a
AN-585
APPLICATION NOTE
E02749–0–5/03(0)
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AD5232–Programmable Oscillator Using Digital Potentiometers
By Alan Li
GANGED TOGETHER
CS
1nF
B
RP
100k⍀
A
W
AD5232
7
3
AD5232
+
ωo =
V+
AD711/AD
6
–
V–
1 5
V–
R2A
2.21k⍀
R2B
100k⍀
To sustain oscillation, we set the loop gain to unity
such that
D1
D2
D1N4148
R2
=2
R1
Figure 1. Programmable Wien-Bridge Oscillator with
Amplitude Stabilization
This digital potentiometer is so versatile that it can be
used in various filter and waveform generation applications. In such applications, users should be aware that its
bandwidth is a function of programmable resistance at a
given setting. As a result, the bandwidth performance of
the digital potentiometer is usually published in the data
sheets. If the application does not violate the bandwidth
limitation, the digital potentiometer can be applied in a
programmable oscillator where the programmable resistance can be used to set the programmable frequency.
Figure 1 shows one implementation where a popular
Wien-Bridge oscillator with amplitude stabilization is furnished by D1 and D2. If we set CP = CS = C, RP = RS = R, and
R2 = R2A || (R2B + RDIODE), it can be shown that the loop
gain equation simplifies to
1 + R 2 / R1
L(s ) =
3 + sCR + 1 / sCR
(3)
is the end-to-end resistance.
D1N4148
R1
10k⍀
1
CR
R=
N1
4
(5)
By using 1 nF for C, 100 kΩ for R (end-to-end resistance of
the digital potentiometer), and with D set to 0, 128 , and
196, oscillations occur, respectively, at 1.59 kHz, 3.18 kHz,
and 6.36 kHz with amplitude of ±1 V (See Figure 2).
8
7
D = 0, VO + 6V
6
5
4
D = 128, VO + 3V
3
2
D = 196, VO + 0V
1
0
–1
–2
0
0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.010
TIME – Sec
(1)
Figure 2. For D = 0, 128, and 196, Oscillations Occur at
1.59 kHz, 3.18 kHz, and 6.36 kHz, Respectively
© 2003 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective companies.
REV. 0
(2)
256 – D
RAB
(4)
256
where D is the decimal equivalent of the digital code of
the AD5232 dual 256-step digital potentiometer and RAB
VO
N2
2
1 + R2 / R1
3 + j (ωCR + 1 / ωCR )
W
V+
U1
VO – C
CP
1nF
L( jω ) =
RS
A 100k⍀ B