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DRIVING A CAPACITIVE LOAD

THS412x

Output

Output

20

Ω

20

Ω

ACTIVE ANTIALIAS FILTERING

+

-

+

-

THS412x

C3

C3

R4

R2

R4

C1

C1

R2

R3

R3

C2

R1

R1

Vs

THS4120

THS4121

SLOS319D – FEBRUARY 2001 – REVISED OCTOBER 2004

APPLICATION INFORMATION (continued)

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 THS412x 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 10 pF, it is recommended that a resistor be placed in series with the output of

the amplifier, as shown in Figure 21. A minimum value of 20

Ω should work well for most applications. For

example, in 50-

Ω transmission systems, setting the series resistor value to 50 Ω both isolates any capacitance

loading and provides the proper line impedance matching at the source end.

Figure 21. Driving a Capacitive Load

For signal conditioning in ADC applications, it is important to limit the input frequency to the ADC. Low-pass

filters can prevent the aliasing of the high-frequency noise with the frequency of operation. Figure 22 presents a

method by which the noise may be filtered in the THS412x. Proper ground referencing should be considered.

Figure 22. Antialias Filtering

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