Shock Detector Circuit Using Switched Capacitor Filters

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Solution Overview

Problem

Traditional shock detectors for disk drives require numerous external capacitors, which increase pin count and cost, and struggle with DC offset amplification, leading to false outputs and inefficiencies.

Innovation Solution

The implementation of a shock detector using a first stage operational amplifier without external capacitors, followed by a second stage switched capacitor high-pass filter that eliminates DC offset and provides the necessary amplification, utilizing two parallel switched capacitor filters with different cut-off frequencies to prevent aliasing and achieve the required amplification of 600 times.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If external capacitors are used in traditional shock detector circuits, then DC offset suppression and amplifier stability are achieved, but the number of external components and pin count increases

Engineering Contradiction:

Improveamplifier stabilityVSAvoidnumber of external components

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The patent merges the function of external capacitors into an internal capacitor (capacitor 65) within the integrated circuit. This single internal capacitor performs the DC offset suppression function that previously required multiple external capacitors, thereby reducing pin count and external component requirements while maintaining amplifier stability.

Inventive Principle:

Principle #5Merging (Combining)

Solution Approach 2:

The patent introduces a switched-capacitor filter circuit as an intermediary between the first amplifier stage and the second amplifier stage. This switched-capacitor filter (using capacitors 52, 54, 56, 58 and switches 60, 62) acts as a mediator that suppresses DC offset and provides signal conditioning, eliminating the need for external capacitors in the traditional configuration.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Productivity

If DC offset is amplified along with the signal, then amplifier gain is maximized, but false outputs and saturation occur

Engineering Contradiction:

Improveamplifier gainVSAvoidoutput accuracy

Core Design Contradiction:

ProductivityVSReliability

Solution Approach 1:

The patent segments the amplification process into two distinct stages with different functions. The first amplifier stage (op-amp 41) provides high gain (40x) for signal amplification, while the second amplifier stage (op-amp 42) with switched-capacitor filter provides additional gain (15x) and DC offset suppression. This segmentation allows each stage to be optimized for its specific function, preventing DC offset saturation.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent converts the harmful DC offset into a beneficial filtering opportunity by using the switched-capacitor filter. The filter's high-pass characteristic naturally attenuates DC and low-frequency offset components while preserving the desired shock detection signals, effectively turning the DC offset problem into a signal conditioning advantage.

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

3Device complexity

If a single switched capacitor filter is used, then circuit simplicity is maintained, but aliasing problems occur at certain frequencies

Engineering Contradiction:

Improvefilter circuit structureVSAvoidfrequency response accuracy

Core Design Contradiction:

Device complexityVSReliability

Solution Approach 1:

The patent implements dynamic frequency coverage by using two switched-capacitor filters with different cut-off frequencies (first filter with fHP1, second filter with fHP2). The system dynamically selects or combines the outputs of these filters based on the input signal frequency, ensuring accurate shock detection across the entire frequency range from 60 Hz to 15 kHz without aliasing.

Inventive Principle:

Principle #15Dynamics

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

This solution reduces the number of external components, eliminates DC offset amplification issues, and effectively detects shock signals between 60 Hz and 15 KHz without saturating the amplifiers, ensuring accurate output detection.

Implementation Method 1

A piezo transducer 10 to provide an electrical charge proportional to acceleration

Methodology Applied

Scientific EffectPiezoelectric effect: Piezoelectric Effect

Implementation Method 2

a second stage switched capacitor high pass filter that eliminates DC offset

Methodology Applied

Scientific EffectHigh-pass filtering: Filter (electronic)

Data Source

PatentUS8482338B1Shock detector with DC offset suppression using internal components

Publication Date: 2013.07.09 MARVELL ASIA PTE LTD

AI summary

A shock detector, such as for disk drives, which eliminates discrete external capacitors used in prior art devices. A first stage operational amplifier (without external capacitors) provides part of the gain required. This is followed by a second stage switched capacitor high pass filter (without external capacitors) that provides the remaining gain required while filtering out the DC offset of the first stage operational amplifier. In order to cover the range of frequencies expected without aliasing problems, two switched capacitor high pass filters in parallel are used, each designed with a different cut-off frequency.