THS4281 Datasheet, PDF (27/43 Pages) Texas Instruments – VERY LOW-POWER, HIGH-SPEED, RAIL-TO-RAIL INPUT AND OUTPUT VOLTAGE-FEEDBACK OPERATIONAL AMPLIFIER

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8.2 Typical Application
2 kW
649
V_I
1.5 nF
2.61 kW
1 nF
2 kW
5V
_
+
−5V
V_O
R_L
1 kW
Figure 71. Second-Order Sallen-Key 100-kHz Butterworth
Filter, Gain = 2 V/V
THS4281
SLOS432B – APRIL 2004 – REVISED OCTOBER 2015
2.05 kW
270 pF
1.02 kW
V_I
2.2 nF
2.1 kW
5V
_
+
−5V
V_O
R_L
1 kW
Figure 72. Second-Order MFB 100-kHz Butterworth Filter,
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
Supply voltage
Amplifier topology
Gain
Filter requirement
Input/Output Requirements
VALUE
±5 V
Voltage feedback
2 V/V
Second Order 100 KHz Sallen- Key
Butterworth Filter
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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