LMF60 High Performance 6th-Order Switched Capacitor Butterworth Lowpass Filter# Technical Documentation: LMF60CIWM100 - 100 MHz Low-Pass Filter
Manufacturer : NS (National Semiconductor, now part of Texas Instruments)
Component Type : Integrated Continuous-Time, Low-Pass Filter
Document Revision : 1.0
---
## 1. Application Scenarios
### Typical Use Cases
The LMF60CIWM100 is a monolithic, 8th-order, elliptic low-pass filter IC featuring a fixed cutoff frequency of 100 MHz. Its primary function is to provide precise signal conditioning in high-frequency analog signal chains.
* Anti-Aliasing in High-Speed Data Acquisition: Placed before an analog-to-digital converter (ADC) sampling at or above 200 MSPS, it effectively attenuates out-of-band noise and high-frequency signals that would alias into the desired Nyquist band, ensuring ADC accuracy.
* Reconstruction Filtering in DAC Outputs: Following a digital-to-analog converter (DAC), it smooths the staircase output waveform by removing high-frequency spectral images centered at multiples of the DAC update rate, producing a clean, continuous analog signal.
* Channel Selection and Bandwidth Limiting: In communication receivers or wideband instrumentation, it selects a specific channel or limits system bandwidth to reduce noise and interference from adjacent channels or out-of-band signals.
* Signal Integrity Enhancement: Removes high-frequency noise, clock feedthrough, and harmonic distortion from amplifier or mixer outputs, improving the signal-to-noise ratio (SNR) and spurious-free dynamic range (SFDR) of the system.
### Industry Applications
* Test & Measurement Equipment: Used in oscilloscopes, spectrum analyzers, and arbitrary waveform generators for input conditioning and output signal smoothing.
* Communications Infrastructure: Employed in software-defined radios (SDR), base transceiver stations (BTS), and microwave backhaul equipment for intermediate frequency (IF) filtering.
* Medical Imaging: Applied in ultrasound systems and digital X-ray detectors to filter high-frequency switching noise from digital circuits, enhancing image clarity.
* High-Speed Data Links: Used in serializer/deserializer (SerDes) channels and video distribution systems (e.g., HD-SDI) for signal conditioning and EMI reduction.
### Practical Advantages and Limitations
Advantages:
* Precision & Repeatability: As a monolithic IC, it offers excellent accuracy and temperature stability for its 100 MHz cutoff, superior to discrete RC-active implementations.
* Simplified Design: Eliminates the need for designing, tuning, and calibrating a complex 8th-order elliptic filter with discrete components, saving board space and engineering time.
* Fast Time-to-Market: Provides a guaranteed filter response, reducing prototyping cycles.
* Integrated Solution: Includes internal buffer amplifiers, requiring minimal external components (typically just bypass capacitors and a resistor for frequency tuning).
Limitations:
* Fixed Frequency: The cutoff frequency is factory-set to 100 MHz. While tunable ±15% via an external resistor (`R_Q`), it cannot be arbitrarily programmed like a switched-capacitor or digital filter.
* Fixed Response: The elliptic filter response (ripple in passband/stopband) is fixed. Designers cannot alter the filter type (e.g., to Butterworth or Bessel).
* Dynamic Range Constraints: Like all active filters, its performance is bounded by the IC's supply voltage, noise floor, and linearity, which may not suit ultra-high dynamic range applications compared to some passive solutions.
---
## 2. Design Considerations
### Common Design Pitfalls and Solutions
1. Pitfall: Improper Power Supply Bypassing. Leads to poor stopband rejection, peaking in the frequency response, or even oscillation.
* Solution: Use a low-inductance layout. Place 0
