Rainbow Electronics MAX7491 - Manual (Page 10)

MAX7490/MAX7491
Dual Universal Switched-Capacitor Filters
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Table 1. Filter Operating Modes
MODE LP HP BP N L P- N * H P- N * COMMENTS
1 • • •
f CLK /f O 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 f CLK /f O ratios greater than the nominal
value.
2 • • •
Combination of Mode 1 and Mode 3; f CLK /f O 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 f O 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
f CLK The clock frequency applied to the switched-capacitor filter.
f
O
The center frequency of the 2nd-order complex pole pair, f O , is determined by measuring the peak response
frequency at the bandpass output.
f NOTCH The frequency of minimum amplitude response at the notch output.
Q
Quality factor, or Q, is the ratio of f O 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.
H OBP The gain in V/V of the bandpass output at f = f O .
H OLP The gain in V/V of the lowpass output at f →0Hz.
H OHP The gain in V/V of the highpass output at f →f
CLK /2.
H ON1 The notch output gain as f →0Hz.
H
ON2
The notch output gain at f = f
CLK
/2.
LP-N A notch output with H ON1 > H ON2.
HP-N A notch output with H ON1 < H ON2.