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LMV721 Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LMV721 Datasheet(HTML) 11 Page - National Semiconductor (TI) |
11 / 15 page Application Notes (Continued) 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 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 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 filter has a −20dB/decade roll-off after its corner frequency fc. R 2 should be chosen equal to the parallel combination of R 1 and R3 to minimize error due to bias current. The fre- quency 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 frequency ( ≤ 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 inter- est at least 50 times larger than the gain of the filter at this frequency. 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. Here is a LMV721 used as a microphone preamplifier. Since the LMV721 is a low noise and low power op amp, it makes it an ideal candidate as a battery powered microphone preamplifier. The LMV721 is connected in an inverting con- figuration. Resistors, R 1 =R2 = 4.7k Ω, sets the reference half way between V CC = 3V and ground. Thus, this config- ures the op amp for single supply use. The gain of the preamplifier, which is 50 (34dB), is set by resistors R 3 = 10k Ω and R 4 = 500k Ω. The gain bandwidth product for the LMV721 is 10 MHz. This is sufficient for most audio applica- tion since the audio range is typically from 20 Hz to 20kHz. A resistor R 5 =5k Ω is used to bias the electret microphone. Capacitors C 1 =C2 = 4.7µF placed at the input and output of the op amp to block out the DC voltage offset. 10092223 FIGURE 8. Single-Supply Inverting Amplifier 10092224 FIGURE 9. Simple Low-Pass Active Filter 10092225 FIGURE 10. Frequency Response of Simple Low-pass Active Filter in Figure 9 10092244 FIGURE 11. A Battery Powered Microphone Preamplifier www.national.com 11 |
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