OP282/OP482
The circuit shown in Figure 3 can be used to accurately
program the “Q,” the cutoff frequency f
C
, and the gain of a two
pole state-variable filter. OP482s have been used in this design
because of their high bandwidths, 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 all used in the voltage mode so all values
are dependent only on the accuracy of the DAC and not on the
absolute values of the DAC’s resistive ladders. This make this
circuit unusually accurate for a programmable filter.
Adjusting DAC 1 changes the signal amplitude across R1;
therefore, the DAC attenuation times R1 determines the
amount of signal current that charges the integrating capacitor,
C1. This cutoff frequency can now be expressed as:
fc
=
D
1
1
2
π
R
1
C
1
256
where D
1
is the digital code for the DAC.
Gain of this circuit is set by adjusting D
3
. The gain equation is:
Gain
=
R
4
R
5
D
3
256
DAC 2 is used to set the “Q” of the circuit. Adjusting this DAC
controls the amount of feedback from the bandpass 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.
Q
=
R
2
R
3
256
D
2
R7
2k
1/4
DAC8408
V
IN
R5
2k
R4
2k
1/4
DAC8408
R1
2k
C1
1000pF
-
+
1/4
DAC8408
R1
2k
C1
1000pF
1/4
OP482
-
+
1/4
OP482
-
+
1/4
OP482
-
+
1/4
OP482
-
+
1/4
OP482
-
+
LOW
1/4
PASS
HIGH PASS
OP482
R6
2k
R3
2k
R2
1k
1/4
DAC8408
BANDPASS
1/4
OP482
-
+
1/4
OP482
-
+
Figure 3.
REV. B
–5–
AD [ ANALOG DEVICES ]
