Introduction

Devices with analogue inputs and outputs list a rated impedance as part of their specifications. Understanding this impedance is critical for selecting compatible components to ensure proper system operation. In this article, we’ll explain how impedance affects 4–20 mA analogue signals and how to correctly select your devices to avoid common pitfalls.

Quick Rule of Thumb

When working with 4–20 mA systems:

Your sensor or transmitter must be able to drive a load (input impedance) without exceeding its maximum output capability.

In simple terms:

A sensor’s maximum load impedance should be greater than or equal to the input impedance it is driving.

How a 4–20 mA Output Works

A current-output device (sensor or transmitter) controls the current, not the voltage. It will adjust its output voltage to maintain the correct current across the connected load (input impedance). The load’s impedance determines how much voltage the device must generate to maintain the specified current.

For example:

Suppose a sensor outputs 20 mA and the analogue input impedance is 85 Ω.

Using Ohm’s law (V = I × R):
V = 0.02 × 85 = 1.7 V across the input.

If the input impedance were 250 Ω instead:

V = 0.02 × 250 = 5 V across the input.

Thus, for the same 20 mA current, the output device must generate more voltage when driving higher impedance loads.

Why Maximum Impedance Ratings Matter

Every current-output device (sensor or transmitter) has a maximum load impedance it can drive at its maximum current (typically 20 mA). This limit is based on the maximum output voltage the device can produce.

Example:

A transmitter specifies a maximum load of 200 Ω.

An analogue input has an impedance of 250 Ω.

When the transmitter tries to drive 20 mA into 250 Ω, it would need to generate 5 V (0.02 × 250 = 5 V).
But the transmitter can only supply up to 4 V (0.02 × 200 = 4 V), so it won't be able to maintain 20 mA across the higher impedance.
Instead, the maximum current it can deliver becomes:

I = V / R = 4 V / 250 Ω = 16 mA

This means the input signal will max out at 16 mA instead of 20 mA, causing inaccurate readings and reduced measurement range.

Key Selection Guidelines

When designing or selecting 4–20 mA systems:

Always verify that the receiving device’s input impedance is lower than the maximum impedance the transmitter or sensor can drive.

Consult device datasheets carefully — maximum load resistance (for transmitters) and input impedance (for inputs) are usually specified.

Remember that wiring resistance (especially over long cable runs) adds to the total load and must be considered.

Shown below is the rated impedance of our ZX1 displacement sensor.

The below snippet shows the impedance of the NX-AD3208. Since the ZX1 has a maximum impedance of 300Ω and the AD3208 has a impedance of 250Ω this device combination will work. 

Conclusion

Ensuring that the impedance ratings of your analogue inputs and outputs are compatible is critical for reliable 4–20 mA system performance. Always cross-check device specifications, and when in doubt, select components with plenty of headroom to avoid unexpected issues like signal clipping or loss of measurement range.