AD8682/AD8684

Rev. 0 | Page 11 of 16

PROGRAMMABLE STATE VARIABLE FILTER

The circuit shown in Figure 34 can be used to accurately program

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

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

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

R7

2kΩ

Figure 34. Programmable State Variable Filter