Selection and integration of MEMS-based motion processing devices

1: Filtering considerations
Flexible filtering is generally required in motion applications since noise and signal bandwidth requirements typically vary with the action being performed. There are two primary filtering techniques: 1) in the analog domain, typically performed with the analog-to-digital converter (ADC) anti-aliasing filter or an RC (resistor, capacitor) circuit, and 2) in the digital domain, performed with a digital processor after the system ADC. The analog filter is always required to prevent aliasing.

For motion processing applications with variable bandwidths, the optimal choice is to include a programmable digital filter after the analog filtering. As an example of variable bandwidth requirements, consider a gaming application. Certain sports games frequently contain fast movements which demand wider bandwidths. In other games which require drawing or selecting on-screen menu options, higher precision is necessary where a narrower bandwidth and lower noise are preferred.

Motion processing equipped mobile devices will enable multiple applications such as gaming, camera image stabilization, user interface and automotive navigation, each requiring different signal bandwidths. For example, to capture a user's gaming motions up to 10-Hz signal frequencies, a sampling rate of 200 Hz may be required which results in the need for a low-pass filter (LPF) below 100 Hz based on the Nyquist rule (which calls for removal of any signal that is greater than or equal to half the desired sampling frequency). Similarly, to gather automotive navigation heading signals up to 1 Hz, a sampling frequency of 10 Hz with an LPF cutoff frequency of less than 5 Hz is required.

The challenge for anti-aliasing of multiple motion processing functions is that the 100-Hz LPF suitable for gaming applications may result in undesirable noise for the navigation application, while a 5-Hz LPF is too low for gaming and introduces latency. The solution for those applications requiring variable filter bandwidths is to use an anti-aliasing filter that meets the widest bandwidth requirement (gaming in this case), while using a programmable digital filter to meet those applications with more stringent noise requirements.

When selecting a non-integrated motion processing solution (see Figure 3 below), a dedicated microcontroller may be required that would continuously sample at 100 Hz using a 5-Hz digital LPF within the device and output at 10 Hz. The drawback is the additional cost of a microcontroller which precludes their use in cost-sensitive CE applications.

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As part of its signal processing, the fully integrated six-axis motion processing solution (see Figure 4 below) has fixed-frequency anti-aliasing filters as part of its ADC block, followed by programmable digital LPFs which negate the need for external signal conditioning and microcontrollers.

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