Rainbow Electronics MAX7491 - Manual (Page 16)

Wideband Noise
The wideband noise of the filter is the total RMS value
of the device’s noise spectral density and is used to
determine the operating SNR. Most of its frequency
contents lie within the filter’s passband and cannot be
reduced with postfiltering. The total noise depends
mainly on the Q of each filter section and the cascade
sequence. Therefore, in multistage filters, the section
with the highest Q should be placed first for lower out-
put noise.
Multiple Filter Stages
In some designs, such as very narrow band filters, or in
modes where f
O cannot be tuned with resistors, several
2nd-order sections with identical f O may be cascaded
without multiple feedback. The total Q of the resultant
filter (Q
T ) is:
Total Q
T
= Q / (2 N - 1) 1/2
Q is the Q of each individual filter section, and N is the
number of 2nd-order sections. In Table 5, the total Q
and total bandwidth (BW) are listed for up to five identi-
cal 2nd-order sections. B is the bandwidth of each sec-
tion.
Chip Information
TRANSISTOR COUNT: 1439
TECHNOLOGY: BiCMOS
MAX7490/MAX7491
Dual Universal Switched-Capacitor Filters
16 ______________________________________________________________________________________
TOTAL SECTIONS TOTAL BW TOTAL Q
1 1.000 B 1.00 Q
2 0.644 B 1.55 Q
3 0.510 B 1.96 Q
4 0.435 B 2.30 Q
5 0.386 B 2.60 Q
Table 5. Cascading Identical Bandpass
Filter Sections