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LM3916N-1 bảng dữ liệu(PDF) 11 Page - National Semiconductor (TI) |
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LM3916N-1 bảng dữ liệu(HTML) 11 Page - National Semiconductor (TI) |
11 / 27 page Application Hints (Continued) Peak Program Meter The VU meter, originally intended for signals sent via tele- phone lines, has shortcomings when used in high fidelity systems. Due to its slow response time, a VU meter will not accurately display transients that can saturate a magnetic tape or drive an amplifier into clipping. The fast-attack peak program meter (PPM) which does not have this problem is becoming increasingly popular. While several European organizations have specifications for peak program meters, the German DIN specification 45406 is becoming a de facto standard. Rather than respond instantaneously to peak, however, PPM specifications re- quire a finite “integration time” so that only peaks wide enough to be audible are displayed. DIN 45406 calls for a re- sponse of 1 dB down from steady-state for a 10 ms tone burst and 4 dB down fora3ms tone burst. These require- ments are consistent with the other frequently encountered spec of 2 dB down fora5ms burst and are met by an attack time constant of 1.7 ms. The specified return time of 1.5s to −20 dB requires a 650 ms decay time constant. The full-wave peak detector of Figure 6 satisfies both the attack and decay time criteria. Cascading The LM3916 The LM3916 by itself covers the 23 dB range of the conven- tional VU meter. To display signals of 40 dB or 70 dB dy- namic range, the LM3916 may be cascaded with the 3 dB/ step LM3915s. Alternatively, two LM3916s may be cascaded for increased resolution over a 28 dB range. Refer to the Ex- tended Range VU Meter and High Resolution VU Meter in the Typical Applications section for the complete circuits for both dot and bar mode displays. To obtain a display that makes sense when an LM3915 and an LM3916 are cascaded, the −20 dB output from the LM3916 is dropped. The full-scale display for the LM3915 is set at 3 dB below the LM3916’s −10 dB output and the rest of the thresholds continue the 3 dB/step spacing. A simple, low cost approach is to set the reference voltage of the two chips 16 dB apart as in Figure 5. The LM3915, with pin 8 grounded, runs at 1.25V full-scale. R1 and R2 set the LM3916’s reference 16 dB higher or 7.89V. Variation in the two on-chip references and resistor tolerance may cause a ±1 dB error in the −10 dB to −13 dB transition. If this is ob- jectionable, R2 can be trimmed. The drawback of the aforementioned approach is that the threshold of LED #1 on the LM3915 is only 56 mV. Since comparator offset voltage may be as high as 10 mV, large er- rors can occur at the first few thresholds. A better approach, as shown in Figure 9, is to keep the reference the same for DS007971-14 Design Equations GAIN R5 R6 C2 C3 1 100k 43k 2.0 0.56 µF 10 1M 100k 1.0 0.056 µF FIGURE 7. Full-Wave Average Detector to VU Meter Specifications* www.national.com 11 |
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Mô tả tương tự - LM3916N-1 |
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