Analog Devices AN580 Application Notes

AN-580

a

APPLICATION NOTE

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Programmable Oscillator Uses Digital Potentiometers

by Alan Li, Analog Devices, San Jose, CA

Digital potentiometers are versatile, and can be used in
many filtering and waveform generation applications.
This design describes an oscillator where setting the
resistance of the digital potentiometers programs the
oscillation frequency and amplitude independently.
Figure 1 shows a typical diode-stabilized Wien-bridge
oscillator that can be used to generate accurate sine
waves from 10 k to 200 kHz.

ADJUST TO SAME SETTINGS
FOR FREQUENCY TUNING

V

O

R = R' = 1/2 AD5232 10k⍀
R2b = AD5231 100k⍀
D1 = D2 = 1N4148
U1 = AD8510, OP1177

2.2nF

VP

B

C

W

R

A

10k⍀

VN

R1

1k⍀

10k⍀

A

2.2nF

100k⍀

W

+2.5V

V+

AD8510

V–

–2.5V

R2a

2.1k⍀

R2b

AB

W

AMPLITUDE CONTROL

B

D1

D2

R'

C'

Figure 1. Programmable Wien-Bridge Oscillator with
Amplitude Stabilization

In this classic oscillator circuit, the Wien network (R, R',
C, C') provides positive feedback, while R1 and R2 pro­vide negative feedback with R2 = R2a储(R2b + R

DIODE

). To
establish a sustainable oscillation, the phase shift of the
loop gain should be zero and the magnitude of the loop
gain should be unity. In this circuit, the loop gain,
A(j)(j), can be found by multiplying the amplifier
gain with the transfer function V

. With R = R' and C = C',

P/VO

the loop gain is:

+

121

As s

β

()()

=

RR/

sRC

++

3

1

sRC

(1)

Substituting s = j and rearranging the real and imagi-
nary terms give:

R

+

12

R

Aj j

ωβ ω

()()

=

+

3

1



jRC

–

ω






1




RC

ω

(2)

Since the phase angle of the loop gain is defined as:

phase angle arctan

=



Im

Aj j

ωβ ω

()()




Re

Aj j

ωβ ω

()()









(3)

We force the imaginary term to zero to set the phase shift
to zero. As a result, the oscillation frequency becomes:

ω

1

or

f

=

O

RC

1

=

O

RC

2π

(4)

where R is the programmable resistance as:

D

256

R

–

256

R

AB

(5)

=

and D is the decimal equivalent of the digital code
programmed in the AD5232. R

is the end-to-end resis-

AB

tance of AD5232.

To sustain oscillation, the bridge must be in balance. If
the positive feedback is too large, oscillation amplitude
will increase until the amplifier saturates. If the negative
feedback is too large, the oscillation amplitude will be
damped out. According to equation (2), the attenuation
of the loop gain is 3 at resonance. Thus setting:

RR2

= 2

1

(6)

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
ensures R2/R1 to be smaller than 2 momentarily and
therefore stabilizes the oscillation.

Once the oscillation frequency is determined, the ampli­tude can be tuned independently by R2b since:

2

ODb D

3

2VIR V

=+

(7)

REV. 0

© Analog Devices, Inc., 2002

AN-580

T

R2b = 20k⍀

T

T

R2

R1

1.00V 20.0␮s

M 40.0␮s A CH1 200mV

Tek P re Vu Tri g’d

R = 8.06k⍀
f = 8.8kHz

R = 4.05k⍀
f = 17.6kHz

R = 670⍀
f = 102kHz

R3

REF2

–80.0000ns

R2b can simply be shorted which gives oscillation ampli­tude of approximately ±0.6 V. On the other hand, V
and V

are interdependent variables. With proper selec-

D

O

, ID,

tion of R2b, equilibrium can be reached such that V
converges. However, R2b should not be too large to
saturate the output. In this circuit, we applied a sepa­rate 100 kΩ digital potentiometer as R2b. By adjusting
the resistance setting from the minimum scale to 35 kΩ,
we were able to adjust the oscillation amplitude from
±0.6 V to ±2.3 V.

Finally, using 2.2 nF for C and C', 10 kΩ dual digital
potentiometer with R and R' set to 8 kΩ, 4 kΩ, and 670 Ω,
oscillation can be tuned to 8.8 kHz, 17.6 kHz, and 102 kHz
respectively with ±3% error (Figure 2). Higher frequency
is achievable with an increase in error. At 200 kHz, the
error becomes 6%. Although it deviates from the speci­fication, AD8510 was found to be working at ±2.5 V in
this circuit.

Two notes of caution: In frequency-dependent applica­tions, the bandwidth of the digital potentiometer is a
function of the programmed resistance. Therefore, care
must be taken not to violate the bandwidth limitations. In
addition, the frequency tuning in Figure 1 requires that R
and R' be adjusted to the same setting. Since the two
channels can be adjusted one at a time, an intermediate
state will occur that may not be acceptable for certain
applications. If this becomes an issue, separate devices

can be used in daisy-chain mode so that parts can be
programmed to the same setting simultaneously.

O

E02802–0–2/02(0)

Figure 2. Programmable Frequency

REFERENCES

1. Sergio Franco,

and Analog Integrated Circuits

Design with Operational Amplifiers

, Second Edition,

McGraw- Hill, 1998.

2. Sedra/Smith,

Microelectronic Circuits

, Fourth Edition,

Oxford, 1998.

3. Walter G. Jung,

IC Op-Amp Cookbook

, Third Edition,

Prentice Hall, 1997.

–2–

PRINTED IN U.S.A.

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