Application Hints (Continued)
DS005104-13
FIGURE 5. Low Cost Circuit for 60 dB Display
DS005104-14
FIGURE 6. Improved Circuit for 60 dB Display
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.
450 µA to flow from pin 7 into the divider which means that
the LED current will be at least 5 mA. R1 will typically be be-
tween 1 kΩ and 2 kΩ. To trim the reference voltage, vary R2.
The circuit in Figure 8 shows how to add a LED intensity con-
trol which can vary LED current from 9 mA to 28 mA. The ref-
erence adjustment has some effect on LED intensity but the
reverse is not true.
TIPS ON REFERENCE VOLTAGE
AND LED CURRENT PROGRAMMING
MULTIPLE LM3915s
Figure 9 shows how to obtain a common reference trim and
intensity control for two LM3915s. The two ICs may be con-
nected in cascade for a 60 dB display or may be handling
separate channels for stereo. This technique can be ex-
tended for larger numbers of LM3915s by varying the values
of R1, R2 and R3 in inverse proportion to the number of de-
vices tied in. The ICs’ internal references track within
100 mV so that worst case error from chip to chip is only
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
=
0.1 dB for VREF 10V.
11
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