LM3915 datasheet(10/25 Pages) NSC

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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

100k

R1 = R2 for AV =1

R1 = R2/R10 for AV =10

C1 = 10/R1

FIGURE 2. Precision Half-Wave Rectifier

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