MICHAEL CHIANGEECS 713

12/05 /2013

Presenting: Capacitors

Objective:

Lab exercise for CapacitorsUnderstand bypass/decoupling capacitor

selection.Calculate the capacitors for specific rise

time/knee frequency of a HSDC.Measure characteristics of capacitor

What is a capacitor?

Capacitor is a two terminal electrical component used to store electric charge with conductors separated by an insulator.

d

AC

Capacitor Applications:

Capacitors are used in applications such as energy storage, power systems, noise suppression and coupling.

In a high speed digital device, capacitors are used for uniform voltage distribution (bypass) and shunt current to the return path (decoupling) to provide low impedance path between power and ground or ground connections between gates.

Every capacitor has a parasitic series inductance (lead inductance, package inductance, or mounting inductance – ESL), parasitic series resistance (equivalent Series resistance – ESR) which is real impedance, and self-resonate frequency (SRF):

Circuit Issues

For uniform voltage distribution, a shunt capacitor is connected to the power supply in the frequency range where wired inductance can be a problem.

This bypass capacitor provides low impedance between ground and power plane. Bypass capacitor loses effectiveness at high frequency where it becomes inductance.

This problem is resolved by an assortment of capacitors (large and smaller) used to provide low impedance power source for every logic device across various operating frequency range.

Measuring Capacitor Characteristics

Using Test JigUsing Impedance Analyzer

Test Jig Setup

Materials Oscillocope: Tektronix DPO 4054 Pulse Gen: SYSTRON 101 Pulse Generator Resistors:

100Ω, 0805, 1% 10Ω, 0805, 1%

Capacitor: 1µF, 0603, X5R, 10V

Cu Board Analyzer Probes

Test Jig: Cu Board

Test Jig: Lead Inductance

Rs = source resistance of test jig, Ω = 4.35ΩA = area under spike, Vs = 844.5pVs∆V = Open-circuit Step Voltage of test jig, V = 0.996VL = lead inductance, H

Open Circuit test Load: 1µF Capacitor

Test Jig: ESR

Rs = source resistance of test jig, Ω = 4.35Ω

X = measured step voltage after spike, V = 0.064V

∆V = Open-circuit Step Voltage of test jig, V = 0.996V

Test Jig: Capacitance

∆V = Open-circuit Step Voltage of test jig, V = 0.996V

Rs = source resistance of test jig, Ω = 4.35Ω

X = measured step voltage after spike, V = 0.064V

dV/dt = charge rate of ramp, V/s = 0.236V/998ns = 236472.9V/s

C = capacitance, F

Impedance Analyzer: Resonance Frequency

Materials Impedance Analyzer: HP 4194A Bread board Capacitors:

0.1µF, 0603, X7R, 16V 1µF, 0402, X5R, 10V 1µF, 0603, X5R, 10V 10µF, 0603, X5R, 6.3V

Impedance Analyzer: Resonance Frequency

Impedance Analyzer: Results

1µF, 0603, X5R, 10VLs @ 5MHz ESR @1.93MHz Cs @ 1MHz

8.102nH 10.1Ω 1.47µF

Resonance ESR1000pF, 0603, X7R, 50V 51.5MHz 9.44Ω0.1µF, 0603, X7R, 16V 4.7MHz 7.53Ω1µF, 0402, X5R, 10V 1.9MHz 10.1Ω1µF, 0603, X5R, 10V 1.53MHz 1.18Ω

10µF, 0603, X5R, 6.3V 534kHz 6.81Ω

Resonance Frequency:

When the frequency is below the SRF it behaves capacitive.

When the frequency is above the SRF it behaves inductively.

The inductance in systems induces voltage change between the logic components and supply or ground plane that introduces noise.

Therefore, a capacitor selected must behave capacitively.

1µF Ceramic Capacitor (0603)

From Test Jig

From Impedance Analyzer

Reference:

Johnson, Howard, et al. “High Speed Digital Design: A Handbook of Black Magic” . Pearson Education, New Delhi, 1993, p253-255 & 275-299.

http://www.farnell.com/datasheets/1525783.pdf

http://people.eecs.ku.edu/~callen/713/713_Vias-F13.ppt