Application Notes
(Continued)
4.4 Active Filter
4.4.1 Simple Low-Pass Active Filter
The simple low-pass filter is shown in
Figure 9.
Its low-pass
frequency gain (ω
→
o) is defined by −R
3
/R
1
. This allows
low-frequency gains other than unity to be obtained. The fil-
ter has a −20dB/decade roll-off after its corner frequency fc.
R
2
should be chosen equal to the parallel combination of R
1
and R
3
to minimize error due to bais current. The frequency
response of the filter is shown in
Figure 10.
DS100922-22
FIGURE 7. Two-op-amp Instrumentation Amplifier
4.3 Single-Supply Inverting Amplifier
There may be cases where the input signal going into the
amplifier is negative. Because the amplifier is operating in
single supply voltage, a voltage divider using R
3
and R
4
is
implemented to bias the amplifier so the input signal is within
the input common-common voltage range of the amplifier.
The capacitor C
1
is placed between the inverting input and
resistor R
1
to block the DC signal going into the AC signal
source, V
IN
. The values of R
1
and C
1
affect the cutoff fre-
quency, fc =
1
⁄
2
π
R
1
C
1
.
As a result, the output signal is centered around mid-supply
(if the voltage divider provides V
+
/2 at the non-inverting in-
put). The output can swing to both rails, maximizing the
signal-to-noise ratio in a low voltage system.
DS100922-24
FIGURE 9. Simple Low-Pass Active Filter
DS100922-25
DS100922-23
FIGURE 10. Frequency Response of Simple Low-pass
Active Filter in
Figure 9
Note that the single-op-amp active filters are used in to the
applications that require low quality factor, Q(≤ 10), low fre-
quency (≤ 5KHz), and low gain (≤ 10), or a small value for the
product of gain times Q(≤ 100). The op amp should have an
open loop voltage gain at the highest frequency of interest at
least 50 times larger than the gain of the filter at this fre-
quency. In addition, the selected op amp should have a slew
rate that meets the following requirement:
Slew Rate
≥
0.5 x (ω
H
V
OPP
) X 10
−6
V/µsec
Where
ω
H
is the highest frequency of interest, and V
OPP
is
the output peak-to-peak voltage.
FIGURE 8. Single-Supply Inverting Amplifier
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