Flexible PLC/DCS analog output module with only two analog components

L0504-Murata muRata common mode inductor 90Ω 150mA

Circuit function and advantage

The circuit shown in Figure 1 is a full-featured, flexible, programmable analog output solution that uses only two analog devices to meet most of programmable logic controller (PLC) and distributed control system (DCS) applications. Claim. The AD5660-1 is a low power (2.8 mW @ 5 V), rail-to-rail output, 16-bit nanoDAC®. The AD5750-1 is an industrial current/voltage output driver that combines all typical Current and voltage output range, 16-bit resolution with no missing codes, 0.05% linearity, and output error less than 0.1%. The circuit also has some important features that support industrial applications, such as on-chip output fault detection, CRC checking for packet error prevention (PEC), and flexible power-up options, making it ideal for building robust industrial control systems. In high-volume production, it maintains consistent performance without the need for external precision resistors or calibration procedures, making it ideal for PLC or DCS modules.

Figure 1. Basic analog output circuit for a single channel (schematic diagram, not showing all connections and protection circuits)

Circuit description

The AD5750/AD5750-1 are single-channel, low-cost, precision voltage/current output drivers designed to meet the needs of industrial process control applications. The voltage output range can be programmed for the standard output range of PLC and DCS applications: 0 V to 5 V, 0 V to 10 V, −5 V to +5 V, and −10 V to +10 V. For the standard range, a 20% overrange setting is also provided, resulting in the following options: 0 V to 6 V, 0 V to 12 V, −6 V to +6 V, and −12 V to +12 V.

The current outputs are provided on separate pins and can be programmed to the following ranges: 4 mA to 20 mA, 0 mA to 20 mA, −20 mA to +20 mA, 0 mA to 24 mA, and −24 mA to +24 mA. The unipolar range has an overrange setting of 2%. Because the current output of the AD5750/AD5750-1 can be either source or sink, it can interface with a wide range of sensors or actuators. If desired, the voltage and current output pins can be tied together to configure the end system as a single channel output.

The AD5660-1 is a single, low cost, low power, rail-to-rail voltage buffered output nanoDAC with on-chip integrated 1.25V, 5 ppm/°C reference. The AD5660-1 incorporates a power-on reset circuit that ensures that the DAC output powers up to 0 V and remains there until a valid write is performed.

The interface between the AD5660-1 DAC and the AD5750-1 driver is simple and requires no external components. The output voltage range of the AD5660-1 is 0 V to 2.5 V, matching the input range of the AD5750-1. In addition, the reference output voltage of the AD5660-1 is 1.25 V, which exactly matches the reference input requirements of the AD5750-1.

The ESD protection and overvoltage protection required for devices used in PLC and DCS applications is generally much higher than the formal recommendations. The AD5750-1 has an ESD protection diode on each pin to prevent 3 kV transient damage to the device (human body model). However, industrial control environments can subject I/O circuits to much higher transients. The EVAL-CN0203-SDPZ board contains an external 30 V/600 W transient voltage suppressor (TVS), 50 mA/30 V resettable fuse (PolySwitch) and Schottky power diode for higher voltage ESD protection. 50 mA overcurrent protection and 30 V overvoltage protection. The schematic of Figure 1 does not show an optional external protection circuit, but can be found in the detailed schematic of the CN0203 Design Support Package (EVAL-CN0203-SDPZ-SCH pdf file):

This circuit must be built on a multilayer board with a large area ground plane. For optimal performance, proper layout, grounding, and decoupling techniques must be used (see Tutorial MT-031 - "Implementing the Grounding of Data Converters and Unlocking the Mystery of AGND and DGND") and Tutorial MT-101 - "Decoupling technology").