Comparing Frequency Response Characteristics Of Various Decoupling Capacitors

Purpose of Decoupling Capacitors

Decoupling capacitors are used in electronic circuits to suppress noise, filter voltage spikes, and improve stability in power supply lines. Proper selection of decoupling capacitors is critical to ensure the capacitors can effectively perform these functions over the necessary frequency ranges in an application.

This article compares key parameters that affect the frequency response of different types of decoupling capacitors to provide guidelines on selecting the best capacitors for noise filtering and power integrity in circuit designs.

Parameters that Affect Frequency Response

The main specifications of a decoupling capacitor that determine its frequency response characteristics are:

Table of Contents

Equivalent Series Inductance (ESL)
Equivalent Series Resistance (ESR)
Capacitance Value

These parameters influence the impedance profile of the capacitor over frequency. The impedance determines how effectively the capacitor can shunt noise at different frequencies. By understanding each parameter’s role and typical values for different capacitor types, the frequency response can be compared to select the best fit decoupling capacitors.

Equivalent Series Inductance (ESL)

All capacitors have some small amount of internal inductance, modeled as equivalent series inductance (ESL). At higher frequencies, the impedance of ESL begins to dominate and causes the capacitor’s impedance to rise, reducing its effectiveness at shunting noise. Minimizing a capacitor’s ESL is key to maintaining low impedance across higher frequency ranges.

Different capacitor materials and construction substantially impact ESL values:

Electrolytic capacitors – High ESL in the 10 nH range
Tantalum capacitors – Lower ESL around 100 pH range
MLCC ceramic capacitors – Very low ESL achieving few 10s of pH

Advanced MLCC materials like X7R and newer dielectric formulations continue pushing ESL values lower. However, equivalent series resistance tends to rise with ultra-low ESL capacitors.

Equivalent Series Resistance (ESR)

Equivalent series resistance (ESR) represents the resistive losses within a capacitor. Lower ESR improves frequency response by keeping the capacitor’s impedance low across wider bandwidths. However, ESR cannot be reduced indefinitely without impacting other parameters like ESL and capacitance.

Typical ESR values for popular decoupling capacitor varieties are:

Aluminum electrolytic – ESR from 100 mΩ range up to 100s mΩ
Tantalum – Lower ESR around 10s of mΩ
X7R MLCC Ceramic – Very low ESR down to 100 mΩ range
Hi-CV MLCC ceramic – Ultra-low ESR down below 10 mΩ

So while Hi-CV chip capacitors can achieve ESR on the order of single digit milliohms, their low-ESR frequently comes at the expense of higher vs. X7R dielectric MLCCs.

Capacitance Value

A decoupling capacitor’s capacitance value affects its frequency response in two key ways:

Impedance – Higher capacitance directly lowers impedance, improving shunting of noise frequencies below the capacitor’s resonant frequency.
Resonance Frequency – The capacitor’s impedance reaches a minimum at its self-resonant frequency, proportional to 1/(2π√(LC)) for the device’s inductance L and capacitance C values.

Electrolytic capacitors offer high capacitance values in the μF to 100s μF range but their poor ESL and ESR characteristics limit high frequency performance. MLCC ceramic chip capacitors offer a broad range of values with the lowest ESL and ESR combinations to push resonance frequencies to GHz levels for superior high frequency response.

Frequency Response Comparison

Using the above performance characteristics and typical parameters for different capacitor types, the frequency response of various common decoupling capacitors can be compared:

Aluminum Electrolytic Capacitors

High capacitance, up to 1000 μF range
ESR from 100 mΩ range up to 100s mΩ
High ESL around 10 nH range
Effective from 120 Hz down to kHz levels

Aluminum electrolytic capacitors offer the highest capacitance but have limitations for decoupling usage. Their high ESL and ESR lead to impedance peaks making them ineffective above a few MHz. Useful where large bulk capacitance is needed for lower frequency decoupling.

Tantalum Capacitors

Capacitance range from μF up into 10s μF
ESR in 10s mΩ range
ESL around 100 pH
Effective from 500 kHz up to ~100 MHz levels

Tantalum capacitors improve upon electrolytic with lower ESL and ESR properties. Their intermediate levels extend usefulness to 10s MHz but still reach limits at high frequencies. Can be selected over electrolytic where board space constraints exist.

X7R MLCC Ceramic Capacitors

Capacitance range from nF to μF range
ESR down to 100 mΩ range
ESL down to 10s pH
Effective from 10 MHz up to 1 GHz

Modern X7R dielectric chip capacitors offer a robust decoupling solution. MLCC construction significantly reduces ESL to less than 10% of tantalum levels. An ESR-ESL balance supports low impedance up to GHz frequencies. Widely usable chip capacitor technology for SMPS output filtering.

Hi-CV MLCC Ceramic Capacitors

Capacitance mid-range from nF to 100s nF
ESR below 10 mΩ
Low ESL down to 10 pH levels
Effective from 50 MHz to > 10 GHz

Hi-CV chip materials optimized for very low ESR to support stable impedance to 10 GHz levels. Best performance where ultra-wideband response up to mmWave frequencies is critical. Tradeoffs come in higher DC bias sensitivity and lower capacitance density than X7R materials.

Example Circuits and Simulations

Comparing example impedance profiles and circuit simulations demonstrates the capable frequency range for real-world applications.

Impedance Modeling

An impedance model contrasts response for common 600 nF decoupling values. The superior low ESL properties of X7R MLCC and Hi-CV capacitors become apparent by 30 MHz and are an order of magnitude lower than tantalum at GHz frequencies.

Decoupling Circuit Frequency Analysis

This test configuration models a typical voltage regulator with ~1 μF decoupling. Gain plots demonstrate the extended noise filtering range achieved stepping up in capacitor performance from electrolytic –> tantalum –> MLCC technology.

Guidelines for Selecting Decoupling Capacitors

With a better understanding of their capabilities, capacitors can be selected appropriately for target application frequencies and placement locations. General tips include:

Electrolytic – Near power inputs where bulk capacitance is key
Tantalum – Intermediate frequency filtering as size allows
X7R MLCC – General SMPS/digital IC decoupling for 100 MHz to GHz protection
Hi-CV MLCC – High density GHz+ decoupling demanding maximum HF bandwidth

Carefully evaluating capacitor parameters allows picking the right component combination to achieve noise suppression and signal integrity across all critical frequency ranges.