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LM3915N Datasheet(PDF) 10 Page - National Semiconductor (TI) |
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LM3915N Datasheet(HTML) 10 Page - National Semiconductor (TI) |
10 / 25 page Application Hints (Continued) Power dissipation, especially in bar mode should be given consideration. For example, with a 5V supply and all LEDs programmed to 20 mA the driver will dissipate over 600 mW. In this case a 7.5 Ω resistor in series with the LED supply will cut device heating in half. The negative end of the resistor should be bypassed with a 2.2 µF solid tantalum capacitor to pin 2. TIPS ON RECTIFIER CIRCUITS The simplest way to display an AC signal using the LM3915 is to apply it right to pin 5 unrectified. Since the LED illumi- nated represents the instantaneous value of the AC wave- form, one can readily discern both peak and average values of audio signals in this manner. The LM3915 will respond to positive half-cycles only but will not be damaged by signals up to ±35V (or up to ±100V if a 39k resistor is in series with the input). It’s recommended to use dot mode and to run the LEDs at 30 mA for high enough average intensity. True average or peak detection requires rectification. If an LM3915 is set up with 10V full scale across its voltage divider, the turn-on point for the first LED is only 450 mV. A simple silicon diode rectifier won’t work well at the low end due to the 600 mV diode threshold. The half-wave peak detector in Figure 1 uses a PNP emitter-follower in front of the diode. Now, the transistor’s base-emitter voltage cancels out the diode offset, within about 100 mV. This approach is usually satisfactory when a single LM3915 is used for a 30 dB display. Display circuits using two or more LM3915s for a dynamic range of 60 dB or greater require more accurate detection. In the precision half-wave rectifier of Figure 2 the effective diode offset is reduced by a factor equal to the open-loop gain of the op amp. Filter capacitor C2 charges through R3 and discharges through R2 and R3, so that appropriate selection of these values results in either a peak or an average detector. The circuit has a gain equal to R2/R1. It’s best to capacitively couple the input. Audio sources frequently have a small DC offset that can cause significant error at the low end of the log display. Op amps that slew quickly, such as the LF351, LF353, or LF356, are needed to faithfully respond to sudden transients. It may be necessary to trim out the op amp DC offset voltage to accurately cover a 60 dB range. Best results are obtained if the circuit is adjusted for the correct output when a low-level AC signal (10 mV to 20 mV) is applied, rather than adjusting for zero output with zero input. For precision full-wave averaging use the circuit in Figure 3. Using 1% resistors for R1 through R4, gain for positive and negative signal differs by only 0.5 dB worst case. Substitut- ing 5% resistors increases this to 2 dB worst case. (A 2 dB gain difference means that the display may have a ±1dB error when the input is a nonsymmetrical transient). The averaging time constant is R5–C2. A simple modification results in the precision full-wave detector of Figure 4. Since the filter capacitor is not buffered, this circuit can drive only high impedance loads such as the input of an LM3915. 00510409 *DC Couple FIGURE 1. Half-Wave Peak Detector 00510410 D1, D2: 1N914 or 1N4148 Average Peak R2 1k 100k R3 100k 1k R1 = R2 for AV =1 R1 = R2/R10 for AV =10 C1 = 10/R1 FIGURE 2. Precision Half-Wave Rectifier www.national.com 10 |
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