A protection stage is necessary in order to protect electronic components if voltage or current values exceed the maximum ratings. ESD (electro static discharge) and EMI (electromagnetic interference) are other important protections to include in the datalogger, in order to avoid permanently damage to the product and to assure accurate measures.
A signal conditioner is a device that converts the analog sensor electric output signal (current, voltage) into a another type of signal (typically voltage or frequency) that meets the input requirements of the next processing stage. It can be an amplifier, a differential amplifier, a current to voltage converter, etc…
According to Shannon-Nyquist theorem, a sampled signal must not contain components at frequencies above half the sampling rate (the Nyquist frequency). So, the highest frequency which can be accurately represented is one-half of the sampling rate. If components above the Nyquist frequency pass to the A/Dc (analog to digital converter), the system exhibits a phenomenon called aliasing. Aliasing is the presence of unwanted components in the converted signal spectrum.
A low pass filter is necessary in order to avoid aliasing. Due to component tolerance and to filters roll off, the low pass filter is calculated at a frequency lower than the Nyquist frequency.
Since the CPU only processes digital information, it requires a digital input signal. Therefore, an analog to digital converter (ADC) is required to convert analog signal from the sensor to a digital signal for the CPU. The conversion is done by sampling the analog signal at discrete points in time. Sampling rate of A/Dc defines the number of samples per second taken from the analog signal to make the digital signal. Resolution of A/Dc defines how many binary digits (bit) represent the sampled signal. A 24 bit A/Dc converts signals into a 24-digit binary number. Since each digit of a binary number can take one of the two values 0 or 1, a 24 bit number can take one of 2^24 = 16777216 values, representing the integers from 0 to 16777215.
When the sensor has a digital output, the signal is compatible with the CPU, but it needs to be filtered and translated. The translation process is performed by an electronic device (signal translator or signal shifter) which changes one signal level to another, compatible with the next processing stage. If the sensor output signal is a simple on/off voltage or a low frequency, it can be directly processed by the CPU.
If the sensor output signal is a message formed by a sequence of bits (1 and 0), it can be processed by the CPU via an UART or USART device. UARTs and USARTs are generally included in the CPU package. An UART (universal asynchronous receiver/transmitter) is an electronic device that translates data between serial and parallel forms. If data are transmitted in a synchronous way (there are both data and clock signals), they are translated by devices called USARTs (universal synchronous/asynchronous receiver/transmitter). The main component of UARTs and USARTs is the shift register, the basic stage of conversion between serial and parallel forms. A parallel datum is directly accepted by the CPU.
A power supply stage is necessary to convert the power supply voltage into all levels of voltage necessary to sensors and all other components. There are two main kinds of power supply units: linear regulators and switching mode power supplies. Since a damage in PSU could cause a damage of all other components, a particular attention is required while designing the power supply unit, specially when selecting appropriate components, such as transient filters, over voltage suppressors, inductors, capacitors and transformers, to ensure safe and reliable operation.
The CPU (central processing unit) is the fundamental part of the datalogger. It takes care of the interpretation of signals and execution of all instructions. It is generally a single microchip. The main tasks are: interpretation of input signals, convert data format to human readable numbers or graphics, communicate with all peripherals, send data to memory and to display, define output signals and communicate with the computer.
Memory is an electronic device used in the datalogger to store data. Built in memory is generally non volatile, so that data is retained also without power supply. Two kinds of memories are commonly used in dataloggers: EEPROM (Electrically Erasable Programmable Read-Only Memory) and FLASH. While in EEPROM memories, each byte can be independently read, erased, and re-written, in FLASH memories, bytes can be written and read in blocks (or pages).
It is the main clock to assure that each measured datum is indexed over the proper date and time.
Display and keyboard are the user interface. They are devices to assure interactions between humans and the datalogger.
It is the communication port (cabled or wireless) to download data to a computer for further organization and processing. It is possible to draw, print and share graphics and numeric tables of the input measured values.