THS4281_14 Datasheet, PDF (20/38 Pages) Texas Instruments – VERY LOW-POWER, HIGH-SPEED, RAIL-TO-RAIL INPUT AND OUTPUT VOLTAGE-FEEDBACK OPERATIONAL AMPLIFIER

THS4281
SLOS432A – APRIL 2004 – REVISED NOVEMBER 2009
APPLICATION CIRCUITS
www.ti.com
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 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
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.
2 kW
649 W
V_I
1.5 nF
2.61 kW
1 nF
2 kW
5V
_
+
−5V
V_O
R_L
1 kW
Figure 73. Second-Order Sallen-Key 100-kHz
Butterworth Filter, Gain = 2 V/V
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 74. Second-Order MFB 100-kHz
Butterworth Filter, Gain = 2 V/V
10
0
Sallen-Key
Response
−10
−20
−30
−40
−50
MFB
−60
Response
−70 ^VS ^{= 3 V, 5 V, ±5 V, 15 V,}
V_O = 100 mV_PP
−80
1k 10k 100k 1M 10M
f − Frequency − Hz
100M
Figure 75. Second-Order 100-kHz Active Filter
Response
Driving Capacitive Loads
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
be recommended to improve A/D linearity. A
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 open-loop output
resistance is considered, this capacitive load
introduces an additional pole in the feedback path
that decreases the phase margin. When the primary
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 (for example, R_(ISO) = 100 Ω for C_LOAD = 10
pF to R_(ISO) = 10 Ω for C_LOAD = 1000 pF) between the
amplifier output and the capacitive load.
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