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, place the

section with the highest Q first for lower output noise.

Multiple Filter Stages

In some designs, such as very narrow band filters, or in

modes where fO cannot be tuned with resistors, several

2nd-order sections with identical fO may be cascaded

without multiple feedback. The total Q of the resultant

filter (QT) is:

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 identical

2nd-order sections. B is the bandwidth of each section.

Chip Information

PROCESS: BiCMOS

Total Q

Q

T

N

/

//

=−

()

21

11 2

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