LPV321 Datasheet, PDF(11/21 Page) National Semiconductor (TI) – General Purpose, Low Voltage, Low Power, Rail-to-Rail Output Operational Amplifiers

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LPV321 Datasheet, PDF (11/21 Pages) National Semiconductor (TI) – General Purpose, Low Voltage, Low Power, Rail-to-Rail Output Operational Amplifiers

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Application Notes (Continued)

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.

R4 should equal to R1 and R3 should equal R2.

DS100920-7

DS100920-11

FIGURE 5. Difference Amplifier

4.2 Instrumentation Circuits

The input impedance of the previous difference amplifier is

set by the resistor R1, R2, R3, and R 4. To eliminate the prob-

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

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 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, VIN. The values of R1 and C1 affect the cutoff fre-

quency, fc = 1/2Ï R 1C1.

As a result, the ouptut 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.

DS100920-85

FIGURE 6. Three-op-amp Instrumentation Amplifier

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 100Mâ¦. The gain of this instrumen-

tation amplifier is set by the ratio of R2/R 1. R3 should equal

R1 and R4 equal R2. Matching of R3 to R1 and R4 to R2 af-

fects the CMRR. For good CMRR over temperature, low drift

resistors should be used. Making R4 Slightly smaller than R

2 and adding a trim pot equal to twice the difference between

R 2 and R4 will allow the CMRR to be adjusted for optimum.

DS100920-13

FIGURE 8. Single-Supply Inverting Amplifier

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(Ï â o) is defined by âR3/R1. This allows

low-frequency gains other than unity to be obtained. The fil-

ter has a â20dB/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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