THS6042 Datasheet, PDF (28/33 Pages) Texas Instruments – 350 mA, ±12 V ADSL CPE LINE DRIVERS

THS6042, THS6043
350 mA, ±12 V ADSL CPE LINE DRIVERS
SLOS264G – MARCH 2000 – REVISED DECEMBER 2001
APPLICATION INFORMATION
driving a capacitive load
Driving capacitive loads with high performance amplifiers is not a problem as long as certain precautions are
taken. The first is to realize that the THS6042/3 has been internally compensated to maximize its bandwidth
and slew rate performance. When the amplifier is compensated in this manner, capacitive loading directly on
the output decreases the device’s phase margin leading to high frequency ringing or oscillations. Therefore, for
capacitive loads of greater than 5 pF, it is recommended that a resistor be placed in series with the output of
the amplifier, as shown in Figure 49. Keep in mind that stray capacitance on the output is also considered
capacitive loading, whether or not it is there on purpose. A minimum value of 5 Ω should work well for most
applications. In ADSL systems, setting the series resistor value to 12.4 Ω both isolates any capacitance loading
and provides the proper line impedance matching at the source end.
750 Ω
100 Ω
_
Input
+
12.4 Ω
Output
C(Stray) + CL
Figure 49. Driving a Capacitive Load
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 and is not recommended. The THS6042/3,
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, integrators and 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 50).
RG
RF
VI
R1
–
+
C1
ǒ Ǔǒ Ǔ ^VO
VI
+
1
)
RF
RG
1
1 ) sR1C1
VO
f_–3dB
+
1
2pR1C1
Figure 50. Single-Pole Low-Pass Filter
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