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AD5280 데이터시트(PDF) 23 Page - Analog Devices

부품명 AD5280
상세설명  Single/Dual,15 V/5 V,256-Position I2C-Compatible Digital Potentiometer
PDF  28 Pages
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제조업체  AD [Analog Devices]
홈페이지  http://www.analog.com
Logo AD - Analog Devices

AD5280 데이터시트(HTML) 23 Page - Analog Devices

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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
O
O
V
ω
=
2
O
O
i
S
Q
S
V
ω
+
ω
+
(11)
R1R2C1C2
O
1
=
ω
(12)
R2C2
R1C1
Q
1
1
+
=
(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.
VI
U1
VO
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
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, fO,
the overall phase shift is 0, and the positive feedback causes the
circuit to oscillate. With R = R', C = C', and R2 = R2A//(R2B +
Rdiode), the oscillation frequency is
RC
f
or
RC
o
O
π
ω
1
1
=
=
2
(14)
where R is equal to RWA 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 R2B because
D
B
D
O
V
R2
I
V
+
=
3
2
(17)
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 prevent saturation of the output.
FREQUENCY
ADJUSTMENT
C
2.2nF
CI
2.2nF
RI
10kΩ
R
10kΩ
BA
A
W
V+
V–
B
B
W
W
D1
D2
VO
U1
VP
A
VN
–2.5V
+2.5V
R1
1kΩ
R2B
10kΩ
R2A
2.1kΩ
R1 = R1I = R2B = AD5282
D1 = D2 = 1N4148
AMPLITUDE
ADJUSTMENT
OP1177
Figure 64. Programmable Oscillator with Amplitude Control



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