Good partners for simplifying PLC/DCS module design

Program Logic Control (PLC) or Distributed Control System (DCS) is a quite important component in industrial control application, in which Analog Digital Converter (ADC) has a decisive influence on the accuracy and stability of PLC/DCS module. This text will introduce AD4111 analog digital converter launched by Analog Devices, Inc. (ADI) and help you have a more exact knowledge of its functions and characteristics.

Highly integration for reducing development cost of PLC/DCS module

PLC and DCS modules commonly operate in a hostile industrial environment and must bear electromagnetic interference (EMI). In the process of designing input modules with the function of electromagnetic compatibility (EMC), complexity must be added because the rated value of most devices is not suitable for EMC. Thus, the design of input protection and filter circuit become complex, which may obviously increase the time for designing and test development. The rent for the EMC lab is very expensive and the failure of tests may be equal to a long-time delay. Redesigning and retesting the circuit board will largely improve the development cost of products.

In the process of designing input modules for process control applications like PLC or DCS module etc., cost performance is usually the major trade-off. Traditionally, this application field applies bipolar ±15V power rail to provide active front-end components for the attenuation or transmission gain of input signals. But it may influence the cost of Bill of Material (BOM) and setting up isolated bipolar power supplies will increase the design complexity. In order to save cost, another method is to use single 5V power supply design architecture. The single 5V power rail obviously reduces the design complexity of analog front-end isolated power supply, but it will introduce other disadvantages like reducing the accuracy of measurement solution. The AD4111 launched by ADI has conducted many integration works required by voltage and electric current measurement, which solves the limitation of 5V power supply solution.

Being a 24-bit sigma delta (Σ-Δ) analog digital converter with low power consumption and low noise, the AD4111 integrates an analog front end (AFE) which is suitable for fully differential or single-ended high impedance (≥1 MΩ) bipolar ±10V voltage input and the current input from 0mA to 20mA. It also integrates key analog and digital signal conditioning blocks that can input eight kinds of independent settings of channel configuration for each analog. For completely stable data, the maximum channel scanning rate of AD4111 is 6.2 kSPS (161 µs).

With embedded 2.5V low drift (5 ppm/) band gap internal reference voltage source (with output reference buffer), the AD4111 can reduce the number of external components. Its application of digital filter can achieve flexible setup including synchronous 50 Hz and 60 Hz restrain in the output data rate of 27.27 SPS. Users can make choices among different filter settings according to each channel requirement in the application. The automatic channel sequence generator enables ADC to switch among each working channel.

Precision performance of the AD4111 can be achieved through integrating the proprietary iPassives™ technology provided by ADI, with the integration of precisely matched sense resistor and resistance divider together with highly matched channel-to-channel simplify calibration requirements. The AD4111 has been calibrated before delivery, which can achieve specified high precision and support as many as eight single-ended voltage inputs and four current inputs. This new product is suitable for the reconfiguration of platform solution and meanwhile reduces PLC and DCS modules’ size, complexity and cost.

In addition, the voltage pin of the AD4111 can provide an absolute maximum rating of ±50V and the current pin can provide the rating of ±50mA. With the unique function of conducting open wire detection for voltage input, it can use single 5V or 3.3V power to fulfill system level diagnosis which can detect in case of the disconnection between external sensor or signal source and system input.

AD4111 uses single power to operate, thus it is convenient for the usage in electrical isolation applications with the specified operating temperature range between 40to +105. Adopting 40-pin 6 mm × 6 mm LFCSP package for the AD4111, the module requiring complete and complex PCB can be replaced by a single device. The AD4111 can be applied in PLC and DCS process control applications, which can prevent the circuit performance from permanent influence of radio frequency (RF) or conduction radio-frequency interference. What’s more, it has been proved to have enough abilities of electro-static discharge (ESD), electrical fast transient (EFT) and surging. While using with the ADuM5411 four-channel digital isolator, AD4111 supports the stability test with EMC/EMI standard and IEC 61000-4-x standard set, and conducts evaluation for the CISPR11 with the radiated emission level far below Class A limiting value.

In order to assist customers to quicken the development speed of products, ADI also launches the EVAL-AD4111SDZ evaluation kit which adopts AD4111 24-bit 31.25 kSPS analog digital converter with the input voltage range of ±10 V (4 differential channels or 8 single-ended channels) and open wire detection function. The operating current range of 4 current channels is from 0mA to 20mA and all channels have onboard over-voltage and over-current protection.

The EVAL-AD4111SDZ board includes reference voltage source, power source and data insulation, which can be linked to SDP-B system demonstration platform (EVAL-SDP-CB1Z) of ADI. SDP-B board can be linked to a PC via USB port and can also supply power for the EVAL-AD4111SDZ board from the PC USB port. The evaluation software named AD411X Eval+ used for controlling and data analysis can configure the AD4111 device function as well as provide open wire detection and DC time-domain analysis for ADC performance evaluation in the forms of waveform graphs, histograms and relevant noise analysis.

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