LM3915 datasheet(12/25 Pages) NSC

12 / 25 page

Application Hints (Continued)

in both the precision rectifier and the 30 dB gain stage.

Alternatively, instead of amplifying, input signals of sufficient

amplitude can be fed directly to the lower LM3915 and

attenuated by 30 dB to drive the second LM3915.

To extend this approach to get a 90 dB display, another

30 dB of amplification must be placed in the signal path

ahead of the lowest LM3915. Extreme care is required as the

lowest LM3915 displays input signals down to 0.5 mV! Sev-

eral offset nulls may be required. High currents should not

share the same path as the low level signal. Also power line

wiring should be kept away from signal lines.

TIPS ON REFERENCE VOLTAGE

AND LED CURRENT PROGRAMMING

Single LM3915

The equations in Figure 7 illustrate how to choose resistor

values to set reference voltage for the simple case where no

LED intensity adjustment is required. A LED current of 10 mA

to 20 mA generally produces adequate illumination. Having

10V full-scale across the internal voltage divider gives best

accuracy by keeping signal level high relative to the offset

voltage of the internal comparators. However, this causes

450 µA to flow from pin 7 into the divider which means that

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FIGURE 5. Low Cost Circuit for 60 dB Display

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FIGURE 6. Improved Circuit for 60 dB Display

LM3915 Datasheet(PDF) 12 Page - National Semiconductor (TI)