A modulated current is provided by the integrated rotational speed sensor KMI 15/x. This current signal must be converted to ground referenced voltage signal, matching the logic levels of the processing unit for digital signal processing. This signal conditioning can be done by using this circuit. This circuit consist of protective elements to suppress line conducted interference and low pass filter in front of the comparator input. Here is the circuit :
This circuit uses first-order RC low pass filter as signal filter. This circuit is made of C4 and R3 with cut off frequency of 10kHz. This cut off frequency is used to achieve an optimum absorption of noise.
To block negative interference pulses and protect the sensor and electronics against reverse polarity of the supply voltage, this circuit uses the series diode D1. The suppressor diode D2 is used to limit positive interference pulses. Fast negative and positive interference pulses are absorbed by the capacitor C2. To supply the sensor during short supply voltage breakdown because of negative pulses, the electrolytic capacitor C3 stores energy. [Source: Philips Semiconductors Application Note]