Since the digital-to-analog converter (DAC) directly interfaces with the controlled device, it is susceptible to interference through shared grounding. To prevent this, isolation techniques are essential. Optocouplers are commonly used for this purpose, as they provide optical coupling between the controller and the controlled object, ensuring electrical isolation without direct signal paths. An optocoupler consists of a light-emitting diode (LED) and a photodiode housed in a single package. The LED’s input and the photodiode’s output exhibit characteristics similar to those of a transistor, making them suitable for signal transmission with isolation. 02 Converter Interface Isolation Methods 1. Analog Signal Isolation Method. By operating the optocoupler in its linear region, the DAC's output voltage can be converted into a current signal, thereby achieving analog signal isolation. A two-stage optocoupler configuration is often used to convert the DAC output voltage into a current, which allows both isolation and voltage-to-current conversion. For optimal linearity and accuracy, it is crucial to select two optocouplers with matching transfer ratios that operate consistently within their linear range during application. The main advantage of this method is the low component count and cost. However, it requires careful selection and tuning of the optocouplers, as improper choices can lead to reduced accuracy and non-linear performance. Fortunately, integrated linear optocoupler chips are now available, containing two high-linearity optocouplers with identical transfer ratios, simplifying the design and debugging process significantly. 2. Digital Signal Isolation Method. This approach leverages the switching behavior of optocouplers to isolate digital control and data signals from the DAC. Input signals are fed into the optocoupler, and the output is connected to the DAC, effectively isolating the digital signal path. The primary benefit of this method is ease of implementation and minimal impact on signal accuracy and linearity. However, optocouplers designed for digital applications tend to be more expensive compared to their analog counterparts. 03 Overview of I/V Conversion Many transmitters generate output signals in the form of 0–10 mA or 4–20 mA currents. Since most analog-to-digital converters (ADCs) require a voltage input, current signals must first be converted into voltage signals before being processed. This is where an I/V (current-to-voltage) conversion circuit becomes essential. Let’s explore how this conversion can be implemented. (1) Passive I/V Conversion. This method uses passive components such as resistors to perform the conversion, often including additional protection features like filtering and output limiting. For example, with a 0–10 mA input, using R1 = 100 Ω and R2 = 500 Ω (a precision resistor), the output voltage will range from 0 to 5 V. Similarly, for a 4–20 mA input, R1 = 100 Ω and R2 = 250 Ω would produce a 1–5 V output. (2) Active I/V Conversion. In this approach, operational amplifiers are used alongside resistors to create a more accurate and stable I/V conversion. For instance, with R3 = 100 kΩ, R4 = 150 kΩ, and R1 = 200 Ω, a 0–10 mA input will result in a 0–5 V output. Alternatively, with R3 = 100 kΩ, R4 = 25 kΩ, and R1 = 200 Ω, a 4–20 mA input will yield a 1–5 V output. 04 Preamplifier Circuit The preamplifier is responsible for boosting small analog input signals to a level suitable for ADC processing. To accommodate various small signals, a variable gain amplifier can be designed. However, many modern transmitters already output standard voltage or current signals, so preamplifiers are not always necessary in ADC circuits. 05 Multi-Channel Analog Switch A multi-channel analog switch allows the selection of one specific analog input signal from multiple sources. Field-effect transistors (FETs) are widely used due to their fast switching speed, low on-resistance (5–25 Ω), and high off-resistance (up to 10^10 Ω), making them ideal for such applications. 06 Principle of Sample and Hold Circuit A sample-and-hold circuit captures and maintains the value of an analog input signal at a given moment. During the sampling phase, the circuit follows the input signal, while during the hold phase, it retains the last sampled value. When the switch is closed, the capacitor charges quickly, tracking the input signal. Once the switch opens, the capacitor discharges slowly, and the op-amp maintains the voltage level at the output, preserving the signal value until the next sampling cycle.
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