LPV321 Datasheet, PDF (14/22 Pages) National Semiconductor (TI) – General Purpose, Low Voltage, Low Power, Rail-to-Rail Output Operational Amplifiers

Application Information (Continued)
Two-op-amp Instrumentation Amplifier
A two-op-amp instrumentation amplifier can also be used to
make a high-input-impedance DC differential amplifier (Fig-
ure 7). As in the three-op-amp circuit, this instrumentation
amplifier requires precise resistor matching for good CMRR.
R₄ should equal to R₁ and R₃ should equal R₂.
ACTIVE FILTER
Simple Low-Pass Active Filter
The simple low-pass filter is shown in Figure 9. Its low-
frequency gain(ω → o) is defined by −R₃/R₁. This allows
low-frequency gains other than unity to be obtained. The
filter has a −20 dB/decade roll-off after its corner frequency
fc. R₂ should be chosen equal to the parallel combination of
R₁ and R₃ to minimize errors due to bais current. The
frequency response of the filter is shown in Figure 10
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FIGURE 7. Two-op-amp Instrumentation Amplifier
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 R₃ and R₄ is
implemented to bias the amplifier so the input signal is within
the input common-common voltage range of the amplifier.
The capacitor C₁ is placed between the inverting input and
resistor R₁ to block the DC signal going into the AC signal
source, V_IN. The values of R₁ and C₁ affect the cutoff fre-
quency, fc = 1/2π R ₁C₁.
As a result, the output signal is centered around mid-supply
(if the voltage divider provides V⁺/2 at the non-inverting
input). The output can swing to both rails, maximizing the
signal-to-noise ratio in a low voltage system.
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FIGURE 9. Simple Low-Pass Active Filter
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FIGURE 8. Single-Supply Inverting Amplifier
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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
frequency (≤ 5 kHz), 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 inter-
est at least 50 times larger than the gain of the filter at this
frequency. In addition, the selected op amp should have a
slew rate that meets the following requirement:
Slew Rate ≥ 0.5 x (ω_HV _OPP) X 10^−6V/µsec
Where ω_H is the highest frequency of interest, and V_OPP is
the output peak-to-peak voltage.
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