When current is used as the main variable, it’s much easier to maintain over long cables and distances. This is why using the current signal, known as 4-20mA current loop, has become a technology with longevity.
Process control and numerous other applications utilize 4-20mA current loops. The AD74416H chip keeps up the current loops trend and provides numerous features to simplify the current loop configuration. Here is how it can be accomplished.
Easy Set Up Current Output (IOUT)
Any of the 4 channels can be independently programmed to Current Output mode for demonstration purposes. I picked the Channel A (CHA) position in the bottom right corner of the EV-AD74416H-ARDZ.
To demonstrate the Current Output capabilities, a simple 1kΩ resistor can be used. Since every 1 mA of current flowing through the load causes a 1V voltage drop, one can easily interpret the output current by simply measuring voltage. The direct proportionality between current and voltage simplifies observation and validation of the output behavior.
It is important to note that in practical applications, loads typically range from 0Ω up to 800Ω.
Figure 1: Resistor Connection diagram (Channel A of EV-AD74416H-ARDZ)
Once the physical load is connected to the channel, set up the Current Output (IOUT) using the dropdown menu, and set the output voltage to the desired value by modifying DAC_CODE. Configuring ADC is optional in this mode; it provides additional voltage monitoring capabilities, making it easier for the user to check the current being output. Figure 2, below, highlights the configurations mentioned.
Figure 2: AD74416H ACE plugin, Voltage Output enabled
The green ellipse, in Figure 2 (above), highlights the value being read, while the red ellipse highlights the values being configured by the user.
Current Output (IOUT) - Open Circuit Detection
Open circuit detection is triggered and notifies the user when the set current cannot be supplied to the load. The AD74416H chip has this feature fully built in. An open circuit can happen for several reasons, such as a fault in the connected load, a disconnected or damaged cable, or low headroom voltage on the output driver of the AD74416H chip.
The open circuit detection can be easily simulated by disconnecting the 1kΩ resistor from the load on the channel. An alert will remain flagged until the user manually clears it, ensuring important warnings are not missed.
Figure 3: Detected Open Circuit in Current Output
For the alert to be cleared, connect the load first. Then, click on the relevant alert status. Note: each alert can also be masked, so if an alert is triggered, the user will not be notified. This allows them to proceed onward, without interruption, to optimize the application.
Current Output (IOUT) - Slew Rate
In addition to basic functionality, IOUT offers adjustable slew rate options. This feature helps the user transition between output current values in a controlled and predictable manner. A controlled ramp of the output current is especially useful for reducing signal overshoot and oscillations in inductive loads .
Two options for slew rates are available:
- Programmable slew rate (SLEW_EN = SLEW_LINEAR) of the output channel. Settling times can be adjusted via SLEW_LIN_STEP and SLEW_LIN_RATE. Timing description and example can be found in the AD74416H datasheet.
- HART Compatible slew rate (SLEW_EN = SLEW_HART_COMP) is designed for use in IOUT_HART mode, when the output channel needs to be configured to operate with the integrated HART modem.
Simplicity and various applications help keep the simple current loop approach alive and thriving despite the technology’s age. New technologies may emerge, but numerous industry controllers and sensors using this technology are being deployed in the field, requiring backward compatibility to ensure the longevity of 4 - 20mA loops.
Will current loops ever become obsolete? For the sake of progress, I hope so, but it may take years or decades. Perhaps in the future we may see all end nodes IP (Internet Protocol) addressable. “But until then, industrial loops have their gain.”
Read all the blogs in the Configurability IQ series.