4116R-R2R-503 datasheet(45/62 Pages) BOURNS

45 / 62 page

Specifications are subject to change without notice.

321

FOR PRODUCT SPECIFICATIONS, SEE PAGES 305 AND 306.

Reducing EMI/RFI

The radiation of electromagnetic interference and radio fre-

quency interference (EMI/RFI) to the environment is a pressing

concern for many manufacturers of electronic equipment.

According to FCC regulations (Parts 15 and18), emissions must

not exceed certain maximum levels depending on whether the

equipment is for strictly industrial use or also for residential use.

A graphical representation of these limits is shown in Figure 1.

Similar restrictions apply to equipment sold in Europe (VDE

0871, a West German standard), Japan (VCCI), and to the mili-

tary (MIL-STD-461/462.)

Several approaches are available today to control EMI/RFI

emissions, including grounded metal enclosures, shielded

cables, judicious component placement and interconnect

designs, power-supply decoupling, and low-pass filtering of sig-

nal lines.

Low-pass filtering can be effective for EMI/RFI filtering when

the noise components to be rejected occur at frequencies high-

er than the signal frequency (to be passed). For these situations,

Bourns has developed low-pass resistor-capacitor filter net-

works which are ideal for board-level EMI/RFI filtering.

A typical application would be to filter signal lines between

RS-232 drivers and their corresponding connectors. In such low

to medium frequency applications, these networks represent a

more useful (and economical) solution than inductive type filters

such as ferrite beads. In fact, ferrite beads become mostly inef-

fective below 10MHz.

The basic “T” configuration (Figure 2) is a standard R-C net-

work available in versions for 7 or 8 input lines. The 8 input-line

version is available in both through-hole DIP and surface-mount

models.

Under steady state conditions, the capacitor C offers an infi-

nite impedance to the DC component of the input waveform

(which will be assumed for the moment to be entering from the

left side). Thus, the DC component of the signal voltage is

passed to the load, but reduced in value by the voltage drop

across the two resistors.

The impedance of C becomes lower at higher (noise) frequen-

cies. Thus, the noise component of the signal faces a voltage

divider consisting of the first resistor (R) and C. At the high fre-

quencies of the noise component, R will be much greater than

the impedance of C, therefore, most of the noise voltage will be

dropped across the resistor. Almost no noise current flows

through the load and, therefore, will hardly affect the DC volt-

ages (i.e., the signal) across the load.

Since the filter is symmetric, its principle of operation is the

same for waveforms traveling in the opposite direction, in which

case the voltage divider is formed by the second resistor and

the capacitor. Such a symmetrical design is useful for filtering

signals on a bidirectional bus.

Assuming purely resistive source and load impedances, the

transfer function is given by:

Vout

Vin

ωC(R + RS)(R + RL) + (RS + RL + 2R)

Vin

Vout

Figure 2.

basic T-Filter configuration

225

200

175

150

125

100

108

136

174

216

300

400

470

1000

FREQUENCY IN MEGAHERTZ

F.C.C. LIMIT FOR CLASS A (INDUSTRIAL) EQUIPMENT AT 30 METERS.

F.C.C. LIMIT FOR CLASS B (MASS MARKET) EQUIPMENT AT 3 METERS.

Figure 1.

F.C.C. radiation limits for class A

and class B computing devices

EMI/RFI Filters 601 Series

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