THS6184_16 Datasheet, PDF (23/39 Pages) Texas Instruments – DUAL-PORT, LOW-POWER DIFFERENTIAL xDSL LINE DRIVER AMPLIFIERS

THS6184
www.ti.com ................................................................................................................................................. SLLS635D – AUGUST 2005 – REVISED JANUARY 2009
R_G
R_F
V_I
R1
−
+
C1
V_O
ǒ Ǔǒ Ǔ ^VO
VI
+
1
)
RF
RG
1
1 ) sR1C1
f–3dB
+
1
2pR1C1
Figure 56. Single-Pole Low-Pass Filter
If a multiple pole filter is required, the use of a Sallen-Key filter can work very well with CFB amplifiers. This is
because the filtering elements are not in the negative feedback loop and stability is not compromised. Because of
their high slew rates and high bandwidths, CFB amplifiers can create very accurate signals and help minimize
distortion. An example is shown in Figure 57.
C1
V_I
R1
+
R2
_
C2
R_F
R_G
R1 = R2 = R
C1 = C2 = C
Q = Peaking Factor
(Butterworth Q = 0.707)
f–3dB
+
1
2pRC
( ) ^{RG =}
R_F
1
2− _Q
Figure 57. 2-Pole Low-Pass Sallen-Key Filter
PCB DESIGN CONSIDERATIONS
Proper PCB design techniques in two areas are important to assure proper operation of the THS6184. These
areas are high-speed layout techniques and thermal-management techniques. Because the THS6184 is a
high-speed part, the following guidelines are recommended.
• Ground plane – It is essential that a ground plane be used on the board to provide all components with a low
inductive ground connection. Although a ground connection directly to a terminal of the THS6184 is not
necessarily required, it is recommended that the thermal pad of the package be tied to ground. This serves
two functions. It provides a low inductive ground to the device substrate to minimize internal crosstalk and it
provides the path for heat removal. Note that the BiCOM1 process is an SOI process and thus, the substrate
is isolated from the active circuitry.
• Input stray capacitance – To minimize potential problems with amplifier oscillation, the capacitance at the
inverting input of the amplifiers must be kept to a minimum. To do this, PCB trace runs to the inverting input
must be as short as possible, the ground plane should be removed under any etch runs connected to the
inverting input, and external components should be placed as close as possible to the inverting input. This is
especially true in the noninverting configuration.
• Proper power supply decoupling – Use a minimum of a 6.8-µF tantalum capacitor in parallel with a 0.1-µF
ceramic capacitor on each supply terminal. It may be possible to share the tantalum among several amplifiers
depending on the application, but a 0.1-µF ceramic capacitor should always be used on the supply terminal of
every amplifier. In addition, the 0.1-µF capacitor should be placed as close as possible to the supply terminal.
As this distance increases, the inductance in the connecting etch makes the capacitor less effective. The
designer should strive for distances of less than 0.1 inches between the device power terminal and the
ceramic capacitors.
• For a differential configuration as shown in Figure 1, it is recommended that a 0.1-µF or 1-µF capacitor be
added across the power supplies (from V_CC+ to V_CC- ) as close as possible to the THS6184. This allows for
differential currents to flow properly, slightly reducing even-order harmonic distortion. The 0.1-µF capacitors to
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