B
+
R
diode
), the oscillation frequency is
RC
for
RC
oO
π
ω
11
==
2
(14)
where
R is equal to R
WA
such that
AB
R
D
R
256
256
=
−
(15)
2=
R1
R2
(16)
In practice, R2/R1 should be set slightly larger than 2 to ensure
that oscillation can start. On the other hand, the alternate turn-
on of Diode D1 and Diode D2 ensures that R2/R1 are smaller
than 2 momentarily and, therefore, stabilizes the oscillation.
Once the frequency is set, the oscillation amplitude can be
tuned by R2
B
because
DBDO
VR2IV +=
3
2
(17)
V
O
, I
D
, and V
D
are interdependent variables. With proper
selection of R2
B
, an equilibrium is reached such that V
O
converges. R2
B
can be in series with a discrete resistor to
increase the amplitude, but the total resistance cannot be
too large to prevent saturation of the output.
FREQUENCY
ADJUSTMENT
C
2.2nF
C
I
2.2nF
R
I
10k
R
10k
BA
A
W
V+
V–
B
B
W
W
D1
D2
V
O
U
1
VP
A
VN
–2.5V
+2.5V
R1
1k
R2
B
10k
R2
A
2.1k
R1 = R1
I
= R2B = AD5282
D1 = D2 = 1N4148
AMPLITUDE
ADJUSTMENT
02929-064
OP1177
Figure 64. Programmable Oscillator with Amplitude Control