Circuit functions and advantages

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

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

The ADR444 is a low drift (3 ppm/°C maximum, Grade B), high initial accuracy (0.04% maximum, Grade B), low noise (1.8 μV pp typical, 0.1 Hz to 10 Hz) voltage reference , providing a reference voltage for AD5751 and AD5662, ensuring that the circuit has ultra-low noise, high accuracy, and low temperature drift characteristics. 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 needed between the microcontroller and the analog signal chain. The ADuM5401 also provides an isolated 5 V power supply. The circuit also has some important features that 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. It maintains consistent performance in high-volume production without the need for external precision resistors or calibration procedures, making it ideal for PLC or DCS modules.

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 operating voltage range of the AD5751 is 10.8 V to 55 V, and the voltage output can be as high as 44 V. The voltage output range can be programmed to the standard output range and 20% over-range settings for PLC and DCS applications: 0 V to 5 V, 0 V to 10 V, 0 V to 6 V, and 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 on 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 are also 2% over-range settings, 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 as a single-channel output.

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

The ADuM1301 is a three-channel digital isolator. The ADuM5401 is a quad-channel digital isolator with an integrated isoPower® isolated DC/DC converter. They are based on iCoupler® technology and are used to provide isolation between the signal chain and the system microcontroller, with an isolation rating of 2.5 kV rms. The ADuM5401 provides 5 V isolated power to all circuitry on the secondary side.

Digital power and analog power should be separated and connected through ferrite beads. Each supply pin is decoupled with a 10 μF capacitor and a 0.1 nF ceramic capacitor in parallel. See the schematics in the CN0204 Design Support Package for more information.

Devices used in PLC and DCS applications generally require much more ESD protection and overvoltage protection than are formally recommended. Each pin of the AD5751 has built-in ESD protection diodes to prevent transient damage to the device. However, industrial control environments can subject I/O circuits to much higher transients. The EVAL-CN0204-SDPZ circuit board has built-in external 64 V/1500 W transient voltage suppressor (TVS), 50 mA/30 V resettable fuse (PolySwitch) and Schottky power diode to provide higher voltage ESD protection, 50 mA overcurrent protection and 64 V overvoltage protection. The optional external protection circuit is not shown in the schematic of Figure 1, but can be found in the detailed schematic of the CN0204 Design Support Package (EVAL-CN0204-SDPZ-SCH pdf file): www.analog.com/CN0204-DesignSupport

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

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 application needs. The INL, DNL and output error measurement results of this circuit are shown in Figure 2, Figure 3 and Figure 4 respectively. Measurement conditions are 25°C, voltage output mode and using internal current sense resistor. The AD5751 range is set to 0 V to 5 V. Test results for all other ranges are listed in Table 1.

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

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 over the 0 mA to 20.4 mA range includes gain error, which can be eliminated by customer calibration.