Texas Instruments THS4281 Evaluation Module THS4281EVM THS4281EVM 데이터 시트
제품 코드
THS4281EVM
_
+
5 V
− 5 V
2.1 k
W
2.05 k
W
2.2 nF
V
O
R
L
1 k
W
V
I
270 pF
1.02 k
W
−80
−70
−60
−50
−40
−30
−20
−10
0
10
1k
10k
100k
1M
10M
100M
Signal Gain − dB
Sallen-Key
Response
MFB
Response
V
S
= 3 V, 5 V,
±
5 V, 15 V,
V
O
= 100 mV
PP
f − Frequency − Hz
_
+
5 V
− 5 V
2 k
W
2 k
W
1 nF
2.61 k
W
1.5 nF
649
W
V
O
R
L
1 k
W
V
I
SLOS432A – APRIL 2004 – REVISED NOVEMBER 2009
www.ti.com
APPLICATION CIRCUITS
Active Filtering with the THS4281
High-performance active filtering with the THS4281 is
achievable due to the amplifier's good slew rate, wide
bandwidth,
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
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
Figure 74. Second-Order MFB 100-kHz
Butterworth Filter, Gain = 2 V/V
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
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
and
. One of the advantages of
the MFB filter, a much better high-frequency rejection,
is clearly shown in the response shown in
is clearly shown in the response shown in
.
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
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
Figure 75. Second-Order 100-kHz Active Filter
sensitivity in component variation. This is important
Response
when using real-world components where capacitors
can easily have ±10% variations.
can easily have ±10% variations.
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
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
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
Figure 73. Second-Order Sallen-Key 100-kHz
introduces an additional pole in the feedback path
Butterworth Filter, Gain = 2 V/V
that decreases the phase margin. When the primary
considerations
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
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.
20
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