Digital type of transmitters digitize the measured analog signal, and use a microprocessor. The analog signal entering the transmitter is digitized with an analog to digital converter (ADC).
Due to the time required to quantify a signal, digital instruments do not perform a continuous measurement, only sample of the signal are taken.
Below we will see a classification of digital transmitters. They are totally digital, including the output.
Although it can generate digital output, can also have an analog output of
4-20 mA. Fieldbus instruments are of this type.
1. Intelligent Transmitter
By having a microprocessor, it performs functions that analog ones do not linearize, compensate based on another measured variable or algorithms, and others.
2. Smart Transmitter
Its output is analog from 4-20 mA, and it can communicate with a “hand-held” communicator using modulation on the output.
Differences between digital and analog transmitter
In an analog instrument, the electronic components are different. Linear circuits such as OPAMPs are used.
In digital instruments, microprocessors, ADC converters, and DAC are used if analog outputs are available.
The analog output signal is generated differently. A digital instrument comes from a DAC.
Advantages of Digital Transmitters
They are flexible in their functions. Digital instruments have more functions due to the ease of manipulating numbers by a microprocessor. The functions can be modified or extended by changing the firmware.
The 4 to 20 mA analog output is independent of the measurement circuit, its range may be different from that of the instrument.
Calibration and idealization are performed digitally.
Idealization can be characterized by the equalization of a particular sensor.
Digital conditioning techniques are more powerful.
Compensates for sensor drift with the help of a temperature sensor.
Communication capacity, as in smart and digital instruments.
The following figure shows the independence of the measurement circuit and the output circuit, the adjustments are made independently.
Advantages of Analog Transmitters
They work in real-time. In digital instruments, samples are taken in the order of 2 to 20 sample (scan time 50ms to 500ms). Therefore, in very fast processes, digital instruments cannot be used, analog ones must be used.
Disadvantages of Analog Transmitters
- For changing the measurement ranges, calibration is required. Personnel should be well trained and experience is necessary.
- In order to carry out the calibration process, the instrument must be removed from line.
- Drift occurs when components such as potentiometers are used. Components, such as potentiometers, experience “drift”.
- Idealization is fixed for a single type of sensor.
- The following tables compare an analog and a digital transmitter, and an analog and a “smart” transmitter.
Analog instruments work in real time. Digital instrument are considered to work in real time if the “scan time” is much less than the delay constants of the controlled process. The digital instrument ha dead time introduced by the ADC and by the microprocessor program execution time.