AD713
Figure 18. A Programmable State Variable Filter Circuit
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).
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.
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.
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.
The filter of Figure 19 can be scaled for any center frequency by
using the formula:
1.11
2πRC
fC
=
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.
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
Figure 19. A 1/3 Octave Filter Circuit
–11–
REV. C
ADI [ ADI ]
