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Keywords: Switched-Capacitor IC Forms Notch Filter
APPLICATION NOTE 431
Switched-Capacitor IC Forms Notch Filter
Nov 01, 2000
Switched-capacitor filters (SCFs) are renowned for ease of use. They are accurate, require no external
components, and maintain a predictable response over all specified operating conditions. For integrated-
circuit SCFs, their tightly matched and trimmed internal capacitors produce a fixed frequency and phase
response that is proportional solely to the external clock frequency.
To construct a notch (band-reject) or passband response, the more flexible universal filter is needed.
Universal filters (either SCF or continuous-time types) allow any combination of low-pass, high-pass,
bandpass, or band-reject response, but they cost $3 to $10 each (1k pcs) and come in 14- to 16-pin
DIPs or large surface-mount packages. They usually require a dual-voltage power supply.
The low-pass SC filter of Figure 1, on the other hand, has features that enable a notch-filter
configuration. This fifth-order Butterworth low-pass filter operates on a single supply, has rail-to-rail I/O,
fits in a tiny 8-pin µMAX® package half the size of an 8-pin SO, and costs less than $1.50. Its internal
architecture (Figure 2) has summing nodes similar to those used for feedback-error generation in analog
signal-processing stages. When operating, it low-pass-filters the quantity (V
IN
- V
COM
) and adds V
OS
at
the output:
V
OUT
= (V
IN
-V
COM
)
LPF
+ V
OS
,
where (typically) V
COM
= V
DD
/2.
Figure 1. These simple connections produce a band-reject (notch) filter response.
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