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 (VOL). Otherwise, the output is equal to the Low level output voltage (VOH).

The output model is shown in the following illustration.

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:

  1. Connect the output pin to the base of an NPN Bipolar Transistor or PNP Bipolar Transistor block.

  2. 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, VOL, 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, VOH, 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 (VOHVOH1)/IOH1, where VOH1 is the reduced output high voltage when the output current is IOH1. 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.

Ports

This block has the following ports:

+

Positive electrical input port

-

Negative electrical input port

OUT

Electrical output port

See Also

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