Digital Data Acquisition Systems: Principles and Applications

SADD Description

The acquisition system captures signals from field sensors and transducers, sending them to the SADD MDIO for data transmission. The data is processed, and addresses are assigned to facilitate transmission through the distribution system.

Digital vs. Analog Controllers

Digital controllers are justified in applications requiring microprocessors or high-frequency responses.

Analog Controller

Operates in continuous time; a dedicated circuit performs the required work.

Digital Controller

Operates in discrete time, executing software on a microcomputer-based system equipped with ADC (Analog-to-Digital Converter) and DAC (Digital-to-Analog Converter) interfaces.

Successive Approximation ADC

The use of an ADC based on successive approximation is justified for applications requiring high precision or specific conversion speeds.

ADC Integration

• Performance: Slower than RAMP or monitoring methods, but significantly more accurate.
• Quantization Error: Occurs due to the discretization of analog signals, influenced by the signal range and the number of bits used for discretization.

Advantages of Digital SADD

Digital SADD systems are preferred over analog counterparts due to their simplicity and flexibility.

Error and Uncertainty

Uncertainty depends on the conversion time (Tc) of the ADC and the rate of change of the analog signal.

Trapezoidal Approximation

Integration via the trapezoidal method is an effective approximation. It utilizes the bilinear transformation, which is ideal when the controller requires a discrete frequency response similar to an analog protocol.

Key System Components

• Sampling: Captures the magnitude of a continuous signal at a defined moment.
• Quantization: Bounds the continuous signal within defined intervals.
• Aliasing Filter: Prevents frequencies above half the sampling frequency from passing.
• MUX (Multiplexer): Enables the controller to manage multiple control loops.
• Sample & Hold: Demonstrates a variable in a very short time and retains it. This solves uncertainty issues caused by slow, high-resolution ADCs.

ADC and RAMP Approximations

The Successive Approximation Register (SAR) converter compares the input signal to a reference, starting at the MSB (Most Significant Bit). If the digital value exceeds the analog input, the bit is set to zero, and the process continues to the next bit until the LSB (Least Significant Bit) is reached. Unlike the RAMP approach, the conversion time (Tc) is constant and does not depend on signal amplitude.