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OP282 Datasheet(PDF) 5 Page - Analog Devices

Part No. OP282
Description  Dual/Quad Low Power, High Speed JFET Operational Amplifiers
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OP282 Datasheet(HTML) 5 Page - Analog Devices

 
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OP282/OP482
REV. B
–5–
The circuit shown in Figure 3 can be used to accurately
program the “Q,” the cutoff frequency fC, 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
=
1
2
πR
1C1
D
1
256


where D1 is the digital code for the DAC.
Gain of this circuit is set by adjusting D3. 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


R5
2k
1/4
OP482
-
+
LOW
PASS
R6
2k
R7
2k
BANDPASS
R1
2k
1/4
OP482
-
+
VIN
1/4
DAC8408
R4
2k
1/4
DAC8408
1/4
OP482
-
+
1/4
OP482
-
+
HIGH PASS
1/4
DAC8408
-
+
1/4
OP482
R1
2k
1/4
DAC8408
1/4
OP482
-
+
1/4
OP482
-
+
R2
1k
R3
2k
1/4
OP482
-
+
C1
1000pF
C1
1000pF
Figure 3.


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