Dual Universal Switched-Capacitor Filters

10

______________________________________________________________________________________

Table 1. Filter Operating Modes

MODE

LP

HP

BP

N

LP-N*

HP-N*

COMMENTS

1

••

•

fCLK/fO ratio is the nominal value. Good for bandpass filters

with identical sections cascaded, higher order Butterworth filters,

high-Q bandpass, low-Q notches.

1B

••

•

Same as Mode 1 with fCLK/fO ratios greater than the nominal

value.

2

••

•

Combination of Mode 1 and Mode 3; fCLK/fO ratios always

less than the nominal value. Less sensitivity to resistor tolerances

than Mode 3.

2N

•

Extension of Mode 2 that allows higher frequencies. Highpass

and lowpass outputs are summed with external op amp and

two resistors. Good for lowpass elliptic filters.

3

••

•

Adjustable fO above and below the nominal frequency.

Commonly used for multiple-pole Chebyshev filters, all-pole

higher order bandpass, lowpass, and highpass filters.

3A

••

•

•

•

Extension of Mode 3 that needs an external op amp and

two additional resistors. Commonly used for lowpass or higher

elliptic or Cauer filters.

*

LP-N = lowpass notch, HP-N = highpass notch. Both require an external op amp. See Definition of Terms (Table 2).

Table 2. Definition of Terms

TERM

DEFINITION

fCLK

The clock frequency applied to the switched-capacitor filter.

fO

The center frequency of the 2nd-order complex pole pair, fO, is determined by measuring the peak response

frequency at the bandpass output.

fNOTCH

The frequency of minimum amplitude response at the notch output.

Q

Quality factor, or Q, is the ratio of fO to the -3dB bandwidth of the 2nd-order bandpass filter. Q also determines

the amount of amplitude peaking at the lowpass and highpass outputs, but is not measured at these outputs.

HOBP

The gain in V/V of the bandpass output at f = fO.

HOLP

The gain in V/V of the lowpass output at f

→0Hz.

HOHP

The gain in V/V of the highpass output at f

→fCLK/2.

HON1

The notch output gain as f

→0Hz.

HON2

The notch output gain at f = fCLK/2.

LP-N

A notch output with HON1 > HON2.

HP-N

A notch output with HON1 < HON2.