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| 型號： | EVAL-AD5233SDZ |
| 廠商： | Analog Devices Inc |
| 文件頁數： | 19/32頁 |
| 文件大?。?/td> | 0K |
| 描述： | BOARD EVAL FOR AD5233 |
| 標準包裝： | 1 |
| 系列： | * |
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AD5233
Rev. B | Page 26 of 32
PROGRAMMABLE OSCILLATOR
In a classic Wien-bridge oscillator, shown in Figure 55, 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 zero, and the positive feedback
causes the circuit to oscillate. If the op amp is chosen with a
relatively high gain bandwidth product, the frequency response
of the op amp can be neglected.
+2.5V
OP1177
V+
V–
VO
–2.5V
R2A
2.1k
D1
D2
R2B
10k
VN
R1
1k
A
B
W
R = R' = R2B = 1/4 AD5233
D1 = D2 = 1N4148
C'
2.2nF
R'
10k
AB
W
VP
C
2.2nF
R
10k
A
B
W
U1
AMPLITUDE
ADJUSTMENT
FREQUENCY
ADJUSTMENT
02
79
4
-05
6
Figure 55. Programmable Oscillator with Amplitude Control
With R = R, C = C, and R2 = R2A||(R2B + RDIODE), the
oscillation frequency is
RC
O
1
=
ω
or
RC
fO
π
=
2
1
(13)
where R is equal to RWA such that
AB
R
D
R
64
64
=
(14)
At resonance, setting
2
=
R1
R2
(15)
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 R1/R2 is smaller than 2 momentarily and,
therefore, stabilizes the oscillation.
Once the frequency is set, the oscillation amplitude can be
turned on by R2B, because
D
O
V
R2B
I
V
+
=
3
2
(16)
where 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 or it saturates the output. In this configuration, R2B can be
adjusted from minimum to full scale with amplitude varied
from ±0.6 V to ±0.9 V. Using 2.2 nF for C and C, 10 kΩ dual
digital potentiometer, with R and R set to 8.06 kΩ, 4.05 kΩ,
and 670 Ω, oscillation occurs at 8.8 kHz, 17.6 kHz, and
102 kHz, respectively (see Figure 56).
1V/DIV
R = 8.06k
f = 8.8kHz
R = 4.05k
f = 17.6kHz
R = 670
f = 102kHz
1V/DIV
02794-
057
Figure 56. Programmable Oscillation
frequency tuning requires that both RDACs be adjusted 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 all instructions (5, 7, 13, and 15) can be used.
Different devices can also be used in daisy-chain mode so that
parts can be programmed to the same setting simultaneously.
PROGRAMMABLE VOLTAGE SOURCE WITH
BOOSTED OUTPUT
For applications that require high current adjustment, such as a
laser diode driver or tunable laser, a boosted voltage source can
be considered (see Figure 57).
AD5233
V+
V–
W
U2
AD8601
A
B
VIN
VOUT
RBIAS
IL
SIGNAL
CC
LD
2N7002
02
79
4
-05
8
Figure 57. Programmable Boosted Voltage Source
In this circuit, the inverting input of the op amp forces the VOUT
to be equal to the wiper voltage set by the digital potentiometer.
The load current is then delivered by the supply via the N-
channel FET N1. N1 power handling must be adequate to
dissipate (Vi VO) × IL power. This circuit can source a
maximum of 100 mA with a 5 V supply.
For precision applications, a voltage reference such as ADR421,
ADR03, or ADR370 can be applied at Terminal A of the digital
potentiometer.
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