Analog and digital sensors — advantages of each type

Sensor detection originates in the analog world, where the desired information is a continuously changing parameter. Typically, the signal from the sensitive element was amplified, compensated, and converted to a linear form according to the system for which the sensor was intended, and all actions were performed in analog form. Starting from the mid-1990s, there was a transition from analog form to digital protocols, signal processing, and interfaces, and today this transition is in full swing. The speed of devices transitioning to analog-to-digital conversion varies depending on the industry, but currently, digital technologies dominate many application areas. This does not mean that the analog converter segment will shrink and disappear, as there are still many applications where the advantages of analog converters will always outweigh the digital method, for example, in environments with high levels of electromagnetic/radio frequency interference or in legacy but still operating analog systems.

Pressure sensor with digital or analog output signal

Digital and analog pressure sensors differ in technology, interface, and type of output signal. KELLER has developed an industrial pressure sensor available in both digital and analog versions.

Both versions have the following advantages:

• Perform measurement and data transmission functions for gas or liquid pressure in systems, pipes, and tanks.
• Mechanically identical and interchangeable.
• Use a flexible membrane, silicon strain gauge, and Wheatstone bridge to convert pressure into an analog voltage signal.
• Convert the analog signal to digital for further processing by the sensor electronics.

The similarities between the versions end here. The differences between analog and digital converters will be described not only functionally but also in terms of capabilities and advantages.

Analog output — continuously varying signal

In the analog version of the sensor, correction of the digital signal from the built-in analog-to-digital converter is performed in several ways. Calibration coefficients are applied to confirm the sensor's accuracy characteristics. Then, temperature compensation adjusts the signal. Finally, zero and span calibration is performed to set the output signal within the desired range according to the sensor's serial number. After this, the signal is converted back to analog using a built-in digital-to-analog converter, passes through a unity gain amplifier, and is sent to the output terminals. The output signal continuously changes exactly as the pressure applied to the sensor changes. In older technologies, such as foil strain gauges, the signal is adjusted by resistors located on the printed circuit board based on the sensitive element's pressure and temperature characteristics. Since microprocessors and microchips have become more compact, many have been integrated into pressure sensors, resulting in higher accuracy, better design, and, in some cases, reduced costs.

The analog pressure converter 23SY has the following types of output signals:

Digital signal - data output in PC language

The digital version of the transducer involves the use of various compensation schemes during signal processing. The processed data in digital form is stored in registers for subsequent transmission to the system. The most common digital data transmission protocol used for sensors and transducers is Inter-Integrated Circuit (IIC or I2C). This communication technique is designed so that the sensor does not transmit the pressure measurement value to the system until a data request is received from the controller. Since the need to obtain data from the sensor is periodic, the sensor can enter "sleep mode" (low power mode) between data requests. This ensures energy savings, which is especially important for battery-powered and wireless applications.

Almost all KELLER digital transducers measure temperature. Temperature information is important as it is used for signal compensation to ensure maximum measurement accuracy. The sensor digitizes the temperature data for further compensation and stores it in a register, which can also be accessed just like the pressure measurement values. The result is two measurement values from one sensor. For efficiency, the data is stored and transmitted in digital form in hexadecimal format, but for ease of review in graphs and tables, decimal format is used.

Many KELLER sensors, including the 23D model, measure temperature using a Wheatstone bridge. This technique provides more accurate data on the temperature of the medium exerting pressure on the sensor for compensation of readings.

Comparison of Analog and Digital Transducer Characteristics

The table below provides a comparison of the main technical characteristics and operating parameters of the analog and digital 23SY and 23D transducers.

1. FSO = Full Scale Output
2. N/A = no data
3. N/S = not specified by technical conditions
4. Range depends on P/N and output configuration

The overall measurement accuracy of a system using analog sensors can be affected by errors at the sensor output as well as the method of their correction. In digital systems, errors may occur due to vibrations, jitter, and resolution issues.

Choosing the Right Sensor Based on Design Specifications

Availability of analog or digital pressure transducer 23SY/23D significantly expands the possibilities for design engineers. Considering that the customer's project may involve unique and rare parameters, meeting them will not be a problem for the 23SY/23D sensors, as a range of modifications includes a wide variety of different standard versions of the 23SY/23D converter, and the likelihood that one of them will fit the specifications is high. The table below lists the key design characteristics and the choice for each of them.

Analog Sensor Selection

In some cases, the task conditions contain special requirements that must be considered when choosing an analog or digital converter. A good example is pressure control in chemical processes at an oil refinery. Depending on the plant layout, the pressure sensor may be located in different places, for example, very far from the control system to which it is connected; in such cases, a long connecting cable is required. Such cables can be subject to electromagnetic and radio interference, as well as electrostatic discharges from adjacent cables and equipment. These electrical interferences can suppress or distort the high-resistance analog signal in the cable.

A common technology to solve this problem is the method of transmission with high current and low resistance, immune to interference sources. The 4-20 mA current loop is ideal for this task. The pressure sensor uses the signal to control the current strength received from the power source located in the same place as the control system. At zero signal level, the current strength is 4 mA, at maximum level - 20 mA. The signal level within this range will be proportional to the corresponding current strength value. This technology is an ideal way to solve problems related to electromagnetic and radio interference. In the analog 4-20 mA version, the 23SY pressure sensor can be made with 22 different standard pressure ranges (from vacuum pressure to 1000 bar gauge).

Digital Sensor Selection

Industry is gradually transitioning to the new IIoT technology - Internet of Things or Industry 4.0. The main idea is to use sensors to provide continuous monitoring of every key parameter of the industrial process in production. The collected data is transmitted to a central computer or cloud, where it is processed and stored further. All processes are carried out in digital format to improve the efficiency of data transmission, analysis, and storage. In this case, using a converter with a digital interface for data transmission ensures cost savings on the system, as well as optimal interaction of the sensor with the control system.

Freedom of design choice with the 23SY/23D sensor

Regardless of whether the task is in the analog or digital domain, the availability of sensors capable of operating in both cases provides significant flexibility for system developers. Converters in both analog and digital versions have the same core functions of accurate pressure measurement and data transmission to the control system. The option to transmit data in both analog and digital formats implies that the system design will be efficient and cost-effective, thereby ensuring the desired result that meets any design requirements.

Conclusion

KELLER is a technological leader in the field of pressure sensor manufacturing, providing connectivity capabilities in a modern world with an increasing number of various interconnections. KELLER has vast experience: from supplying measuring instruments to enterprises in mechanical engineering, industry, commercial transportation, aerospace, and defense sectors to solving various tasks in the medical field and handling unique custom requests.