LM13600 datasheet(18/24 Pages) NSC

18 / 24 page

Additional Applications (Continued)

into capacitor C whenever the input to A1 is brought high,

giving a ramp-rate of about 1 V/ms for the component values

shown.

The true-RMS converter of

Figure 27 is essentially an auto-

matic gain control amplifier which adjusts its gain such that

the AC power at the output of amplifier A1 is constant. The

output power of amplifier A1 is monitored by squaring ampli-

fier A2 and the average compared to a reference voltage

with amplifier A3. The output of A3 provides bias current to

the diodes of A1 to attenuate the input signal. Because the

output power of A1 is held constant, the RMS value is con-

stant and the attentuation is directly proportional to the RMS

value of the input voltage. The attenuation is also propor-

tional to the diode bias current. Amplifier A4 adjusts the ratio

of currents through the diodes to be equal and therefore the

voltage at the output of A4 is proportional to the RMS value

of the input voltage. The calibration potentiometer is set such

that V

O reads directly in RMS volts.

DS007980-32

FIGURE 25. Sample-Hold Circuit

DS007980-33

FIGURE 26. Ramp and Hold

DS007980-34

FIGURE 27. True RMS Converter

LM13600 Datasheet(PDF) 18 Page - National Semiconductor (TI)