AD5280BRU200 查看數據表（PDF） - Analog Devices
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AD5280BRU200 Datasheet PDF : 20 Pages
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AD5280/AD5282
R2B in theory can be made as small as needed to achieve the
current needed within A2’s output current driving capability. In
this circuit, OP2177 can deliver ± 5 mA in either direction, and
the voltage compliance approaches 15 V. It can be shown that
the output impedance is:
ZO
=
R1′ × R2B (R1 + R2A )
R1 × R2′ – R1′(R2A + R2B )
(10)
This output impedance can be infinite if resistors R1' and R2'
match precisely with R1 and R2A + R2B, respectively. On the
other hand, it can be negative if the resistors are not matched.
As a result, C1 in the range of 1 pF to 10 pF, is needed to pre-
vent the oscillation.
Programmable Low-Pass Filter
In A/D conversion applications, it is common to include an
antialiasing filter to band-limit the sampling signal. Dual-chan-
nel digital potentiometers can be used to construct a second
order Sallen Key low-pass filter (see Figure 21). The design
equations are:
VO
Vi
=
S2
+
ωo2
ωo S
Q
+
ωo2
(11)
1
ωO = R1R2C1C2
(12)
1
1
Q=
+
(13)
R1C1 R2C2
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 desirable
bandwidth.
C1
C
+2.5V
R1
R2
AB AB
Vi
W
R
W
R
V+
AD8601
VO
V–
C2 C
U1
–2.5V
ADJUSTED TO
SAME SETTING
Figure 21. Sallen Key Low-Pass Filter
PROGRAMMABLE OSCILLATOR
In a classic Wien-bridge oscillator (Figure 22), the Wien net-
work (R, R', C, C') provides positive feedback, while R1 and R2
provide negative feedback. 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:
1
1
ωO = RC or fO = 2πRC
(14)
where R is equal RWA such that:
256 – D
R = 256 RAB
(15)
At resonance, setting:
R2
=2
(16)
R1
balances the bridge. In practice, R2/R1 should be set slightly
larger than 2 to ensure the oscillation can start. On the other
hand, the alternate turn-on of the diodes D1 and D2 ensures
that R2/R1 are smaller than 2 momentarily, and therefore stabi-
lizes the oscillation.
Once the frequency is set, the oscillation amplitude can be
tuned by R2B since:
2
3VO
=
ID R2B
+ VD
(17)
V0, ID, and VD are interdependent variables. With proper
selection of R2B, an equilibrium will be 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
R
VP 2.2nF 10k⍀
C
2.2nF
B
R
10k⍀ W
AB
+2.5V W
A
V+
U1
OP1177
VO
R1 = R1' = R2B = AD5282
D1 = D2 = 1N4148
VN
R1
1k⍀
V–
–2.5V
R2A
R2B 2.1k⍀ D1
10k⍀
BA
D2
W
AMPLITUDE
ADJUSTMENT
Figure 22. Programmable Oscillator with
Amplitude Control
REV. 0
–17–
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