LM3916 datasheet(12/27 Pages) NSC

12 / 27 page

Application Hints (Continued)

both drivers (10V in the example) and

amplify the input sig-

nal by 16 dB ahead of the LM3915. Alternatively, instead of

amplifying, input signals of sufficient amplitude can be fed di-

rectly to the LM3916 and

attenuated by 16 dB to drive the

LM3915.

To extend this approach to get a 70 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 2 mV! Several

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 Driver

The equations in

Figure 10 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 1

DS007971-15

VREF2 ≅ 7.89V

FIGURE 8. Low Cost Circuit for 40 dB Display

DS007971-16

FIGURE 9. Improved Circuit for 40 dB Display

LM3916 Datasheet(PDF) 12 Page - National Semiconductor (TI)