One-wire interface over power pin for sensor calibration

Calibration is a key step in the manufacturing process of many sensors (or transmitters) such as pressure, temperature and position sensors. One key component needed for calibration is the communication interface between the sensor and the calibration system. This communication interface involves hardware and software, both in the sensor and calibration system. The communication interface must have the ability to calibrate multiple sensors simultaneously.

In the context of sensor calibration, reducing the number of sensor pins needed specifically to support communication during calibration – and correspondingly the number of harness wires or cables – is a benefit because it reduces costs and the sensor solution hardware size (including pins and wires). The one-wire communication interface (OWI) provides these benefits by allowing communication with the device to take place over one wire. Moreover, in the case of two-wire transmitters, OWI over the power line provides even further benefits.

Two-wire transmitters eliminate the need for an additional pin because data and power are sent over the same wire. In this article we specifically discuss the communication interfaces used during sensor calibration. Additionally, we address OWI for two-wire transmitters focusing on the challenges related to OWI over the power line, and present a solution to overcome these challenges.

Calibration of sensor signal conditioners
Calibrating sensor signal conditioners is a crucial step in the sensor – or transmitter – manufacturing process. Figure 1 shows the block diagram of a sensor and calibration system. This block diagram shows that the sensor has two main components: 1) the sense element which converts the physical quantity of interest into electrical signal; and 2) the sensor signal conditioner that processes the sense element using mathematical algorithms. The block diagram also shows the "calibration system." The calibration system, which is installed in the final sensor manufacturing line, is a collection of hardware and software used to calibrate the sensor. In this article, we use the terms sensor and transmitter interchangeably because they imply the same end device.

Sense elements typically have non-idealities, such as offset, non-linearities, and gain errors. Mathematical algorithms implemented in the sensor signal conditioner are used to correct for these non-idealities. Calibrating a sensor is the process of determining the parameters – or coefficients – of the mathematical algorithms. It is during the calibration process that the calibration system collects data from the sensor signal conditioner and determines the coefficients. The calibration system then transfers the algorithm parameters to the sensor signal conditioner. As presented in reference [3], calibration is usually a two-step process and, depending on the desired final accuracy of the sensor, it could be time-consuming – especially if temperature calibration is included.

Regardless of the final accuracy of the sensor, the calibration system has to receive from and transmit data to the sensor signal conditioner. This data includes [3]: back-end data, front-end data and sensor signal conditioner configuration and algorithm parameters.

This calibration system transmits data to the sensor signal conditioner and receives data from registers in the sensor signal conditioner. The back-end and front-end data is collected at multiple temperature points based on the end-accuracy target of the system. For example, steps "a" and "b" are repeated for each of the temperatures used for calibration. Note that coefficients and configuration settings are typically stored in non-volatile memory, such as EEPROM, in the sensor signal conditioner.

Communication interfaces used for calibration
The calibration system can communicate with the sensor signal conditioner using communication interfaces supported by the sensor signal conditioner. Some commonly used communication interfaces are: serial peripheral interface (SPI), inter-integrated circuit (I2C), universal asynchronous, receiver/transmitter (UART) and one-wire interface (OWI).