CN0435 Analog I/O System Reference Design

ADI's CN0435 reference design focuses on demonstrating the development of a PLC/DCS system controlled by a Modbus master and exemplifies how the latest features of key components are used. A single node system is often referred to as a PLC, whereas a larger system is often referred to as a DCS. Each node can control up to 16 analog field devices, sensors, or actuators, either highway addressable remote transducer (HART)-compatible or analog-only, and systems can be extended to include as many as 16 individual nodes. The system can also be used for general-purpose precision analog data acquisition applications, such as instrumentation, analog data logging, or test and measurement.

Analog Input Board
The CN-0414 is designed to measure four fully differential or eight single-ended voltage and four current signals. The heart of the circuit is an AD4111 low power, low noise 24-bit, Σ-Δ analog-to-digital converter (ADC) with integrated ±10 V and 20 mA analog front ends. The voltage inputs support an input range of up to ±10 V. The AD4111 incorporates a unique feature that enables open wire detection on ±10 V voltage inputs while operating on a 5 V or 3.3 V single power supply; the previous solutions would typically require a supply greater than ±10 V.

The current inputs support input ranges of 0 mA to 24 mA. The input impedance of the circuit is 250 Ω (60 Ω internal to the AD4111), with all inputs referenced to isolated ground. The 250 Ω input impedance on the current inputs is necessary to make the AD5700-1 HART-compliant modem to work in conjunction with the AD4111.

The analog front end of the circuit, the AD4111, and the AD5700-1 are isolated from the processing side through ADuM5411 and ADuM3151 to offer significant space savings over discrete transformer-based solutions. The CN-0414 board is powered by a 9.5 V_DC to 36 V_DC power supply that is typical of industrial automation systems making it easy for customers to retrofit it into their systems.

Analog Output Board
The CN-0418 is a quad-channel voltage and current output board based on the AD5755-1 digital-to-analog-converter (DAC) with dynamic power control. This circuit provides 4 mA to 20 mA current outputs, as well as unipolar or bipolar voltage outputs (±10 V). The board also includes the AD5700-1 HART modem, to give a complete analog output solution with HART connectivity. External transient protection circuitry is also included, which is important for applications located in harsh industrial environments.

The current and voltage outputs are available on separate pins, but only one is active at a time, thus allowing both output pins to be tied together and connected to a single terminal. Analog outputs are short-circuited and open-circuit protected. The AD5755-1 contains integrated dynamic power control using a dc-to-dc boost converter circuit, allowing reduced power consumption in the current output mode. It has four CHART pins corresponding to each of the four output channels. The HART signal can be coupled into these pins and appears on the corresponding output if that output is enabled.

RS-485 Transceiver Board
The CN-0416 is an isolated and non-isolated RS-485 transceiver board, which allows easy implementation of data transmission between multiple systems or nodes, especially over long distances. The circuit uses the ADM2682E RS-485 transceiver for isolated communications and the LTC2865 for non-isolated RS-485 communications. Both can be configured to either full-duplex or half-duplex operation and with open or terminated transmission lines. The circuit has on-board RJ-45 jacks, which allow the use of common CAT5 Ethernet cables for fast physical wiring of nodes. The termination resistance is set by default to the CAT5 cable characteristic impedance of 100 Ω but can be configured to support the standard RS-485 cable impedance of 120 Ω.

The ADM2682E is capable of a data rate of up to 16 Mbps and has true fail-safe receiver inputs with an adjusted differential voltage threshold. It provides 5 kV signal isolation using the iCoupler data channel and 5 kV power isolation using the isoPower integrated dc-to-dc converter. The LTC2865 is capable of a data rate of up to 20 Mbps and has full fail-safe receiver inputs. An internal window comparator determines the fail-safe condition without the need to adjust the differential input voltage thresholds.