How to Design Analog/ Digital Sensor interface circuit for Automotive Applications

Introduction

I would like to share few of the basics for designing sensor circuit input configuration for Automotive application, which can also be used for other applications such as Industrial applications with some adaptations.

           To begin with, let me start by dividing commonly used sensors in Automotive into 3 types.

1) Analog Sensor
2) Digital Sensor

3) Frequency Sensor

In this article lets concentrate on Analog & Digital Sensors. I will write a separate article for Frequency Sensors.

Different types of Circuits on Sensor line:

        A typical interface circuit between Sensor & Micro controller. You will understand the importance of each component below:

• ESD suppression circuit: Common type of circuit found in the Analog/Digital types of Sensors are ESD suppression circuit . This is a section of the circuit  This is very critical circuit which will protect the PCB's or the tracks of internal components from ESD. There are different sources of ESD. Couple of them are Human Body model, Machine model etc. For more details you can refer this link: 

• Current limiting Resistor:  If a sensor interface circuit is connected to micro controller care must be taken that the current through the sensor line is not more than the maximum input current of the Micro controller. The current can be limited by placing a resistor in series on the line to the MicroController. 
• Resistor value is                                     R = V/I              

            V= Operating Voltage 5V/14V (Operating voltage or Battery voltage)

            I = Typical current carrying capacity of MC.

            While taking the Voltage value care must be taken to consider the max voltage possible on the line for ex: Short circuit to Battery etc. This circuit is common for both Analog /Digital.

         

• Filter Circuit: Filter circuit can be implemented by a simple RC combination circuit also called Low Pass filter. Value of RC components should be chosen in such a way that any unwanted frequencies can be filtered out. Cut off frequency of Low Pass Filter is fc = 1/(2*pi*RC). R is already chosen above as current limiter. This circuit is common for both Analog /Digital.

• Pull Up/ Pull Down resistor: In case of a Switch which is active Low  a pull up Resistor is used with a Voltage of Battery Voltage(14V for ex: ) or Micro controller Power supply Voltage (5V for ex). Battery Voltage is used suppose in case Sensor used has been powered by Battery voltage.  Pull Up/ Pull Down resistors also help in executing the default voltage seen by the Micro when sensor is not connected or open load. Predominantly used for Digital circuits. Used in Analog sensors when default value is needed if sensor is disconnected.

• Pull up resistor with Diode: A diode to be used just in case if the signal is not directly coming from Sensor but from other control unit with a different voltage source. This will help in preventing unwanted signal at the micro when one of the Controller is not Turned ON.

• Voltage divider circuit: If Voltage level of the input signal or voltage connected to the Pull up resistor is more than Micro controller voltage to which signal is connected it is advisable to use voltage divider circuit to reduce the impact of high voltage on the internal circuit of the Micro over long run.

Conclusion: Above are the typical circuits found commonly in Automotive control units for Analog & Digital circuits. I will write a separate blog for Frequency Sensor input configuration. Please do comment .