EVAL-AD5235EBZ Analog Devices Inc, EVAL-AD5235EBZ Datasheet - Page 27

• Manufacturer Part Number: EVAL-AD5235EBZ
• Description: BOARD EVALUATION FOR AD5235
• Manufacturer: Analog Devices Inc

Specifications of EVAL-AD5235EBZ

• Main Purpose: Digital Potentiometer
• Utilized Ic / Part: AD5235
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Secondary Attributes: -
• Embedded: -
• Primary Attributes: -

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.
The design equations are
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).
Q =
ω
V
V
V
O
O
I
Figure 57. Programmable Oscillator with Amplitude Control
I
=
=
R1
S
2.2nF
1
R = R' = AD5235
R2B = AD5231
D1 = D2 = 1N4148
2
R1
CONCURRENTLY
C1
A
+
R
C
R1
ADJUSTED
R2
ω
+
Q
ADJUSTMENT
W
Figure 56. Sallen-Key Low-Pass Filter
ω
FREQUENCY
B
f
1
R2
C1
f
S
25kΩ
2
1
+
C2
C2
A
R
B
A
R
ω
R2
f
2
W
C2
B
W
VP
B
R1
1kΩ
2.2nF
10kΩ
W
R2B
+
–
OP1177
–2.5V
C1
C'
+2.5V
V+
V–
ADJUSTMENT
A
AMPLITUDE
25kΩ
U1
W
A
AD8601
R'
+2.5V
–2.5V
2.1kΩ
R2A
B
V+
V–
D2
D1
U1
V
O
V
O
(10)
(11)
(12)
Rev. D | Page 27 of 32
At the resonant frequency, f
the positive feedback causes the circuit to oscillate. With R = R
C = C
where R is equal to R
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
V
selection of R2B, an equilibrium is reached such that V
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.
O
, I
D
ω
R
2
3
',
, and V
WA
O
V
and R2 = R2A /(R2B + R
O
=
(
=
D
RC
1
)
I
D
D
=
R2B
are interdependent variables. With proper
or
1024
1024
f
+
O
WA
−
V
=
D
D
such that :
2
π
×
1
RC
R
O
, the overall phase shift is zero, and
AB
DIODE
+
R
W
), the oscillation frequency is
AD5235
O
(13)
(14)
(15)
'
,