_

+

5 V

− 5 V

2.1 kW

2.05 kW

2.2 nF

1 kW

270 pF

1.02 kW

_

+

5 V

− 5 V

2 kW

2 kW

1 nF

2.61 kW

1.5 nF

649

1 kW

THS4281

www.ti.com

SLOS432B – APRIL 2004 – REVISED OCTOBER 2015

8.2 Typical Application

Figure 71. Second-Order Sallen-Key 100-kHz Butterworth

Figure 72. Second-Order MFB 100-kHz Butterworth Filter,

Filter, Gain = 2 V/V

Gain = 2 V/V

8.2.1 Design Requirements

Table 1 shows example design parameters and values for the typical application design example in Figure 71.

Table 1. Design Parameters

DESIGN PARAMETERS

VALUE

Supply voltage

±5 V

Amplifier topology

Voltage feedback

Gain

2 V/V

Second Order 100 KHz Sallen- Key

Filter requirement

Butterworth Filter

Input/Output Requirements

Rail to Rail

8.2.2 Detailed Design Procedure

8.2.2.1 Active Filtering With the THS4281

High-performance active filtering with the THS4281 is achievable due to the amplifier's good slew rate, wide

bandwidth, and voltage-feedback architecture. Several 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 filter design program called FilterPro™.

FilterPro is 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 determine a 2-pole Butterworth response filter with a corner (–3-dB)

frequency of 100 kHz, which is shown in Figure 71 and Figure 72. 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 frequencies is severely reduced and

allows the high-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.

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