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LMV722M Datasheet(PDF) 10 Page - National Semiconductor (TI) |
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LMV722M Datasheet(HTML) 10 Page - National Semiconductor (TI) |
10 / 15 page Application Notes (Continued) 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 output 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-pass 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 error due to bais current. The frequency response of the filter is shown in Figure 10. Note that the single-op-amp active filters are used in to the applications that require low quality factor, Q( ≤ 10), low fre- quency ( ≤ 5KHz), and low gain (≤ 10), or a small value for the product of gain times Q( ≤ 100). The op amp should have an open loop voltage gain at the highest frequency of interest at least 50 times larger than the gain of the filter at this fre- quency. In addition, the selected op amp should have a slew rate that meets the following requirement: Slew Rate ≥ 0.5x(ω H VOPP)X10 −6V/µsec Where ω H is the highest frequency of interest, and VOPP is the output peak-to-peak voltage. DS100922-22 FIGURE 7. Two-op-amp Instrumentation Amplifier DS100922-23 FIGURE 8. Single-Supply Inverting Amplifier DS100922-24 FIGURE 9. Simple Low-Pass Active Filter DS100922-25 FIGURE 10. Frequency Response of Simple Low-pass Active Filter in Figure 9 www.national.com 10 |
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