CN0204

The AD5751 is a single channel, low cost, precision voltage/current output driver developed to meet the requirements of industrial process control applications. The AD5751 is specified to operate with a power supply range from 10.8 V to 55 V, and the voltage output can be up to 44 V. The voltage output range can be programmed for the standard output ranges and 20% overrange settings for PLC and DCS applications: 0 V to 5 V, 0 V to 10 V, 0 V to 6 V, 0 V to 12 V. In addition, two high voltage output ranges are also provided: 0 V to 40 V and 0 V to 44 V.

The current output, which is provided on a separate pin, can be programmed for the standard ranges of 4 mA to 20 mA, 0 mA to 20 mA, 0 mA to 24 mA. There is also a 2% overrange setting which provides 3.92 mA to 20.4 mA, 0 mA to 20.4 mA, and 0 mA to 24.5 mA. The voltage and current output pins can be tied together to configure the end system as a single-channel output if desired.

The AD5662 is a single channel, low cost, low power, rail-to-rail voltage buffered output nanoDAC device. The AD5662 guarantees ±1 LSB DNL under a wide range of reference voltages that can vary from 0.75 V to the VDD supply. In the circuit of Figure 1, the AD5751 and AD5662 operate from a common reference source of 4.096 V, provided by the ADR444. The whole system will benefit from the ultralow noise and low temperature coefficient of the ADR444. The ADR44x family of references are ideal for driving either the reference or the power input of converters because they can source and sink current. The AD5662 also incorporates a power-on reset circuit to ensure that the DAC output powers up to either midscale or zero and remains there until a valid write takes place.

The ADuM1301 is a triple channel digital isolator. The ADuM5401 is a quad channel digital isolator with isoPower^® integrated, isolated, dc-to-dc converter. They are both based on the iCoupler^® technology. They 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 an isolated 5 V power supply for all the circuit in the secondary side.

The digital power and analog power should be separated and connected with a ferrite bead. Each power is well decoupled by a 10 μF paralleled with a 0.1 μF ceramic capacitor. See the schematics in the CN0204 Design Support package for more details.

Devices for PLC and DCS applications generally need ESD protection and overvoltage protection much higher than the formal recommended specifications. The AD5751 has integrated ESD protection diodes internal to each pin that can prevent damage from transients. However, the industrial control environment can subject I/O circuits to much higher transients. External 64 V, 1500 W transient voltage suppressors (TVS); a 50 mA, 30 V PolySwitch; and power Schottky diodes are built into the EVAL-CN0204-SDPZ circuit board to provide higher voltage ESD protection, 50 mA overcurrent, and 64 V overvoltage protection. The optional external protection circuits are not shown in the simplified schematic of Figure 1 but can be found in the detailed schematic (EVAL-CN0204- SDPZ-SCH pdf file) in the CN0204 Design Support package: www.analog.com/CN0204-DesignSupport.

The circuit must be constructed on a multilayer PC 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 Convertersand Solving the Mystery of "AGND" and "DGND," and Tutorial MT-101, Decoupling Techniques).

Measurements

Integral nonlinearity (INL), differential nonlinearity (DNL), and output error are the most important specifications for PLC, DCS, and other process control systems. The AD5751 has highly flexible and configurable output ranges that can be tailored to meet the needs of the application. Measurements of INL, DNL, and output error for the circuit are shown in Figure 2, Figure 3, and Figure 4, respectively. This data was taken at 25°C in the voltage output mode using the internal current sense resistor. The AD5751 range was set for 0 V to 5 V. The test results for all the other ranges are listed in Table 1.

The test results shown in Table 1 were made at 25°C using the EVAL-CN0204-SDPZ board powered by an Agilent E3631A dc supply and measured with an Agilent 34401A digital multimeter.

Note that the output ranges 0 mA to 20.4 mA and 0 mA to 24.5 mA are both designed to be trimmed by the customer to precisely match the 0 mA to 20 mA and 0 mA to 24 mA ranges. The 1.70% FSR output error measurement in the 0 mA to 20.4 mA range includes the gain error, which can be removed by customer calibration.