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Suppress jitter in Hall-effect sensor design

Posted: 12 Mar 2015 ?? ?Print Version ?Bookmark and Share

Keywords:Hall-effect sensors? Functional safety? signal path? static switching? filtered sampling?

Hall-effect sensors are integral to the automotive industry. These are used to detect end-position or to measure linear or angular movements in an extremely wide range of chassis, safety, body, security, and powertrain applications. An important topic currently dominating R&D discussions in the automotive industry is Functional Safety. Functional safety affects the design and feature set of components in all application systems including Hall sensors.

Hall sensors are increasingly preferred sensing solutions in many applications thanks to their contactless measurement principle and resulting high reliability.

For instance, one trend shows the increasing use of Hall-effect sensors over mechanical switches due to their insensitivity against environmental conditions (e.g. dust, humidity, and vibrations), their constant measurement results under the harshest environmental temperature conditions from -40C up to 150C, and their highly repeatable operation without degradation over time. All while maintaining a high level of quality for unlimited use.

In order to fulfil this ever evolving safety and reliability feature, highest accuracy of the switching threshold is the essential parameter in the Hall switch specification.

The actual switching operation, triggered by a magnetic signal crossing the switching threshold, is affected by different effects like switching delay, sampling jitter and threshold noise. All of these are undesirable as an ideal switch should react instantaneously, however they cannot be completely avoided due to the Hall IC's internal signal processing.

In order to achieve an optimal switching performance, the signal processing inside the latest Hall-effect switch family from Micronas C the HAL 15xy family, was touted to be designed with the highest suppression of such negative side effects.

This paper provides insight to how the signal path design influences the jitter performance of the output signal and the diverse design approaches taken to solve this problem.

A brief introduction of the signal path and the static switching behaviour of a Hall switch are presented in the following section.

Signal path of Hall switches
The simplified signal path of a Hall switch consists of several basic components, as depicted in figure 1.

Figure 1: Simplified Hall switch signal path.

The integrated Hall sensor converts magnetic flux density into an electrical signal, the optional low-pass filter limits the signal bandwidth and the sampled or non-sampled comparator decides if the signal is above or below the currently active threshold.

A sampled comparator produces new decisions each time it is triggered by the sampling clock, while a non-sampled comparator operates continuously without trigger.

In case a low-pass filter is present, it suppresses frequency components above the useful signal bandwidth in order to lower noise contribution from these frequency ranges.

Most Hall sensor ICs, including the Micronas Hall switch families, employ the well-known spinning-current technique for superior offset performance. Figure 1 omits any spinning-current related blocks for simplicity.

Static switching behaviour with hysteresis
A Hall switch features two different magnetic thresholds, Bon and Boff, forming a hysteresis loop. This behaviour is necessary to avoid unwanted toggling or flickering, which would occur without hysteresis. Figure 2 shows a graph of the static output behaviour versus the magnetic flux density B, assuming a non-inverted output behaviour.

Figure 2: Static hysteresis loop of a Hall Switch.

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