Analog Devices AN585 Application Notes

AN-585

a

APPLICATION NOTE

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AD5232–Programmable Oscillator Using Digital Potentiometers

By Alan Li

GANGED TOGETHER

R

S

100k⍀

AB

W

AD5232

6

N2

D1

D1N4148

D2

D1N4148

V

O

1nF

C

S

1nF

C

P

R

100k⍀

AD5232

B

W

P

A

R1

10k⍀

U1

3

+

AD711/AD

2

–

V+

7

V+

V–

4

V–

2.21k⍀

100k⍀

N1

15

R2A

R2B

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 C
R2 = R2

|| (R2B + R

A

), it can be shown that the loop

DIODE

= CS = C, RP = RS = R, and

P

gain equation simplifies to

RR

/

+

Ls

()

121

=

sCR sCR

++

31

/

(1)

Lj

()

ω

ω

=

o

CR

256

R

=

D

where
the AD5232 dual 256-step digital potentiometer and
is the end-to-end resistance.

To sustain oscillation, we set the loop gain to unity
such that

RR2

2=

1

By using 1 nF for
the digital potentiometer), and with
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

6

5

4

– C

3

O

V

2

1

0

–1

–2

Figure 2. For D = 0, 128, and 196, Oscillations Occur at

1.59 kHz, 3.18 kHz, and 6.36 kHz, Respectively

121

RR

/

=

1

256

+

31

jCR CR

ωω

++

()

D

–

R

AB

/

(2)

(3)

(4)

is the decimal equivalent of the digital code of

R

(5)

C

, 100 kΩ for R (end-to-end resistance of

D

set to 0, 128 , and

D = 0, V

+ 6V

O

D = 128, V

D = 196, VO + 0V

+ 3V

O

0.0010 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.010

TIME – Sec

AB

E02749–0–5/03(0)

REV. 0

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