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    參數(shù)資料
    型號(hào): 4609X-101-681
    英文描述: Dual/Triple Ultra-Low-Voltage SOT23 &#181;P Supervisory Circuits
    中文描述: WIDERSTAND NETZWERK DICKFILM 680R 5ST
    文件頁數(shù): 40/62頁
    文件大小: 1883K
    代理商: 4609X-101-681
    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
    =
    RL
    Vin
    j
    ωC(R + RS)(R + RL) + (RS + RL + 2R)
    Vin
    RS
    R
    C
    RL
    Vout
    Figure 2.
    basic T-Filter configuration
    225
    200
    175
    150
    125
    100
    75
    50
    25
    FIELD
    INTENSITY
    (
    V/M)
    25
    54
    88
    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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