PROGRAMMABLE LOW-PASS FILTER
In analog-to-digital conversions (ADCs), it is common to
include an antialiasing filter to band limit the sampling signal.
Therefore, the dual-channel AD5235 can be used to construct a
second-order Sallen-Key low-pass filter, as shown in Figure 56.
C1
R1
R2
VI
AB AB
W
R
W
R
C2
+2.5V
V+
AD8601
VO
V–
U1
–2.5V
ADJUSTED
CONCURRENT LY
Figure 56. Sallen-Key Low-Pass Filter
The design equations are
VO
VI
=
S2
+
ωf 2
ωf
Q
S + ωf2
(10)
ωO =
1
R1 R2 C1 C2
(11)
Q= 1 + 1
(12)
R1 C1 R2 C2
First, users should select 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 equal R2. As a result, the user
can adjust R1 and R2 concurrently to the same setting to
achieve the desirable bandwidth.
PROGRAMMABLE OSCILLATOR
In a classic Wien bridge oscillator, the Wien network (R||C, R'C')
provides positive feedback, whereas R1 and R2 provide negative
feedback (see Figure 57).
FREQUENCY
ADJUSTMENT
R'
VP C' 25kΩ
C
2.2nF
B
R
25kΩ W
A
2.2nF A B
W
+2.5V
+ V+ U1
OP1177
VO
– V–
R2A
–2.5V 2.1kΩ
R = R' = AD5235
R2B = AD5231
D1 = D2 = 1N4148
R2B
D1
10kΩ
BA
D2
W
R1
1kΩ AMPLITUDE
ADJUSTMENT
Figure 57. Programmable Oscillator with Amplitude Control
AD5235
At the resonant frequency, fO, the overall phase shift is zero, and
the positive feedback causes the circuit to oscillate. With R = R',
C = C', and R2 = R2A /(R2B + RDIODE), the oscillation frequency is
ωO
=
1
RC
or
fO
=
1
2πRC
(13)
where R is equal to RWA such that :
RWA (D)
=
1024 − D
1024
× RAB
+
RW
(14)
At resonance, setting R2/R1 = 2 balances the bridge. In practice,
R2/R1 should be set slightly larger than 2 to ensure that the
oscillation can start. On the other hand, the alternate turn-on
of the diodes, D1 and D2, ensures that R2/R1 is smaller than 2,
momentarily stabilizing the oscillation.
When the frequency is set, the oscillation amplitude can be
turned by R2B because
2
3
VO
=
IDR2B
+ VD
(15)
VO, ID, and VD are interdependent variables. With proper
selection of R2B, an equilibrium is reached such that VO
converges. R2B can be in series with a discrete resistor to
increase the amplitude, but the total resistance cannot be too
large to saturate the output.
In Figure 56 and Figure 57, the frequency tuning requires that
both RDACs be adjusted concurrently to the same settings.
Because the two channels might be adjusted one at a time, an
intermediate state occurs that might not be acceptable for some
applications. Of course, the increment/decrement instructions
(Instruction 5, Instruction 7, Instruction 13, and Instruction 15)
can all be used. Different devices can also be used in daisy-chain
mode so that parts can be programmed to the same settings
simultaneously.
Rev. E | Page 27 of 32