THS4281DBVT datasheet(21/31 Pages) TI

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APPLICATION CIRCUITS

Active Filtering With the THS4281

5 V

− 5 V

2.1 kW

2.05 kW

2.2 nF

1 kW

270 pF

1.02 kW

−80

−70

−60

−50

−40

−30

−20

−10

10k

100k

10M

100M

Sallen-Key

Response

MFB

Response

VS = 3 V, 5 V, ±5 V, 15 V,

VO = 100 mVPP

f − Frequency − Hz

Driving Capacitive Loads

5 V

− 5 V

2 kW

1 nF

2.61 kW

1.5 nF

649 W

1 kW

THS4281

SLOS432 – APRIL 2004

High performance active filtering with the THS4281 is

achievable due to the amplifier's good slew rate, wide

bandwidth, and voltage feedback architecture. Sev-

eral options are available for high-pass, low-pass,

bandpass, and bandstop filters of varying orders.

Filters can be quite complex and time consuming to

design. Several books and application reports are

available to help design active filters. But, to help

simplify the process and minimize the chance of

miscalculations, Texas Instruments has developed a

Figure 74. Second-Order MFB 100-kHz

filter design program called FilterPro™. FilterPro is

Butterworth Filter, Gain = 2 V/V

available for download at no cost from TI's Web site

(www.ti.com).

The two most common low-pass filter circuits used

are the Sallen-Key filter and the Multiple Feedback

(MFB)–aka Rauch filter. FilterPro was used to deter-

mine a 2-pole Butterworth response filter with a

corner (-3 dB) frequency of 100 kHz which is shown

in Figure 73 and Figure 74. One of the advantages of

the MFB filter, a much better high frequency rejection,

is clearly shown in the response shown in Figure 75.

This is due to the inherent R-C filter to ground being

the first elements in the design of the MFB filter. The

Sallen-Key design also has an R-C filter, but the

capacitor connects directly to the output. At very high

frequencies, where the amplifier's access loop gain is

decreasing, the ability of the amplifier to reject high

Figure 75. Second-Order 100-kHz Active Filter

frequencies is severely reduced and allows the high

Response

frequency signals to pass through the system. One

other advantage of the MFB filter is the reduced

sensitivity in component variation. This is important

when using real-world components where capacitors

can easily have

±10% variations.

One of the most demanding, and yet common, load

conditions for an op amp is capacitive loading. Often,

the capacitive load is the input of an A/D converter,

including additional external capacitance, which may

recommended

improve

A/D

linearity.

high-speed, high open-loop gain amplifier like the

THS4281 can be susceptible to instability and

peaking when a capacitive load is placed directly on

the output. When the amplifier's open-loop output

resistance is considered, this capacitive load intro-

duces an additional pole in the feedback path that

Figure 73. Second-Order Sallen-Key 100-kHz

decreases the phase margin. When the primary

Butterworth Filter, Gain = 2 V/V

considerations

are

frequency

response

flatness,

pulse response fidelity, or distortion, a simple and

effective solution is to isolate the capacitive load from

the feedback loop by inserting a small series isolation

resistor (10

Ω to 25 Ω) between the amplifier output

and the capacitive load.

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