A loop fails. The system sees 0 mA. And that’s it. No context. No diagnosis. Just a number. Is it a broken wire? A failing transmitter? A loose terminal slowly degrading over time? Or is it something worse, such as intermittent, hidden, waiting to bring production down?
For decades, we’ve trusted the 4–20 mA loop to deliver reliable measurements. And it does, exceptionally well. It’s simple, robust, and immune to noise over long distances. But when something goes wrong, it still leaves us guessing.
Most input channels do exactly what they are designed to do: They measure current and convert it into a process variable. But they don’t tell you whether the loop is healthy, why the signal changed, or if a failure is about to happen. Turning Measurement into Understanding is possible. Current input becomes more than just a measurement. It becomes intelligent loop monitoring, due to the integration of diagnostics and monitoring features, integrated into a single chip AD74416H.
Let’s revisit the technology of current loops again, as in our previous blog, this time from the perspective of the current input loop-powered function.
Current Input Loop-Powered, Measured, Understood
At its core, a 4–20 mA loop is beautifully simple; however, it can be confusing to determine the loop's source and which side controls the current, which reflects the value of the main process variable. A sensor measures a process variable, such as pressure, temperature, or flow, and modulates the current accordingly. The current loop's power source is the I/O channel itself, which also reads the loop current via the ADC. The loop is typically powered by a 24 V supply. This enables the use of sensors that can be powered entirely by loop current, reducing system complexity because no separate power supply is needed at the sensor side, which can be placed hundreds of meters from the PLC (Programmable Logical Controller) containing the I/O channels.
Figure 1 below highlights the described scenario for the Current Input Loop-Powered function in simple blocks.
Figure 1: PLC I/O CARD in Current Input Loop-Powered I/O channel function powering loop current for SENSOR
Let’s Set Current Input Loop-Powered Mode on EV-AD74416H-ARDZ
To demonstrate the current input loop-powered capability on the physical hardware, use the EV-AD74416H-ARDZ board setup and ACE.
- Connect the RT potentiometer to I/OP_x and I/ON_x terminals to simulate a sensor communicating the main variable (by changing the sensor resistance). The setup is shown in Figure 2
Figure 2: Potentiometer Connection to EV-AD74416H-ARDZ to simulate the sensor
- Once the physical connection to the channel is complete, configure Current Input Loop Powered (IIN_LOOP_PWR) using the dropdown menu.
- Set ADC to convert the input voltage by configuring CONV_SEQ to CONTINUOUS and enabling the corresponding channel conversion by ticking CONV_x_EN. Also configure LOOP CURRENT LIMIT by sliding the DAC slider to the very right to set the current limit to a maximum of 25mA.
- Click the Apply button to load the setting to the device.
The figure below shows an example of channel A configuration; the calculated input current can then be read next to the enabled channel configuration.
Figure 3: AD74416H ACE plugin, Current Input Loop-Powered Configuration
No Need for System Interpretation, it Knows Failure
Short circuits represent a severe failure mode, causing incorrect readings or potential device stress. In the current input loop-powered mode, the AD74416H uses internal comparators to monitor the I/O node behavior. If the I/OP terminal is pulled to an unexpected level (for example, towards ground during a short), the device detects the deviation and flags it as a short-circuit condition.
The corresponding ANALOG_IO_SC flag is set in the CHANNEL_ALERT_STATUS register. It can be easily simulated by adjusting the RT thermistor close to ~0Ω. This detection happens in real time, without external circuitry.
At the same time, internal current limiting protects the device and loop, ensuring controlled behavior, even in fault conditions. The loop current won’t exceed the value set by DAC, The DAC value can be easily adjusted by software based on the I/O card requirements or desired real-time behavior of the system.
Built-in VSENSEP_x diagnostic for the channel provides additional visibility, allowing deeper insight into the loop's voltage state beyond current measurement. It enables the user to further interpret the potential cause of failure or to generally monitor current loop health.
Figure 4: Detail view of Current Input Loop-Powered System connections.
Beyond Measurement: Understanding the Loop
If that loop fails again, it won’t just read the current; it will tell you why. From loop-powered sensing and high-resolution measurement to real-time fault detection, current limiting, and voltage diagnostics, the AD74416H turns a simple loop into an intelligent system. It won’t just be measuring the process variable but monitoring its integrity. Because in modern I/O, the goal isn’t just to read the loop; it’s also to understand it before it fails.
Read all the blogs in the Configurability IQ series.