Tuesday, March 22, 2011

Build accurate Spice models for low-noise, low-power precision amplifiers

Build accurate Spice models for low-noise, low-power precision amplifiers: "
System engineers require accurate models for all types of ICs, and they require Spice models to run comprehensive circuit simulations. Early Spice models had few nonlinear elements, minimizing simulation time at the cost of accuracy, but new methods let you increase the number of nonlinear elements and improve accuracy. You can create a multistage model for low-noise, low-power operational amplifiers. The model employs work from Analog Devices (Reference 1) and requires several architectural changes to model a low-noise, low-power precision amplifier. The model architecture processes the input signal through eight stages. You can easily calculate the parameters for the eight stages with a handheld calculator. To understand the model creation, you must have experience using Spice.

Although higher-speed amplifiers have multiple poles and zeros, this model is for a single-pole, 10-MHz amplifier. It lets you simulate the amplifier’s key ac and dc parameters. The model includes ac parameters for flicker and flatband noise, slew rate, CMRR (common-mode rejection ratio), gain, and phase. The model’s dc parameters are VOS (input offset voltage), IOS (input offset current), quiescent supply current, and output-voltage swing. The model uses the 25°C typical parameters (Reference 2). The closer you model the input stage to the actual amplifier, the more accurate your results will be. You can achieve an accurate ac representation of the amplifier’s performance using a few of the process parameters of the input-stage transistors or MOSFETs. This model’s architecture lets you model amplifiers with split supplies. There is no ground reference in any of the signal-processing blocks. After the differential-to-single-ended conversion, all internally generated node voltages are referenced to the midpoint of the power supplies, much like the actual operation of an amplifier."

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