Selecting the Right Analog Sensor Output

While the automation industry increasingly adopts smart sensors like IO-Link, analog sensors remain foundational technology and are still in wide use today. Analog sensors convert physical measurements, such as temperature, pressure, flow, or distance, into continuous electrical signals. In this post, I'll explore the most common types of analog sensor outputs, their key features, and how to select the right one for your system.

4-20 mA current loop: The industry standard

The 4-20 mA current loop is the most widely used analog signal due to its strong noise immunity and reliable long-distance transmission. This makes the 4-20 mA current loop ideal for harsh industrial environments.

• Range: 4 to 20 milliamps (mA)

• Benefits:

  • Noise resistance: current signals are less affected by electrical noise

  • Live zero: the 4 mA base allows fault detection (e.g., wire breaks)

  • Long distance: signal integrity is maintained over long cable runs

Voltage Signals: 0-10 V, 0-5 V, and 1-5 V

Voltage signals are common, particularly in simpler systems with the most popular ranges, including 0-10 V, 0-5 V, and 1-5 V. They perform best in low-noise environments and when sensors are positioned close to control systems.

• 0-10 V: simple applications with shorter distances

• 1-5 V: includes a live zero (1 V), useful for fault detection

Bipolar voltage signals: -10 to +10 V

For applications requiring both positive and negative measurements, like bidirectional flow or motion, a -10 to +10 V signal is sometimes used. This output is ideal for tracking values that fluctuate above and below zero.

• Range: -10 to +10 volts

• Use case: bidirectional or fluctuating signals

Resistance-based outputs: RTDs and thermistors

Temperature sensors, such as RTDs (e.g., PT1000) and thermistors, output resistance that varies with temperature. They are valued for their accuracy in industrial applications.

• Output: resistance

• TypicaluUse: temperature measurement

Millivolt output: thermocouples

Thermocouples generate a millivolt (mV) signal based on the temperature difference between two junctions. They offer durability and are suited for monitoring extreme temperatures, though you may need additional components to make them compatible with control systems.

• Output: millivolt signal

• Use case: high-temperature or wide-range applications

Choosing the right output

The correct analog output involves several key considerations, including:

• Control system compatibility: First, confirm which types of analog inputs (current vs. voltage) your PLC or analog input hubs support. Ensure the sensor output matches the available input type.

• Distance: For long-distance signal transmission, 4-20 mA is the best option.

• Noise environment: If noise is a concern, choose the 4-20 mA current loop or short-distance voltage signals like 0-10 V for better signal integrity.

• Application type: RTDs or thermocouples are best for temperature applications, while bipolar signals are suited for bidirectional data.

• Measurement type: For accurate temperature monitoring, consider using RTDs, thermistors, or thermocouples specifically designed for precision in temperature applications.

• Signal direction: When monitoring bidirectional signals, such as flow or motion in both directions, a bipolar voltage output (-10 to +10 V) is a suitable choice.

• Cost: Voltage signals are generally more cost-effective than current loops but may require extra shielding or shorter cable lengths to reduce noise. Weigh initial sensor costs against potential infrastructure investments.

While smart sensors continue to grow in popularity, analog sensors remain a staple of industrial automation, offering a reliable way to convert physical measurements into actionable data for control systems. By understanding the features of each output type and considering factors like control system compatibility, distance, noise environment, and cost, you can make an informed decision that ensures optimal performance and reliability in your automation system.