Datasheet

LM13700
www.ti.com SNOSBW2E –NOVEMBER 1999–REVISED MARCH 2013
The Ramp-and-Hold of Figure 47 sources I
B
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 V
O
reads directly in RMS volts.
Figure 46. Sample-Hold Circuit
Figure 47. Ramp and Hold
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