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Application Notes (Continued)

4.2.1 Three-Op-Amp Instrumentation Amplifier

The quad LMV324 can be used to build a three-op-amp

instrumentation amplifier as shown in Figure 8.

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 100 M

Ω. The gain of this instrumen-

tation amplifier is set by the ratio of R2/R1. R

3 should equal

1, and R4 equal R2. Matching of R3 to R1 and R4 to R2

affects the CMRR. For good CMRR over temperature, low

drift resistors should be used. Making R

4 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.

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 9) . As in the three-op-amp circuit, this instrumentation

amplifier requires precise resistor matching for good CMRR.

4 should equal to R1 and R3 should equal R2.

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 R4 is

implemented to bias the amplifier so the input signal is within

the input common-mode voltage range of the amplifier. The

capacitor C

1 is placed between the inverting input and resis-

tor R

1 to block the DC signal going into the AC signal source,

IN. The values of R1 and C1 affect the cutoff frequency, fc =

1/2

πR

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.

4.4 ACTIVE FILTER

4.4.1 Simple Low-Pass Active Filter

The simple low-pass filter is shown in Figure 11. Its low-

frequency gain (

ω → 0) is defined by -R

3/R1. This allows

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

filter has a -20dB/decade roll-off after its corner frequency fc.

2 should be chosen equal to the parallel combination of R1

and R

3 to minimize errors due to bias current. The frequency

response of the filter is shown in Figure 12.

10006085

FIGURE 8. Three-op-amp Instrumentation Amplifier

10006011

10006035

FIGURE 9. Two-Op-amp Instrumentation Amplifier

10006013

10006020

FIGURE 10. Single-Supply Inverting Amplifier

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