Flexible, high-precision, low-drift PLC/DCS analog output module-EEWORLD

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Circuit Functionality and Benefits
The circuit shown in Figure 1 is a full-featured, high voltage (up to 44 V), flexible, programmable analog output solution that meets most requirements for programmable logic controller (PLC) and distributed control system (DCS) applications.

The AD5662 is a low power (0.75 mW @ 5 V), rail-to-rail output, 16-bit nanoDAC® device and the AD5751 is an industrial current/voltage output driver that share the same input and output voltage ranges and reference voltage requirements.

The ADR444 is a low drift (3 ppm/°C maximum for B grade), high initial accuracy (0.04% maximum for B grade), low noise (1.8 μV pp, 0.1 Hz to 10 Hz typical) voltage reference that provides the reference voltage for the AD5751 and AD5662, ensuring the circuit has ultralow noise, high accuracy, and low temperature drift. The circuit provides all typical current and voltage output ranges, 16-bit resolution with no missing codes, 0.05% linearity, and less than 0.2% total output error.

The ADuM1301 and ADuM5401 provide all the signal isolation required between the microcontroller and the analog signal chain. The ADuM5401 also provides a 5 V isolated power supply. The circuit also has some important features to support industrial applications, such as on-chip output fault detection, CRC check to prevent packet errors (PEC), and flexible power-on options, making it ideal for building robust industrial control systems. In high-volume production, it can maintain consistent performance without external precision resistors or calibration procedures, making it an ideal choice for PLC or DCS modules.

Figure 1. Basic analog output circuit for a single channel (simplified schematic; all connections and protection circuitry not shown)

Circuit Description

The AD5751 is a single-channel, low-cost, precision voltage/current output driver designed to meet the needs of industrial process control applications. The AD5751 operates over a 10.8 V to 55 V voltage range and can output voltages up to 44 V. The voltage output range can be programmed for standard output ranges for PLC and DCS applications and 20% overrange settings: 0 V to 5 V, 0 V to 10 V, 0 V to 6 V, 0 V to 12 V. Two high voltage output ranges are also available: 0 V to 40 V and 0 V to 44 V.

The current output is provided via a separate pin and can be programmed to the following standard ranges: 4 mA to 20 mA, 0 mA to 20 mA, 0 mA to 24 mA. There is also a 2% overrange setting providing 3.92 mA to 20.4 mA, 0 mA to 20.4 mA, and 0 mA to 24.5 mA. If desired, the voltage and current output pins can be tied together to configure the end system for a single channel output.

The AD5662 is a single-channel, low-cost, low-power, rail-to-rail voltage buffered output nanoDAC device. The AD5662 guarantees a DNL of ±1 LSB over a wide reference voltage range from 0.75 V to the VDD supply voltage. In the circuit shown in Figure 1, the AD5751 and AD5662 operate from a common 4.096 V reference source provided by the ADR444. The entire system benefits from the ultralow noise and low temperature drift of the ADR444. The ADR44x family of references can source and sink current, making them ideal for driving reference voltages or supply inputs. The AD5662 also incorporates a power-on reset circuit that ensures that the DAC output powers up to midscale or 0 V and remains at that level until a valid write operation is performed.

The ADuM1301 is a three-channel digital isolator. The ADuM5401 is a four-channel digital isolator with an integrated isoPower® isolated dc/dc converter. Both are based on iCoupler® technology and are used to isolate the signal chain from the system microcontroller with an isolation rating of 2.5 kV rms. The ADuM5401 provides 5 V isolated power to all circuits on the secondary side.

The digital power supply should be separated from the analog power supply and connected through a ferrite bead. Each power supply pin is decoupled through a 10 μF capacitor and a 0.1 nF ceramic capacitor in parallel. For more information, see the schematics in the CN0204 Design Support Package.

Devices used in PLC and DCS applications typically require much higher ESD and overvoltage protection than formal recommendations. The AD5751 has internal ESD protection diodes on each pin to prevent transients from damaging the device. However, industrial control environments can subject I/O circuits to much higher transients. The EVAL-CN0204-SDPZ circuit board includes an external 64 V/1500 W transient voltage suppressor (TVS), a 50 mA/30 V resettable fuse (PolySwitch), and Schottky power diodes to provide higher voltage ESD protection, 50 mA overcurrent protection, and 64 V overvoltage protection. The optional external protection circuitry is not shown in the simplified schematic of Figure 1, but can be found in the detailed schematics (EVAL-CN0204-SDPZ-SCH pdf file) of the CN0204 Design Support Package: www.analog.com/CN0204-DesignSupport

This circuit must be constructed on a multilayer circuit board with a large area ground plane. Proper layout, grounding, and decoupling techniques must be used to achieve optimum performance (see Tutorial MT-031, Grounding Data Converters and Solving the Mystery of AGND and DGND, and Tutorial MT-101, Decoupling Techniques).

Measurement

For PLC, DCS and other process control systems, integral nonlinearity (INL), differential nonlinearity (DNL) and output error are the most important performance indicators. The AD5751 has a very flexible and configurable output range to meet the application needs. The INL, DNL and output error measurement results of this circuit are shown in Figure 2, Figure 3 and Figure 4 respectively. The measurement conditions are 25°C, voltage output mode and using the internal current sense resistor. The AD5751 range is set to 0 V to 5 V. The test results of all other ranges are listed in Table 1.

The test results shown in Table 1 were obtained at 25°C using the EVAL-CN0204-SDPZ circuit board and an Agilent E3631A dc power supply, using an Agilent 34401A digital multimeter.

Note that the customer needs to adjust the output ranges 0 mA to 20.4 mA and 0 mA to 24.5 mA to exactly match the 0 mA to 20 mA and 0 mA to 24 mA ranges. The 1.70% FSR output error in the 0 mA to 20.4 mA range includes the gain error, which can be removed by the customer through calibration.

Figure 2. INL for 0 V to 5 V output range.

Figure 3. DNL for 0 V to 5 V output range.

Figure 4. Output error for 0 V to 5 V output range.

Common changes

This circuit shows one implementation of a family of driver and DAC products from which other devices can be selected to achieve the desired performance. 12-bit to 16-bit resolution and 0.1% accuracy are typical requirements for PLC and DCS applications. For applications that do not require 16-bit resolution, the AD5620 (12-bit) and AD5640 (14-bit) can be used, which also have an internal reference.

The AD5623R (12-bit), AD5643R (14-bit), and AD5663R (16-bit) are dual-channel nanoDAC devices, and the AD5624R (12-bit), AD5644R (14-bit), and AD5664R (16-bit) are quad-channel nanoDAC devices, all suitable for multichannel applications.

The AD5750-1 and AD5750 are bipolar analog output drivers that can provide ±10 V and ±20 mA.

Circuit Evaluation and Testing

Equipment requirements (equivalent equipment may be substituted)

System Demonstration Platform (EVAL-SDP-CB1Z)
CN-0204 Circuit Evaluation Board (EVAL-CN0204-SDPZ)
CN-0204 Evaluation Software
Software for controlling external test and measurement equipment (not included on CD)
Agilent 34401A 6.5 Digital Multimeter
Agilent E3631A 0 V-6 V/5 A, ±25 V/1 A triple output DC power supply
PC with USB port (Windows® 2000 or Windows XP)
National Instruments GPIB to USB-B Interface and Cable

Getting Started
Load the evaluation software by placing the CN0204 evaluation software CD into the CD drive of the PC. Open "My Computer", locate the drive that contains the evaluation software CD, and open the Readme file. Follow the instructions in the Readme file to install and use the evaluation software.

Functional Block Diagram
Figure 5 shows a functional block diagram of the test setup. The pdf file “EVAL-CN0204-SDPZ-SCH” contains the detailed circuit schematics for the CN0204 evaluation board. This file is included in the CN-0204 Design Support Package: www.analog.com/CN0204-DesignSupport

Connect
the 120-pin connector on the EVAL-CN0204-SDPZ circuit board to the connector marked “CON A” or “CON B” on the EVAL-SDP-CB1Z (SDP) evaluation board. Nylon hardware should be used to secure the two boards together using the holes provided at the ends of the 120-pin connector. After setting the dc output power supply to +25 V, −25 V, and +6 V outputs, turn off the power supply.

The Agilent E3631A power supply has both ±25 V/1 A and 6 V/5 A output channels. The ±25 V/1 A and 6 V/5 A are isolated from each other. Therefore, the ±25 V/1 A can be used to provide the +50 V power supply for the circuit. There must not be any external connection between the “−” pin of the 6 V/5 A power supply and the “COM” pin of the ±25 V/1 A power supply.

With power off, connect a +25 V power supply to the CN3 pin labeled “12~50VIN” on the board and a −25 V power supply to the CN3 pin labeled “GND_ISO” on the board. Leave the ±25 V/1 A “COM” unconnected. Connect +6 V to CN2 in the same manner. Turn on the power and connect the USB cable that came with the SDP board to the USB port on the PC. NOTE: Do not connect the USB cable to the mini-USB connector on the SDP board before turning on the dc power supply to the EVAL-CN0204-SDPZ.

Figure 5. Test setup functional block diagram

Testing
After setting up the test equipment, connect the CN4 pin labeled "VOUT" or the CN1 pin labeled "IOUT" to the input of the Agilent 34401A. Depending on the input signal type (current or voltage), ensure that the cables on the front panel of the Agilent 34401A are connected correctly. Testing INL, DNL, and total error takes quite a while because all the levels of the AD5662 16-bit DAC need to be set and measured by the 34401A.

The DAC codes can be set via a PC using the software provided on the CD. An automated test program is required to step through the codes and analyze the data. This program is not provided on the CD and must be implemented by the customer based on the requirements of the specific multimeter used in the test setup.

In the test configuration shown in Figure 5, the GPIB output of the 34401A multimeter is interfaced to another USB port on the PC using a National Instruments GPIB to USB-B interface and cable. The multimeter readings can then be mapped to each code in an Excel spreadsheet loaded into the PC. This data is then analyzed according to industry standard definitions to obtain INL, DNL, and total error.

For more detailed information on parameter definitions and how to calculate INL, DNL, and total error from measured data, refer to the Terminology section of the AD5662 data sheet and the following references: Data Conversion Handbook, Chapter 5, “ Testing Data Converters , ” Analog Devices, Inc.

Keywords：AD5751 AD5662 PLC DCS Reference address：Flexible, high-precision, low-drift PLC/DCS analog output module

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