High Performance Dual Switched Capacitor Filter# Technical Documentation: LMF100CCN Switched-Capacitor Filter
Manufacturer : National Semiconductor Corporation (NSC)
Component Type : Universal Dual Switched-Capacitor Filter
Package : 20-Pin Plastic DIP (LMF100CCN)
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## 1. Application Scenarios
### 1.1 Typical Use Cases
The LMF100CCN is a versatile monolithic dual switched-capacitor filter designed for precision analog signal processing applications. Its primary function is to provide tunable, stable filter responses without requiring external precision components.
Primary configurations include:
* State-Variable Filter Topology: Configured to provide simultaneous low-pass, high-pass, and band-pass outputs from a single input. This is its most common application, enabling complex filtering from a simple clock input.
* Biquadratic (Biquad) Filter Topology: Used to implement second-order filter sections with specific characteristics (e.g., Butterworth, Chebyshev, Bessel responses). Multiple LMF100CCN devices can be cascaded for higher-order filters.
* Programmable Clock-Tuned Filter: The center frequency (`f0`) or cutoff frequency (`fc`) is directly proportional to an external clock frequency (`fCLK`), typically with a 100:1 or 50:1 ratio. This allows for digital control of filter characteristics via a variable-frequency clock source (e.g., from a microcontroller or VCO).
### 1.2 Industry Applications
* Telecommunications: Tone detection and generation (DTMF), band-splitting in modems, and sideband filtering.
* Audio Processing: Graphic equalizers, parametric equalizers, crossover networks for bi-amping speakers, and noise reduction systems.
* Test & Measurement: Programmable anti-aliasing filters for data acquisition systems, swept-frequency analyzers, and signal conditioning.
* Biomedical Instrumentation: ECG/EEG signal conditioning for isolating specific frequency bands (e.g., removing power line noise or muscle artifact).
* Industrial Control: Vibration analysis, condition monitoring, and isolating signal components from noisy sensor data (e.g., thermocouples, strain gauges).
### 1.3 Practical Advantages and Limitations
Advantages:
* High Accuracy & Stability: Filter characteristics are determined by capacitor ratios (inherently accurate in IC fabrication) and a stable clock frequency, not by absolute values of external resistors or capacitors which have large tolerances and temperature coefficients.
* Easy Tunability: Filter frequency can be varied over a wide range (typically up to 30 kHz) by simply changing the external clock frequency, enabling electronic or digital programmability.
* Monolithic Integration: Contains two independent, high-performance filter sections and all necessary switching circuitry in a single package, reducing board space and component count.
* Low Sensitivity: Performance has low sensitivity to component variations, ensuring consistent manufacturing yields.
Limitations:
* Clock Feedthrough: The internal switching clock can introduce small amounts of noise at the clock frequency and its harmonics into the output signal. Careful design and post-filtering may be required for very dynamic range applications.
* Frequency Range: Optimal performance is in the audio and sub-audio range (up to ~30 kHz). Performance degrades at very high clock frequencies due to internal settling times.
* Power Supply Sensitivity: Requires clean, well-bypassed power supplies. Noise on the supplies can modulate the internal switches and degrade performance.
* Aliasing: Like any sampled-data system, it can suffer from aliasing if input signals contain frequency components above half the clock frequency (`fCLK/2`).
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## 2. Design Considerations
### 2.1 Common Design Pitfalls and Solutions
* Pitfall 1: Inadequate Clock
