Circuit Simulation

Circuit simulation is performed identically to single device simulation. The circuit usually has more inputs and outputs defined than a single device. In addition, the simulated circuit can use independent or controlled voltage and current sources that are defined within the Circuit Editor. When IC-CAP simulates a circuit, it first builds a complete SPICE deck from the circuit description and the setup table. The source names are built from the source type (V or I) and the nodes to which they are connected. Use of the simulation debugger can improve efficiency in performing successful simulations. Knowledge of how IC-CAP interacts with the SPICE simulators gives you more control over the options available for circuit simulation. For more information, refer to Chapter 3, "SPICE Simulators," in the Reference.

One of the advantages of simulating circuits through IC-CAP is the increased levels of analysis available. IC-CAP allows a sweep of more parameters than with a stand-alone SPICE simulator. For example, it is possible to simulate a circuit's behavior over bias conditions, component values and temperature in the same simulation. Once a simulation is complete, you may further analyze the stimulus and response data with IC-CAP's numerical and graphic capabilities. The two-port simulation features enable you to study the high-frequency characteristics of a circuit using any of the S, H, Y, or Z 2-port parameters.

Design Optimization

Designing a circuit usually follows a path of defining a block level functional description, translating it into discrete circuit components, then optimizing those components for the required performance. This last stage can be simplified with the IC-CAP system. It is possible to specify the desired performance from a circuit and then let the IC-CAP optimizer find the best component and model parameters to satisfy it. The following is an overview of how to do this.

1	Enter a circuit with a first order estimate of the required parameters and component values.

2	Create a setup with the inputs and outputs that simulate the desired region of performance.

3	Simulate the circuit to create a set(s) of output data.

4	Copy the simulated data into the measured data set(s) in the outputs.

a

Type the letter S in the Type field and press Return.

b

Type the letter B in the same field to replace the letter S.

5	Save the desired outputs to files using the Write to File menu choice on each output.

6	Edit the files using any editor. Scan down the file to find the type MEAS data section. This is where the measured data begins. Edit the output values, replacing them with the desired performance values for the circuit. Save the file when done.

7	Read the file (and thus the new data) back into the outputs in IC-CAP using the Read from file command on the desired outputs.

8	Set up an optimization transform to find the required parameter and component values that best match the new measured data.

This type of design optimization can save many hours of iteration in fine-tuning high-performance circuit designs.