Many diagnostic systems require the capturing and analysis of a peak value of an input waveform. In an analog environment, a conventional Sample and Hold circuit (S/H) is commonly used to determine the peak value. However, S/H circuits typically have associated minimum input voltage requirements. In many diagnostic applications, where the input signal has a peak value below the minimum required input voltage, inaccuracies result due to distortion of the S/H circuit. Specifically, below the minimum input voltage, the input signal tends to become distorted by the noise of the S/H circuit, that might be comparable in magnitude to the peak, as shown by FIG. 1. As a result, the S/H circuit is incapable of reliably detecting peak values. Moreover, for waveforms having a first portion having a first rate of change and a second portion having a relatively faster rate of change, (or vice versa) as shown by FIG. 1, the S/H circuit may be incapable of capturing the peak values due to the second portion whose rate of change cannot be detected by the current.
In an analog environment, one solution to overcome the problem is to amplify the signal at a predetermined gain so that the S/H circuit functions reliably. Alternatively, a phase shift circuit may be implemented to detect the falling edge of the input signal, VIN. However, as phase shift circuits are plagued with many of the same problems as S/H circuits, they often tend to fail to function reliably. The remaining corrective measure is amplification of the input signal. In situations where power consumption must be kept to a minimum, such as battery powered diagnostic equipment, amplifying input signals consumes unnecessary amounts of power.
One way to address this problem is by digitally signal sampling the waveform with the use of an analog-to-digital (A/D) converter in order to capture the peak value. As illustrated by FIG. 2, the waveform falls abruptly at a rate faster than the sampling rate. In this regard, a peak value is calculated from the rising edge only. However, as a sample reading does not always capture the peak value, results will tend to be inaccurate. A high sampling rate may be used to overcome this problem, but is undesirable in many diagnostic system applications, as fast A/D converters cost more and require more power to operate. Especially for battery powered diagnostic systems in the field, fast A/D converters are not practical.
Accordingly, a need arises to reliably detect peak voltages of a waveform, especially, an input signal level having high rates of change and/or are low in magnitude.