Modeling designs help engineers verify whether their circuit will function as intended. Modeling programs simulate circuit behavior under different conditions and in accordance with the design requirements, allowing engineers to better plan and build circuits.
Typically, to model components, generic models are used, which produce inaccurate and unrealistic simulations based on ideal conditions. This can cause functionality issues to go undetected until far later in the design process. To confidently simulate an AND or NAND gate SPICE Model, create a model using specifications from a manufacturer’s datasheet.
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What is an AND or NAND Gate?
An AND Gate gives a high output when all the inputs are high while a NAND gate gives a low output when all the inputs are high.
AND Gate
AND gates are commonly used for:
- Boolean Multiplication
- Address Decoding
- Data Selecting
NAND Gate
NAND gates are commonly used for:
- Universal Gates
- Data Latching
- Error Detection
What is Needed for an AND or NAND Gate SPICE Model?
AND and NAND gate models are frequently used for digital applications as both components are essential building blocks in digital circuitry to create more complex digital circuitry and logic functions. To realistically simulate digital circuit functionality and create an AND or NAND SPICE model for simulation, there are three items that must be defined:
Logic Parameters
The inputs and outputs of the AND or NAND gate must be defined.
Timing Characteristics
Timing is critical to digital design operation and performance. Consider the delay caused by the AND or NAND gate.
DC Electrical Characteristics
The DC electrical characteristics define the operating conditions for the AND or NAND gate. This includes:
- The maximum output voltages during high-level and low-level operation
- The voltage levels or thresholds that control the switch from high-level operation to low-level operation
- Hysteresis is often defined for these devices to provide a cushion, ensuring accurate switching between low-level and high-level states even with noisy input signals. It is typically provided by device manufacturers on the device datasheet as VH; however, if not provided it can be calculated by VH = VT+ – VT-
Device Resistance
Input and output resistance of the device is critical to the operation and necessary to create an accurate model representation. If this parameter is not provided on the device datasheet, it can be calculated with Ohm’s law.
Input Resistance (RIN)
Use the maximum input voltage (VIN) and maximum leakage current (commonly IIN or Il) to calculate the input resistance.
Output Resistance (ROUT)
Use the maximum output voltage (VOH or VOL) and the maximum output current (IOH or IOL) to calculate the output resistance.
Once this information is obtained and calculated, these values must be incorporated into the SPICE simulation model which can be achieved by manually creating or editing a text file. Keep in mind if the AND or NAND gate created does not produce the intended outcome and a decision is made to change components, values will need to be re-calculated and edited manually. This manual process to produce an accurate AND or NAND gate model is time consuming and increases the likelihood of errors; however, the PSpice Modeling App provides a fast, easily configurable, and fully integrated method to create buffer or inverter models for simulation.
Creating AND and NAND Gate SPICE Models in PSpice
The AND/NAND Gate modeling application quickly creates AND or NAND gate SPICE models with a wizard-based approach. To create AND or NAND gate models, users can easily input the characteristics, defined by manufacturers, directly into predefined parameters:
Number of Inputs
Define the number of inputs for the AND or NAND gate
Type of Output
Define the type of output for the model:
- Selecting Q will create an AND gate model
- Selecting Qbar will create a NAND gate model
- Selecting Both will create a model that includes an AND and NAND output
Number of Inverted Inputs
Define the number of inverted inputs for the AND or NAND gate
Propagation Delay
Define the time delay between the input and output pins of the device. This is typically defined as tPLH or tPHL on device datasheets.
Threshold (TH)
Define the voltage level that indicates a switch between high-level and low-level operation.
Hysteresis Width
Hysteresis is the difference between the positive input threshold voltage and the negative input threshold voltage. It is typically provided on the device datasheet as VH.
Output Resistance (ROUT)
Specify the resistance of the output pin.
Input Resistance (RIN)
Specify the resistance of the input pin.
Max High Voltage
Define the maximum voltage output during high-level operation. This is typically defined on device datasheets as VOH.
Max Low Voltage
Define the maximum voltage output during low-level operation. This is typically defined on device datasheets as VOL.
Using the inputted information above, the PSpice Modeling App generates a schematic symbol and automatically associates the newly created AND or NAND gate SPICE model without leaving the OrCAD Capture environment. The PSpice Modeling App also automatically manages the simulation profile configuration, eliminating any library set up for simulation.
Download the free trial of OrCAD to try it yourself.
Get the step-by-step instructions for creating an AND and NAND gate SPICE Model here.
