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…
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):
…then the IIT lets us re-write this system as a discrete-time rational system:
…of course, with the property that . is the sampling period (typically, during transformation from continous-time to discrete-time, the continuous-time impulse response is scaled by the sample period).
Here’s an example:
First, we break it down into its partial fraction expansion:
Solving (taking limits of s to +j and -j):
Now, the IIT prescribes how to take single-pole transfer functions and convert them to the z-domain.
…which means that:
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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