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LPV358M Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LPV358M Datasheet(HTML) 11 Page - National Semiconductor (TI) |
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11 / 21 page  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 R 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 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 7). As in the three-op-amp circuit, this instrumentation amplifier requires precise resistor matching for good CMRR. R 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. R 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 www.national.com 11 |
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