amplifying, input signals of sufficient amplitude can be fed di-
rectly to the LM3916 and attenuated by 16 dB to drive the
LM3915.
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
DS007971-15
V
7.89V
REF2
FIGURE 8. Low Cost Circuit for 40 dB Display
DS007971-16
FIGURE 9. Improved Circuit for 40 dB Display
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
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