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Application Notes (Continued)

4.2 Instrumentation Circuits

The input impedance of the previous difference amplifier is

set by the resistor R

1,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

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

Ω. The gain of this instrumen-

tation amplifier is set by the ratio of R

2/R 1.R3 should equal

1 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 R

4 Slightly smaller than R

and adding a trim pot equal to twice the difference between

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 7). 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-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 R1 and C1 affect the cutoff fre-

quency, fc = 1/2

π 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.

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(

ω → o) is defined by −R

3/R1. 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.

2 should be chosen equal to the parallel combination of R1

and R

3 to minimize errors due to bais current. The frequency

response of the filter is shown in

Figure 10

DS100920-7

FIGURE 5. Difference Amplifier

DS100920-85

FIGURE 6. Three-op-amp Instrumentation Amplifier

DS100920-11

FIGURE 7. Two-op-amp Instrumentation Amplifier

DS100920-13

FIGURE 8. Single-Supply Inverting Amplifier

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