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R

PU +

V

P

– V

CC

V

IN

– V

P

1

RG )

V

OUT

– V

P

1

RF

(1)

DRIVING A CAPACITIVE LOAD

THS414x

Output

Output

20

Ω

20

Ω

390

Ω

390

Ω

390

Ω

390

Ω

ACTIVE ANTIALIAS FILTERING

+

-

+

-

THS1050

THS414x

C3

C3

R4

R2

R4

+

C1

+

C1

R2

R3

R3

C2

R1

R1

Vs

THS4140

THS4141

SLOS320F – MAY 2000 – REVISED JANUARY 2006

APPLICATION INFORMATION (continued)

The following equation is used to calculate R

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 THS414x 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 will decrease 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 25. 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 25. 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 26 presents a

method by which the noise may be filtered in the THS414x.

Figure 26. Antialias Filtering

13

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