THS6092SLASH3 datasheet(28/46 Pages) TI1

28 / 46 page

THS6042, THS6043

350 mA, ±12 V ADSL CPE LINE DRIVERS

SLOS264G − MARCH 2000 − REVISED DECEMBER 2001

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

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.

C(Stray) + CL

750

Ω

Input

Output

100

Ω

12.4

Ω

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).

−

–3dB +

pR1C1

)

) sR1C1

Figure 50. Single-Pole Low-Pass Filter

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