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Comparator - Model a comparator behaviorally

Library

Integrated Circuits

Description

The Comparator block is an abstracted behavioral model of a comparator integrated circuit. It does not model an internal transistor-level implementation. Therefore, the block runs quickly during simulation but retains the correct I/O behavior. The block models differential inputs electrically as having infinite resistance and a finite or zero capacitance.

The block models the gate output as a voltage source driving a series resistor and a capacitor that connects to ground. The output pin connects to the resistor-capacitor connection node. If the difference in the inputs is greater than the input threshold voltage, then the output is equal to the High level output voltage ( ). Otherwise, the output is equal to the Low level output voltage ( ).

Basic Assumptions and Limitations

Modeling of the output as a controlled voltage source is representative of a totem-pole or push-pull output stage. To model a device with an open-collector:

Connect the output pin to the base of an NPN Bipolar Transistor or PNP Bipolar Transistor block.
Set the Output resistance parameter to a suitable value.

Dialog Box and Parameters

Inputs Tab

Input offset voltage: The voltage which the difference in the input voltages must be greater than so that the comparator gives a logic output 1. The default value is 5 mV.
Average input capacitance: You can usually find this capacitance value on a manufacturer datasheet. The default value is 0 pF. Setting this value to zero can result in faster simulation times.

Outputs Tab

Low level output voltage: The steady-state output voltage, , when the voltage difference across the inputs is less than or equal to the threshold voltage, and the output current is zero. The default value is 0 V.
High level output voltage: The steady-state output voltage, , when the voltage difference across the inputs is greater than the threshold voltage, and the output current is zero. The default value is 5 V.
Output resistance: This parameter is the ratio of output voltage drop to output current. Set this parameter to , where is the reduced output high voltage when the output current is . The default value is 50 Ω.
Propagation delay: Set this value based on the high-to-low and low-to-high propagation delays. The default value is 0 s.