OP282/OP482

REV. B

–5–

The circuit shown in Figure 3 can be used to accurately

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

D

1

256









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

-

+

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.