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AD5280/AD5282

Rev. C | Page 23 of

PROGRAMMABLE LOW-PASS FILTER

At resonance, setting the following balances the bridge:

In analog-to-digital conversion applications, it is common to

include an antialiasing filter to band-limit the sampling signal.

Dual-channel digital potentiometers can be used to construct

a second-order Sallen key low-pass filter (see Figure 63). The

design equations are

(11)

R1R2C1C2

(12)

R2C2

R1C1

(13)

Users can first select some convenient values for the capacitors.

To achieve maximally flat bandwidth where Q = 0.707, let C1 be

twice the size of C2 and let R1 = R2. As a result, R1 and R2 can be

adjusted to the same settings to achieve the desirable bandwidth.

+2.5V

V+

V–

–2.5V

AD8601

ADJUSTED TO

SAME SETTING

Figure 63. Sallen Key Low-Pass Filter

PROGRAMMABLE OSCILLATOR

In a classic Wien-bridge oscillator (Figure 64), 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 0, and the positive feedback causes the

circuit to oscillate. With R = R', C = C', and R2 = R2A//(R2B +

Rdiode), the oscillation frequency is

(14)

where R is equal to RWA such that

256

−

(15)

(16)

In practice, R2/R1 should be set slightly larger than 2 to ensure

that oscillation can start. On the other hand, the alternate turn-

on of Diode D1 and Diode D2 ensures that R2/R1 are smaller

than 2 momentarily and, therefore, stabilizes the oscillation.

Once the frequency is set, the oscillation amplitude can be

tuned by R2B because

(17)

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 to prevent saturation of the output.

FREQUENCY

ADJUSTMENT

2.2nF

10kΩ

V+

V–

–2.5V

+2.5V

1kΩ

R2B

10kΩ

R2A

2.1kΩ

R1 = R1I = R2B = AD5282

D1 = D2 = 1N4148

AMPLITUDE

ADJUSTMENT

OP1177

Figure 64. Programmable Oscillator with Amplitude Control

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