a AN-580 APPLICATION NOTE One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106 • Tel: 781/329-4700 • Fax: 781/326-8703 • www.analog.com 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. VP C 2.2nF B W C' 2.2nF +2.5V A R 10k⍀ ADJUST TO SAME SETTINGS FOR FREQUENCY TUNING R' A 10k⍀ B W D1 R = R' = 1/2 AD5232 10k⍀ R2b = AD5231 100k⍀ D1 = D2 = 1N4148 U1 = AD8510, OP1177 R = D2 (3) 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 + RDIODE). 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 VP/VO. With R = R' and C = C', the loop gain is: 1+ R 2 / R1 1 3 + sRC + sRC 1 1 or f O = 2πRC RC (4) 256 – D R AB 256 (5) and D is the decimal equivalent of the digital code programmed in the AD5232. RAB is the end-to-end resistance of AD5232. AMPLITUDE CONTROL (1) 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: R2 =2 R1 (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 VO = I D R 2b + V D 3 REV. 0 (2) where R is the programmable resistance as: R2b B100k⍀ A A (s )β(s ) = Im A ( j ω )β( j ω ) phase angle = arctan Re A ( j ω )β( j ω ) ωO = R2a 2.1k⍀ W 1 3 + j ωRC – ωRC Since the phase angle of the loop gain is defined as: VO –2.5V R1 1k⍀ A ( j ω) β ( j ω) = 1+ R2 R1 We force the imaginary term to zero to set the phase shift to zero. As a result, the oscillation frequency becomes: V+ AD8510 V– VN Substituting s = j and rearranging the real and imaginary terms give: (7) © Analog Devices, Inc., 2002 AN-580 can be used in daisy-chain mode so that parts can be programmed to the same setting simultaneously. Tek PreVu Trig’d T R2b = 20k⍀ T R = 8.06k⍀ f = 8.8kHz R1 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. R2 R = 4.05k⍀ f = 17.6kHz R3 R = 670⍀ f = 102kHz M 40.0s REF2 1.00V 20.0s T A CH1 E02802–0–2/02(0) R2b can simply be shorted which gives oscillation amplitude of approximately ±0.6 V. On the other hand, VO, ID, and VD are interdependent variables. With proper selection of R2b, equilibrium can be reached such that VO 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. 200mV –80.0000ns Figure 2. Programmable Frequency 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 REFERENCES 1. Sergio Franco, Design with Operational Amplifiers and Analog Integrated Circuits, Second Edition, McGraw- Hill, 1998. 2. Sedra/Smith, Microelectronic Circuits, Fourth Edition, Oxford, 1998. PRINTED IN U.S.A. 3. Walter G. Jung, IC Op-Amp Cookbook, Third Edition, Prentice Hall, 1997. –2– REV. 0

- Similar pages