To ensure a reliable reading an event must be sampled twice when the gear tooth covers more of the LSENSE coil than the LREF coil (LS<LR) producing a LOW output and equivalently twice when the gear tooth covers more of LREF than LSENSE (LR<LS) producing a HIGH output. The maximum speed can be achieved when the output toggles at a duty cycle rate of 50%. This can be achieved by using a gear where the width of each tooth is the same as the width of the gaps between the teeth. For asymmetric systems, the minimum width of either the gap or the gear tooth determines the maximum detectable speed. For symmetrical systems, the maximum rotational speed that can be reliably detected in revolutions per minute (rpm) for a given number of gear teeth can be determined by the following formula:

Equation 9.

where:

• Gear Speed (rpm) is the calculated speed of the gear
• ƒ_SENSOR is the sensor frequency given by Equation 6
• # gear teeth is the total number of events per rotation

A gear with 8 teeth and sensor frequency of 15MHz could reliably measure a gear rotational speed of 6500 rpm.