1 Sensor and Signal Conditioning Circuits

A sensor is used to sense a change within the environment it is surrounded by and produce the corresponding variation as an equivalent electrical signal. Regardless of the input, the output signal of a sensor is usually a voltage, current, charge or resistance. The dynamic output range of a sensor is typically measured in mV or mA and the output of a sensor can be a single-ended signal or a differential signal.

Depending on the system operating environment where the sensor is mounted, the sensor measurements might be affected by noise induced in the system. Therefore, the resultant sensor output signal might be noisy. Sensors require a signal conditioning circuit to remove noise (if any) from the sensor output signal and convert it to the required voltage range or frequency. The converted sensor output signal is fed to the host microcontroller so that it can be processed, displayed and utilized for feedback-controlled systems.

Few sensors have built-in signal conditioning circuitry along with an Analog-to-Digital Converter as an optional stage, referred to as digital sensors. These sensors can be interfaced directly to the host microcontroller. The host microcontroller requires a serial communication peripheral such as I2C, SPI or UART for interfacing with digital sensors and to read data from them. Figure 1-1 shows the block diagram of a typical sensor or transducer.

Figure 1-1. A Typical Block Diagram of Sensor and Signal Conditioning Stage

Signal conditioning circuits accept raw sensor output signals and convert them into a form that the microcontroller-based system can process further. Based on the sensor output and application use, the following are the typical circuits required for sensor interface:

  • Sensor excitation (constant voltage or current supply)
  • Attenuation (voltage divider circuits)
  • Amplification
  • Signal filtering
  • Signal buffering or high-impedance stage
  • Current-to-Voltage Converter
  • Voltage-to-Frequency Converter