AN1556.0 Datasheet, PDF (1/10 Pages) Intersil Corporation – Building an Accurate SPICE Model for Low Noise

Application Note 1556
Author: Don LaFontaine
Building an Accurate SPICE Model for Low Noise,
Low Power Precision Amplifiers
Abstract
In today's fast moving competitive markets, more and
more customers are requesting SPICE models to run
comprehensive circuit simulations. System engineers are
requiring increasingly accurate models for all types of
integrated circuits. Earlier SPICE models (1980) had to
minimize the number of nonlinear elements to minimize
simulation time, all at the cost of accuracy. Today's
models, thanks to the advancement of computing power,
can increase the number of nonlinear elements and
improve the accuracy of the models. The focus of this
Application Note is to provide a method for developing a
multi-stage SPICE model for low noise and low power
operational amplifiers. The model presented, started with
the work from Mark Alexander and Derek F. Bowers from
Analog Devices (Appnote AN-138, 1990) [1]. The final
model ended up with several key architectural changes
that were required to model today's low noise, and low
power precision amplifiers.
This application note provides a systematic process that
simplifies the understanding of how to build an accurate
straightforward SPICE model. This is accomplished by a
model architecture that processes the input signal
through several stages. The model parameters can easily
be calculated using a hand calculator or Excel
spreadsheet. The application note does not discuss the
process of using SPICE, and assumes the user is familiar
with this software.
The model presented in this application note is the
ISL28127 single-pole 10MHz amplifier. The model
enables the user to simulate important AC and DC
parameters of an amplifier. For higher speed amplifiers,
with multiple poles and zeros, reference AN-138 [1].
The AC parameters incorporated into the model are: 1/f
and flat-band noise, slew rate, CMRR, gain and phase.
The DC parameters are VOS, IOS, total supply current
and output voltage swing. The model uses typical
(+25°C) parameters given in the “Electrical
Specifications” table of the data sheet [2].
Introduction
The key to an accurate model is the input stage. The
closer you model the input stage to the actual amplifier,
the better your results. With only a few of the process
parameters of the input stage transistors or MOSFETs,
you can achieve very accurate AC representation of the
amplifiers performance.
Another advantage of this model's architecture is the
ability to model amplifiers with split supplies. There is no
ground reference in any of the signal processing blocks.
Instead, after the differential to single-ended conversion,
all internally generated node voltages are referenced to
the mid point of the supplies, much like the actual
operation of an amplifier.
Discussed in this application note are the following
topics:
1. The different cascaded stages of the SPICE Model:
- Voltage Noise Stage
- Input Stage
- 1st Gain Stage
- 2nd Gain Stage
- Mid Supply Stage
- Supply Isolation Stage
- Common Mode Gain Stage
- Output Stage
2. How the VCCS stages works
3. How the VCCS output stage works
4. Systematic process for calculating model
parameters
5. Simulation results. Actual device vs simulation
6. Conclusions
Cascaded Stages
Figure 1 is the schematic for the SPICE model and
Figure 2 is the net list. Notice from the schematic, the
only circuitry resembling an amplifier is the Input Stage.
All other stages process the input signal with Voltage
Controlled Current Sources (VCCS) and Voltage
Controlled Voltage Sources (VCVS) along with diodes, DC
supplies, simple resistors, capacitors and inductors.
The circuit schematic is built from eight different
functional blocks. Each block is discussed in the following
sections, with details of the blocks’ functionality and
design considerations.
April 19, 2010
AN1556.0
1
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
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