THS6032_14 Datasheet, PDF (24/35 Pages) Texas Instruments – LOW-POWER ADSL CENTRAL-OFFICE LINE DRIVER

THS6032
SLOS233F – APRIL 1999 – REVISED AUGUST 2009....................................................................................................................................................... www.ti.com
GENERAL CONFIGURATIONS
A common error for the first-time CFB user is to create a unity-gain buffer amplifier by shorting the output directly
to the inverting input. A CFB amplifier in this configuration oscillates, so this is not recommended. The THS6032,
like all CFB amplifiers, must have a feedback resistor for stable operation. Additionally, placing capacitors directly
from the output to the inverting input is not recommended. This is because, at high frequencies, a capacitor has
a very low impedance. This results in an unstable amplifier, and should not be considered when using a
current-feedback amplifier. Because of this, simple low-pass filters, which are easily implemented on a VFB
amplifier, have to be designed slightly differently. If filtering is required, simply place an RC-filter at the
noninverting terminal of the operational-amplifier (see Figure 52).
R_G
R_F
V_I
R1
−
+
C1
V_{O = (1 +} R_{F )[}
1
]
V_I
R_G (1 + sR1C1)
V_O
f_-3dB
=
1
2pR1C1
Figure 52. 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. One implementation of the Sallen-Key filter is shown in Figure 53. For more information on Sallen-Key
filters, refer to the Analysis of the Sallen-Key Architecture (SLOA024A).
C1
R1 = R1 = R
C1 = C2 = C
Q = Peaking Factor
(Butterworth Q = 0.707)
V_I
R1
+
R2
_
C2
R_F
R_G
f_-3dB
=
1
2pRC
R_F
R_G
=
(2
-
1
Q
)
Figure 53. 2-Pole Low-Pass Sallen-Key Filter
Another good use for the THS6032 amplifier is as a video distribution amplifier. One characteristic of distribution
amplifiers is the fact that the differential phase (DP) and the differential gain (DG) are compromised as the
number of lines increases and the closed-loop gain increases. Be sure to use termination resistors throughout
the distribution system to minimize reflections and capacitive loading.
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