LM3915 datasheet(11/25 Pages) NSC

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Application Hints (Continued)

CASCADING THE LM3915

To display signals of 60 dB or 90 dB dynamic range, multiple

LM3915s can be easily cascaded. Alternatively, it is possible

to cascade an LM3915 with LM3914s for a log/linear display

or with an LM3916 to get an extended range VU meter.

A simple, low cost approach to cascading two LM3915s is to

set the reference voltages of the two chips 30 dB apart as in

Figure 5. Potentiometer R1 is used to adjust the full scale

voltage of LM3915 #1 to 316 mV nominally while the second

IC’s reference is set at 10V by R4. The drawback of this

method is that the threshold of LED #1 is only 14 mV and,

since the LM3915 can have an offset voltage as high as

10 mV, large errors can occur. This technique is not recom-

mended for 60 dB displays requiring good accuracy at the

first few display thresholds.

A better approach shown in Figure 6 is to keep the reference

at 10V for both LM3915s and amplify the input signal to the

lower LM3915 by 30 dB. Since two 1% resistors can set the

amplifier gain within ±0.2 dB, a gain trim is unnecessary.

However, an op amp offset voltage of 5 mV will shift the first

LED threshold as much as 4 dB, so that an offset trim may

be required. Note that a single adjustment can null out offset

00510411

D1, D2: 1N914 or 1N4148

FIGURE 3. Precision Full-Wave Average Detector

00510412

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

FIGURE 4. Precision Full-Wave Peak Detector

LM3915 Datasheet(PDF) 11 Page - National Semiconductor (TI)