When does a C-filter work for EMI suppression?

A C-filter provides effective EMI attenuation only when the source impedance (Z_source) is significantly higher than the capacitor impedance (Z_cap) at the noise frequency. The insertion loss is approximately IL = 20*log10(1 + Z_source/(2*Z_cap)). If Z_source = 500 ohms (typical for a long cable or high-impedance switching node) and Z_cap = 5 ohms at 30 MHz (using a 1 nF capacitor), the insertion loss is about 34 dB. But if Z_source = 5 ohms (low-impedance power bus), the C-filter provides essentially zero attenuation. This is why C-filters work well on signal lines and high-impedance I/O pins but fail on low-impedance power rails where an LC or pi-filter topology is needed.

What capacitor value should I use for a C-filter?

The value depends on the target frequency and source impedance. For a 3 dB corner at frequency f with source impedance Z_s: C = 1/(2*pi*f*Z_s). For a high-impedance I/O line (Z_s = 1000 ohms) needing filtering above 10 MHz: C = 1/(2*pi*10e6*1000) = 16 pF. For a 50-ohm signal line at 100 MHz: C = 1/(2*pi*100e6*50) = 32 pF. Keep in mind that the capacitor's self-resonant frequency must exceed the highest noise frequency you need to filter. Above the SRF, the capacitor behaves as an inductor and provides no filtering. Use the smallest physical package (0201 or 0402) to maximize SRF.

What is the difference between a C-filter and a pi-filter?

A C-filter uses a single shunt capacitor and provides first-order (20 dB/decade) roll-off. It works when source impedance is high but fails into low impedance sources. A pi-filter uses a capacitor-inductor-capacitor (C-L-C) topology providing third-order (60 dB/decade) roll-off, and works regardless of source impedance because the series inductor creates its own high-impedance element. Pi-filters provide 30-40 dB more attenuation at high frequencies but occupy 3x the board space and add insertion loss at DC. For EMC on power supply entry points where source impedance is unpredictable, pi-filters are the standard choice. For signal lines with known high source impedance, a C-filter is smaller, cheaper, and sufficient.

What is a C-Filter (EMC)? | Capacitive EMI Filter

Definition & Operating Principle

A C-filter is the simplest EMI filter topology, consisting of a single capacitor connected in shunt between a signal or power line and the ground reference. At frequencies where the capacitor impedance is much lower than the source impedance driving the line, high-frequency noise current is diverted through the capacitor to ground rather than propagating down the line to the load or cable. The filter provides first-order low-pass behavior with a 20 dB per decade attenuation slope above its corner frequency.

The critical design consideration for C-filters is source impedance matching. A shunt capacitor can only divert current if there is sufficient source impedance to create a voltage divider effect. On a 50-ohm transmission line, a 100 pF C-filter at 100 MHz presents 16 ohms, giving only 10 dB of insertion loss. The same capacitor on a 1 kohm impedance line gives 36 dB. This impedance dependence makes C-filters ideal for high-impedance signal lines, sensor inputs, and I/O interfaces, but ineffective on low-impedance power rails where an LC or pi-filter is required to create the necessary source impedance with a series inductor or ferrite bead.

Key Formulas

Insertion Loss (C-filter):

IL(dB) = 20 log₁₀(1 + Z_S / (2 × Z_C))

Z_S = 500 Ω, Z_C = 5 Ω at 30 MHz: IL = 34 dB

Corner Frequency:

f_c = 1 / (2π × Z_S × C)

Z_S = 1 kΩ, C = 100 pF: f_c = 1.6 MHz

Attenuation Slope:

Roll-off = 20 dB/decade (first-order)

At 10× corner frequency: IL increases by 20 dB

EMI Filter Topology Comparison

Topology	Components	Roll-off	Z_source Dependence	Board Space	Best For
C-filter	1 cap (shunt)	20 dB/dec	High (needs high Z_S)	Minimal	Signal lines, I/O
L-filter	1 inductor (series)	20 dB/dec	High (needs low Z_L)	Small	Power lines
LC-filter	1 inductor + 1 cap	40 dB/dec	Moderate	Medium	General purpose
Pi-filter (CLC)	2 caps + 1 inductor	60 dB/dec	Low (Z-independent)	Large	Power entry, connectors
T-filter (LCL)	2 inductors + 1 cap	60 dB/dec	Low (Z-independent)	Large	Filtered connectors

Practical Application

A USB 2.0 interface on an embedded system fails CISPR 32 radiated emissions at 240 MHz (the 4th harmonic of the 480 MHz USB clock). The USB data lines have a characteristic impedance of 90 ohms differential (45 ohms each to ground). A pair of 33 pF C0G 0402 C-filter capacitors placed at the connector pins provide Z_C = 20 ohms at 240 MHz against the ~45 ohm source impedance, yielding approximately 7 dB of insertion loss at the problem frequency. Combined with the common-mode choke already in the USB circuit, the additional C-filter attenuation brings the radiated emission 8 dB below the Class B limit. The 33 pF value is chosen to keep the corner frequency at 107 MHz (above the 480 MHz signaling eye mask requirement after accounting for the 90 ohm differential impedance), ensuring the capacitors do not degrade signal integrity.

Frequently Asked Questions

When does a C-filter work for EMI?

Only when source impedance >> capacitor impedance. IL = 20log(1 + Z_S/(2Z_C)). Z_S = 500 ohms, Z_C = 5 ohms: 34 dB. But Z_S = 5 ohms: ~0 dB. Use LC/pi-filter for low-impedance power rails.

What capacitor value for a C-filter?

C = 1/(2πfZ_S) for the 3 dB corner. 1 kohm line, 10 MHz target: C = 16 pF. 50 ohm line, 100 MHz: C = 32 pF. Keep SRF above highest noise frequency; use 0201/0402 packages for maximum SRF.

C-filter vs. pi-filter?

C-filter: single cap, 20 dB/decade, works only into high-Z sources. Pi-filter: C-L-C, 60 dB/decade, works regardless of source impedance but 3x the space. Pi for power entry; C-filter for known high-Z signal lines.

C-Filter (EMC)