LM13700M datasheet(21/32 Pages) TI1

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LM13700

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SNOSBW2E – NOVEMBER 1999 – REVISED MARCH 2013

The Ramp-and-Hold of Figure 47 sources IB into capacitor C whenever the input to A1 is brought high, giving a

ramp-rate of about 1V/ms for the component values shown.

The true-RMS converter of Figure 48 is essentially an automatic 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 amplifier 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 constant and the attenuation is directly proportional to the RMS value of the input

voltage. The attenuation is also proportional 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 VO reads directly in RMS volts.

Figure 46. Sample-Hold Circuit

Figure 47. Ramp and Hold

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LM13700M Datasheet(PDF) 21 Page - Texas Instruments