Analog inputs are used to measure continuously varying physical quantities (such as voltage, current, temperature, pressure, etc.) and are widely applied in industrial control, sensor measurement, and signal processing. Below are several common types of analog inputs and their characteristics:
Signal Range: Typically 0~5V, 0~10V, ±5V, ±10V, etc.
Features:
Directly measures voltage, suitable for most sensors (e.g., potentiometers, LVDTs, some temperature sensors).
Weak noise immunity; susceptible to interference over long-distance transmission.
Typical Applications:
Industrial PLC analog input modules (e.g., 4~20mA transmitter + 250Ω resistor = 1~5V).
Data acquisition cards (DAQ) for sensor output measurement.
Signal Range: Common standards include 4~20mA, 0~20mA (industrial standard).
Features:
Strong noise immunity, suitable for long-distance transmission (current signals are less affected by line resistance).
Requires an external precision resistor (e.g., 250Ω) to convert to voltage for ADC reading.
Typical Applications:
Industrial sensors (pressure transmitters, flow meters).
Process control (PID regulation).
Measurement Method: Measures resistance via constant current excitation or voltage divider method.
Common Sensors:
RTD (Resistance Temperature Detector, e.g., PT100, PT1000): Measures temperature with good linearity.
Thermistor (NTC/PTC): Nonlinear; requires lookup tables or formulas for calculation.
Strain Gauge: Used for force/pressure measurement; requires a Wheatstone bridge.
Typical Circuits:
Constant current source + ADC (e.g., 1mA current source for PT100).
Voltage divider method (e.g., NTC with a fixed resistor).
Features:
Measures the voltage difference between two signal lines (e.g., A+ and A-), suppressing common-mode noise.
Suitable for high-noise environments (e.g., motor control, industrial sites).
Typical Applications:
Bridge outputs (e.g., strain gauges, load cells).
High-speed ADC acquisition (e.g., oscilloscopes, medical devices).
Principle: Based on the Seebeck effect, measures the temperature-dependent voltage (μV~mV level) between two dissimilar metals.
Challenges:
Weak signal (requires high-precision amplifiers, e.g., AD8495).
Requires cold junction compensation (CJC) to correct for ambient temperature effects.
Common Types: Type K, J, T thermocouples.
Applications: Piezoelectric sensors (e.g., accelerometers, microphones).
Features:
Outputs high-impedance charge signals, requiring a charge amplifier to convert to voltage.
Not purely analog but often used for analog measurement:
Examples include speed sensors (Hall/optical encoders) outputting PWM or frequency signals.
Can be processed via frequency-to-voltage conversion (FVC) or MCU timer capture.
| Type | Advantages | Disadvantages | Typical Use Cases |
|---|---|---|---|
| Voltage Input | Simple, versatile | Poor noise immunity | Short-distance, low-noise environments |
| Current Input | Strong noise immunity, long-distance suitable | Requires extra resistor | Industrial sensors (4~20mA) |
| Resistance Input | Direct temperature/force measurement | Requires excitation, may be nonlinear | RTDs, strain gauges |
| Differential Input | High noise immunity | Complex circuitry | Bridge sensors, high-noise environments |
| Thermocouple | High-temperature measurement (up to 1800°C) | Requires CJC, weak signal | Metallurgy, boiler monitoring |
Voltage/Current Inputs are the most versatile analog signals, suitable for most sensors.
Resistance Inputs are used for temperature/pressure measurement, requiring attention to linearity and excitation methods.
Differential Inputs are ideal for high-noise environments, such as industrial control.
Thermocouples and Charge Inputs are specialized analog signals, requiring dedicated signal conditioning circuits.
When designing analog input circuits, factors such as signal range, noise immunity, and ADC resolution must be considered. Amplifiers, filters, or isolation techniques may be necessary to improve accuracy.
