Inside each PNI Sensor’s ASIC is a measurement systems designed to work with PNI’s MI-sensors, such as the RM3100 magnetometer sensor. PNI’s sensors when used with our ASIC drive circuit respond like an external magnet field dependent inductor. This inductance is measured by creating an L/R oscillator. The frequency of this “sensor oscillator” is also dependent on the external bias resistors and where chosen to give the best dynamic range and linearity. Since the time constant of an L/R oscillator is proportional to the inductor, the inductance of the RM3100 sensor in this case, which itself is proportional to the external magnetic field, the period of oscillation of the sensor oscillator is then proportional to the external magnetic field incident to the sensor. In PNI Sensor’s measurement circuit, the period is measured over multiple cycles of the sensor oscillator waveform, which is selectable via the Cycle Count Register. The timebase used in PNI’s ASICs for period measurement is a precision high-speed oscillator, which is dependent on the external timing resistor (33.2k-Ohm 1%).
The Cycle Count Register (CCR) is basically the only control the user needs to adjust. Both the R-Bias resistors and R-Ext resistor values of 120/121-Ohms and 33k/33.2k-Ohms respectively are optimized for best overall performance. The CCR affects both the gain and measurement interval time. A larger CCR value results in larger gain and longer measurement interval. A cycle count value of 200 typically yields the best noise to measurement interval performance trade-off. Noise will continue to reduce with increasing CC values, however, it will not decrease as much as the measurement interval increases in contrast to the rate of change of both parameters up to 200CC. At 200CC the typical gain is 75 counts/uT, or 13nT resolution with a measurement interval of just under 22ms per axis with a noise performance of typically 15nT RMS. Using a CCR value of less than 200 will yield faster measurements and the resolution will become larger than the noise floor of the sensor. A good CCR value for high-speed measurements is 50. This will yield 50nT resolution in about 625uS/Axis.
Please refer to the attached white paper for more information.