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Resettable Integrator Macro
The Int macro within Micro-Cap is one of the most useful macros for system modeling. It pro-
duces an output signal which is the integral of the input signal to the macro. With the current Int
macro, the output is the integral of the waveform over the entire period of the simulation. There is
no means of resetting the macro. This article will describe a revised macro circuit that adds a
resetting capability to the integrator macro.
The standard integrator macro that is distributed with Micro-Cap is shown in Figure 1. Two
parameters are passed to this macro: SCALE and VINIT. The SCALE parameter multiplies the
integral by its defined value, and the VINIT parameter defines an initial value for the integration.
Fig. 1 - Standard Integrator macro circuit
The operation of the macro is fairly simple and takes advantage of the integrated nature of a
capacitive voltage. The input voltage to the macro is converted by the G1 source into an equiva-
lent current multiplied by the SCALE parameter. This current is then used to charge a capacitor.
Since the value of the capacitor is 1 Farad, the voltage across it will be the integral of the current
though the capacitor. The VINIT parameter is used to set the initial condition of the capacitor in
order to provide an initial value that the integrated voltage is then built upon. The parallel resistor
keeps the voltage finite when there is a DC voltage input. The E1 source then provides a buffered
output which reproduces the integrated voltage across the capacitor at the output pin.
The standard integrator macro will be used as the basis for the resettable integrator macro. The
resettable integrator macro circuit is shown in Figure 2. It passes the same two parameters,
SCALE and VINIT, as the standard integrator macro. The basic operation of each of the macros
is the same.
