OP282/OP482

Rev. F | Page 13 of 16

PROGRAMMABLE STATE-VARIABLE FILTER

The circuit shown in Figure 46 can be used to accurately

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 used in the voltage mode; therefore, many

values are dependent on the accuracy of the DAC only and not

on the absolute values of the DAC’s resistive ladders. This makes

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

⎟

⎠

⎞

⎜

⎝

⎛

=

256

2π

1

D1

R1C1

fc

where D1 is the digital code for the DAC.

The gain of this circuit is set by adjusting D3. 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.

⎟

⎠

⎞

⎜

⎝

⎛

=

D2

R3

R2

Q

256

R5

2k

Ω

1/4

OP482

1/4

DAC8408

HIGH PASS

C1

1000pF

R4

2k

Ω

R6

2k

Ω

R7

2k

Ω

1/4

OP482

R1

2k

Ω

1/4

OP482

1/4

DAC8408

1/4

OP482

R1

2k

Ω

1/4

OP482

1/4

DAC8408

1/4

OP482

C1

1000pF

LOW

PASS

1/4

OP482

1/4

OP482

1/4

DAC8408

BAND PASS

R2

2k

Ω

R3

2k

Ω

Figure 46.