AD707

By using a pulse-width-height modulation technique, this circuit implements a 0.015%-accurate multi-plier. The circuit's output equals V_XV_Y/10. An AD581 voltage reference, an AD630 commutating amplifier, and an integrator comprising an AD707 op amp, 2000-pF capacitor, and 150-kΩ resistor first generate a precision triangle wave. For a given state of the AD630's output (+V_REF at TP₁, for example) the integra-tor ramps until its output reaches -11V. Then, TP) changes state and the integrator begins ramping toward +11V. The triangle wave's period is 4.4RC or 1.32ms, where R and C are the values of the integrator components.

The circuit uses a second AD630, driven by the variable V_X to compare the triangle waveform at TP₂ to the signal at V_Y The duty cycle, T₁+T₂, at the output of this second commutating amplifier is:

During T₁, the voltage at TP₄ equals -1.1V_X. During the remaining period, T₂, the pulse height will equal+1.1V_X. V_OUT is the average, obtained by low-pass filtering, of this T₁ and T₂ combined waveform and equals:

You can use a higher bandwidth filter and a higher carrier frequency to build a faster multiplier.