An article explaining the design of simplified analog input modules for process control

When designing analog input modules for process control applications such as Programmable Logic Controller (PLC) or Distributed Control System (DCS) modules, the primary trade-off is often price/performance. Traditionally, this application area has used bipolar ±15 V rails to provide active front-end components for attenuation or gain of the input signal. This impacts the bill of materials (BOM) cost, and creating an isolated bipolar power supply adds complexity to the design. Another way to save cost is to design the architecture with a single 5 V supply. A single 5 V supply rail significantly reduces the complexity of the analog front-end isolated power supply design.But it introduces other pain points that may reduce measurement resolution

Introduction

When designing analog input modules for process control applications such as Programmable Logic Controller (PLC) or Distributed Control System (DCS) modules, the primary trade-off is often price/performance. Traditionally, this application area has used bipolar ±15 V rails to provide active front-end components for attenuation or gain of the input signal. This impacts the bill of materials (BOM) cost, and creating an isolated bipolar power supply adds complexity to the design. Another way to save cost is to design the architecture with a single 5 V supply. A single 5 V supply rail significantly reduces the complexity of the analog front-end isolated power supply design. But it introduces other pain points and may reduce the accuracy of the measurement solution. The AD4111 does much of the integration required for voltage and current measurements and addresses the limitations of 5 V supply solutions.

Integrated front end

The AD4111 is a 24-bit sigma-delta ADC that reduces development time and design cost by implementing an innovative and simple signal chain. It utilizes ADI’s proprietary iPassives™ technology to fuse the analog front end and ADC together. This enables the AD4111 to accept ±10 V voltage inputs and 0 mA to 20 mA current inputs while operating from a single 5 V or 3.3 V supply without external components. The voltage input is specified for an overrange of ±20 V, within which the device still provides valid transitions and an absolute maximum specification of ±50 V on the voltage pin. The current input is specified for a range of -0.5 mA to 24 mA, enabling accurate current measurement close to 0 mA, providing accurate 24 mA conversion. The voltage input to the AD4111 guarantees a minimum impedance of 1MΩ. This removes the ±15 V external buffer, further saving board space and BOM cost. 5 V designs require that each voltage input must have a high impedance divider, which takes up board space. Designing a discrete solution requires a trade-off between the cost and accuracy of precision resistors. To solve this problem, the AD4111 uses a high-impedance precision voltage divider at each input, as shown in Figure 3.

An article explaining the design of simplified analog input modules for process control

Figure 1. AD4111 functional block diagram.

Open circuit detection

Typically, the limitation of a single 5 V design is the lack of open circuit detection, typically using a high impedance resistor to the 15 V rail, pulling the open connection to an out-of-range voltage. The AD4111 overcomes this problem by providing a unique open-circuit detection feature using either a 5 V or 3.3 V supply. This method separates open circuit detection from out-of-range faults, further simplifying diagnosis. By including this feature inside the AD4111, there is no need for a pull-up resistor on the front end, and therefore a 15 V supply, as shown in Figure 2. Elimination of ±15 V supplies reduces isolation circuit complexity, area, and emissions. For applications that do not require open-circuit detection, another general-purpose AD4112 can be used. This device has all the advantages of the AD4111, but without open circuit detection.

system level solution

The AD4111 integrates a voltage reference and an internal clock, helping to further reduce board size and BOM cost, while allowing the use of external components for situations where higher accuracy and lower temperature error conversions are required. Figures 2 and 3 show typical high-end and low-end solutions, respectively. The proportion of the signal chain that can be completely replaced by the AD4111 is highlighted in Figures 2 and 3. The total unadjustable error (TUE) accuracy specification for the AD4111 is designed to meet system-level requirements. For many solutions, the accuracy may be sufficient to omit any additional calibration. In existing high-precision solutions, modules are typically calibrated on a per-channel basis. The AD4111 is designed with highly matched inputs, so calibrating one input provides similar accuracy on all inputs.

An article explaining the design of simplified analog input modules for process control

Figure 2. Typical high-end solution.

An article explaining the design of simplified analog input modules for process control

Figure 3. Typical low-end solution.

EMC testing

PLC and DCS modules often operate in harsh industrial environments and must withstand electromagnetic interference (EMI) conditions. This adds complexity when designing an electromagnetic compatibility (EMC) capable input module, since most devices are not rated for EMC, complicating the design of input protection and filtering circuits. This can significantly increase design and test development time. EMC labs are expensive to rent, and test failures can mean long delays until boards can be redesigned and retested. The AD4111 has been designed as a printed circuit board (PCB) to demonstrate a proven EMC solution. The board is characterized to ensure that circuit performance is not permanently affected by radiated radio frequency (RF) or conducted RF interference, and has been shown to have adequate immunity to electrostatic discharge (ESD), electrical fast transients (EFT) and surges capability, in accordance with the IEC 61000-4-x standard set. The board is also evaluated against CISPR 11, and its radiated emission levels are well below the Class A limit. For more information on the AD4111 EMC PCB, see AN-1572.This application note details all the necessary information on the test procedure used, as well as the circuit board design schematic and layout for designing an EMC certified input module for the AD4111

in conclusion

The AD4111 is a highly integrated system-level ADC with full configurability. Capable of accepting ±10 V voltage inputs and 0 mA to 20 mA current inputs, it operates from a single 5 V or 3.3 V supply, has open-circuit detection and many other features, providing a unique solution for analog input module designs. Available in a 6 mm × 6 mm, 40-pin LFCSP package, modules that previously required a complete complex PCB can now be replaced by a single device.

Author: Yoyokuo