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 (fC); 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

⎟

⎠

⎞

⎜

⎝

⎛

=

256

2π

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.

⎟

⎠

⎞

⎜

⎝

⎛

=

D2

R3

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

VIN

R7

2kΩ

DAC 3

DAC 1

DAC 4

DAC 2

Figure 35. Programmable State Variable Filter