LPV324M National Semiconductor, LPV324M Datasheet - Page 11

• Manufacturer Part Number: LPV324M
• Description: General Purpose / Low Voltage / Low Power / Rail-to-Rail Output Operational Amplifiers
• Manufacturer: National Semiconductor

Application Notes
4.2 Instrumentation Circuits
The input impedance of the previous difference amplifier is
set by the resistor R
lems of low input impedance, one way is to use a voltage fol-
lower ahead of each input as shown in the following two in-
strumentation amplifiers.
4.2.1Three-op-amp Instrumentation Amplifier
The quad LPV324 can be used to build a three-op-amp in-
strumentation amplifier as shown in Figure 6
The first stage of this instrumentation amplifier is a
differential-input, differential-output amplifier, with two volt-
age followers. These two voltage followers assure that the
input impedance is over 100M . The gain of this instrumen-
tation amplifier is set by the ratio of R
R
fects the CMRR. For good CMRR over temperature, low drift
resistors should be used. Making R
2
R
and adding a trim pot equal to twice the difference between
1
FIGURE 6. Three-op-amp Instrumentation Amplifier
2
and R
and R
4
4
equal R
will allow the CMRR to be adjusted for optimum.
FIGURE 5. Difference Amplifier
2
1
. Matching of R
, R
2
, R
3
, and R
(Continued)
4
3
4
. To eliminate the prob-
Slightly smaller than R
2
to R
/R
1
1
. R
DS100920-7
and R
3
should equal
4
DS100920-85
to R
2
af-
11
4.2.2 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.
R
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
implemented to bias the amplifier so the input signal is within
the input common-common voltage range of the amplifier.
The capacitor C
resistor R
source, V
quency, fc = 1/2 R
As a result, the ouptut signal is centered around mid-supply
(if the voltage divider provides V
put). The output can swing to both rails, maximizing the
signal-to-noise ratio in a low voltage system.
4.4 Active Filter
4.4.1 Simple Low-Pass Active Filter
The simple low-pass filter is shown in Figure 9 . Its
low-frequency gain(
low-frequency gains other than unity to be obtained. The fil-
ter has a −20dB/decade roll-off after its corner frequency fc.
R
and R
response of the filter is shown in Figure 10
4
2
FIGURE 7. Two-op-amp Instrumentation Amplifier
should be chosen equal to the parallel combination of R
should equal to R
3
FIGURE 8. Single-Supply Inverting Amplifier
to minimize errors due to bais current. The frequency
1
IN
. The values of R
to block the DC signal going into the AC signal
1
is placed between the inverting input and
1
1
C
and R
1
.
o) is defined by −R
3
1
should equal R
and C
+
/2 at the non-inverting in-
1
affect the cutoff fre-
DS100920-13
3
/R
2
1
.
. This allows
3
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and R
DS100920-11
4
is
1