LM3916 datasheet(10/27 Pages) NSC

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Application Hints (Continued)

For precision full-wave averaging use the circuit in

Figure 5.

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 ±1dBer-

ror when the input is a nonsymmetrical transient). The aver-

aging time constant is R5

•C2. A simple modification results

in the precision full-wave detector of

Figure 6. Since the filter

capacitor is not buffered, this circuit can drive only high im-

pedance loads such as the input of an LM3916.

AUDIO METER STANDARDS

VU Meter

The audio level meter most frequently encountered is the VU

meter. Its characteristics are defined as the ANSI specifica-

tion C165. The LM3916’s outputs correspond to the meter

indications specified with the omission of the −2 VU indica-

tion. The VU scale divisions differ slightly from a linear scale

in order to obtain whole numbers in dB.

Some of the most important specifications for an AC meter

are its dynamic characteristics. These define how the meter

responds to transients and how fast the reading decays. The

VU meter is a relatively slow full-wave averaging type, speci-

fied to reach 99% deflection in 300 ms and overshoot by 1 to

1.5%. In engineering terms this means a slightly under-

damped second order response with a resonant frequency

of 2.1 Hz andaQof 0.62.

Figure 7 depicts a simple rectifier/

filter circuit that meets these criteria.

DS007971-11

D1, D2: 1N914 or 1N4148

Average

Peak

100k

R1 = R2 for AV = 1

R1 = R2/10 for AV = 10

C1 = 10/R1

FIGURE 4. Precision Half-Wave Rectifier

DS007971-12

D1, D2: 1N914 or 1N4148

FIGURE 5. Precision Full-Wave Average Detector

DS007971-13

D1, D2, D3, D4: 1N914 OR 1N4148

Attack and decay time to DIN PPM spec. Response down 1 dB for 10 ms

tone burst. Decays 20 dB in 1.5s.

FIGURE 6. Precision Full-Wave Peak Detector

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