AD5263BRUZ200-R7 datasheet(26/28 Pages) AD

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AD5263

Data Sheet

Rev. F | Page 26 of 28

PROGRAMMABLE LOW-PASS FILTER

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 65). The design

equations are

(10)

(11)

(12)

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, the user can adjust

R1 and R2 to the same settings to achieve the desired bandwidth.

+2.5V

V+

V–

–2.5V

AD8601

ADJUSTED TO

SAME SETTING

Figure 65. Sallen-Key Low-Pass Filter

PROGRAMMABLE OSCILLATOR

In a classic Wien bridge oscillator (Figure 66), 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 zero, and the positive feedback causes the circuit to oscillate.

With R = R′, C = C′, and R2 = R2A||(R2B + RDIODE), the oscillation

frequency is

, or

(13)

where R is equal to RWA, such that

256

256 −

(14)

At resonance, setting

(15)

balances the bridge. In practice, R2/R1 should be set slightly

greater than 2 to ensure that the oscillation can start. On the

other hand, the alternating turn-on of the diodes D1 and D2

ensures that R2/R1 is momentarily less than 2, thereby

stabilizing the oscillation.

Once the frequency is set, the oscillation amplitude can be

tuned by R2B because

R2B

(16)

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 should not be so large that it saturates

the output.

FREQUENCY

ADJUSTMENT

2.2nF

R’

10kΩ

V+

V–

–2.5V

+2.5V

1kΩ

R2B

10kΩ

R2A

2.1kΩ

R1 = R1’ = R2B = AD5263

D1 = D2 = 1N4148

AMPLITUDE

ADJUSTMENT

OP1177

Figure 66. Programmable Oscillator with Amplitude Control

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