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5962-9063301MCA

Quad Precision, Low Cost, High Speed, BiFET Op Amp

Analog Devices 

AD713

Figure 18. A Programmable State Variable Filter Circuit

FILTER APPLICATIONS

A Programmable State Variable Filter

For the state variable or universal filter configuration of Figure

18 to function properly, DACs A1 and B1 need to control the

gain and Q of the filter characteristic, while DACs A2 and B2

must accurately track for the simple expression of fC to be true.

This is readily accomplished using two AD7528 DACs and one

AD713 quad op amp. Capacitor C3 compensates for the effects

of op amp gain-bandwidth limitations.

This filter provides low pass, high pass and band pass outputs

and is ideally suited for applications where microprocessor

control of filter parameters is required. The programmable

range for component values shown is fC = 0 to 15 kHz and

Q = 0.3 to 4.5.

GIC and FDNR FILTER APPLICATIONS

The closely matched and uniform ac characteristics of the

AD713 make it ideal for use in GIC (gyrator) and FDNR (fre-

quency dependent negative resistor) filter applications. Figures

19 and 21 show the AD713 used in two typical active filters.

The first shows a single AD713 simulating two coupled inductors

configured as a one-third octave bandpass filter. A single section

of this filter meets ANSI class II specifications and handles a

7.07 V rms signal with <0.002% THD (20 Hz–20 kHz).

Figure 21 shows a 7-pole antialiasing filter for a 2 ϫ oversam-

pling (88.2 kHz) digital audio application. This filter has <0.05

dB pass band ripple and 19.8 ± 0.3 µs delay, dc-20 kHz and will

handle a 5 V rms signal (VS = ± 15 V) with no overload at any

internal nodes.

The filter of Figure 19 can be scaled for any center frequency by

using the formula:

fC

=

1.11

2πRC

where all resistors and capacitors scale equally. Resistors R3–R8

should not be greater than 2 kΩ in value, to prevent parasitic

oscillations caused by the amplifier’s input capacitance.

REV. C

Figure 19. A 1/3 Octave Filter Circuit

–11–

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