Impulse Invariant Transform


Introduction

The impulse invariant transform (IIT) is a method of taking a continuous-time system H(s) and converting it to a discrete-time system. There are multiple ways of doing this, but the IIT does so with the constraint that the impulse response of the discrete-time system is a sampled version of the impulse response of the continuous-time system.

Here’s an illustration:

…gets converted to…



…with the characteristic that the discrete-time impulse response is a sampled version of the continuous-time:


Rational Systems

This doesn’t seem like a big deal—nor very accurate. The illustration above implies that we’re taking an IIR response from the continuous-time system and sampling as a discrete-time FIR.

However, the IIT actually does something better: if H(s) is rational (composed of a numerator and denominator):

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…then the IIT lets us re-write this system as a discrete-time rational system:

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…of course, with the property that Cannot render equation. Use Firefox instead.. Cannot render equation. Use Firefox instead. is the sampling period (typically, during transformation from continous-time to discrete-time, the continuous-time impulse response is scaled by the sample period).


Example

Here’s an example:

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First, we break it down into its partial fraction expansion:

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Solving (taking limits of s to +j and -j):

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Now, the IIT prescribes how to take single-pole transfer functions and convert them to the z-domain.

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…which means that:

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Application

The impulse invariant transform is useful in modelling continuous-time sigma-delta, allowing one to analyze the mixed-mode continuous-time sigma-delta as a purely discrete-time system.

Most notably, in considering the stability of continuous-time sigma-delta ADC’s, the transformation is useful in allowing one to replace the continuous-time noise-shaping filter with a discrete-time equivalent. One could then perform a closed-loop analytic analysis on the system. This is the procedure advocated in Delta-Sigma Data Converters: Theory, Design, and Simulation. However, the book only prescribes a rule-of-thumb, and in general, one must simulate the sigma-delta rigorously to gain confidence of stability.

Arguably, this transformation was more useful in the past, when high-level mixed-mode simulators (Simulink) were not available. In that case, the only way to simulate the continous-time sigma-delta was to model at a discrete-time. Nowadays (in my experience) Simulink is fast enough that it’s easier to keep the mixed-mode nature of the system intact (i.e. not model it as a purely discrete-time system). However, I can imagine a case of either very long simulations or a regression analysis system where the cycle-accurate discrete-time model may become useful again.


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One Comment

  1. Saumil
    Posted April 28, 2009 at 10:51 am | Permalink

    Good Explanation. Thanks.
    Can you also explain the sampling theorem funda which pops up in IIT (related to the primary and complimentary strips in the s domain when mapped into the z domain).

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