Datasheet

AD8682/AD8684
Rev. B | Page 11 of 16
PROGRAMMABLE STATE VARIABLE FILTER
The circuit shown in Figure 35 can be used to accurately program
the Q factor; the cutoff frequency (f
C
); and the gain of a two-
pole state variable filter. The AD8684 has been used in this
design because of its high bandwidth, low power, and low noise.
This circuit takes only three packages to build because of the
quad configuration of the op amps and DACs.
The DACs shown are used in voltage mode; therefore, many
values are dependent on the accuracy of the DAC only and not
on the absolute values of the DAC resistive ladders. As a result, this
makes the circuit unusually accurate for a programmable filter.
Adjusting DAC 1 changes the signal amplitude across R1; therefore,
the DAC attenuation × R1 determines the amount of signal current
that charges the integrating capacitor, C1.
This cutoff frequency can be expressed as
⎟
⎠
⎞
⎜
⎝
⎛
=
2562π
1 D1
R1C1
f
C
where
D1 is the digital code for the DAC.
DAC 3 is used to set the gain. The gain equation is
⎟
⎠
⎞
⎜
⎝
⎛
=
256
D3
R5
R4
Gain
DAC 2 is used to set the Q of the circuit. Adjusting this DAC
controls the amount of feedback from the band-pass node to
the input summing node. Note that the digital value of the
DAC is in the numerator; therefore, zero code is not a valid
operating point.
⎟
⎠
⎞
⎜
⎝
⎛
=
D2R3
R2
Q
256
1/4
AD8684
1/4
DAC8408
R5
2kΩ
R4
2kΩ
1/4
AD8684
1/4
AD8684
1/4
DAC8408
R1
2kΩ
C1
1000pF
1/4
AD8684
1/4
AD8684
1/4
DAC8408
R1
2kΩ
C1
1000pF
1/4
AD8684
1/4
DAC8408
R2
2kΩ
R3
2kΩ
1/4
AD8684
1/4
AD8684
R6
2kΩ
HIGH PASS
BAND PASS
LOW
PASS
V
IN
R7
2kΩ
06278-035
DAC 3
DAC 1
DAC 4
DAC 2
Figure 35. Programmable State Variable Filter