Circuit functions and advantages

The circuit shown in Figure 1 is a full-featured, flexible, programmable analog output solution using only two analog devices that meets most programmable logic controller (PLC) and distributed control system (DCS) applications. Require.   The AD5660-1 , a low power (2.8 mW @ 5 V), rail-to-rail output, 16-bit nanoDAC®, and the AD5750-1 , an industrial current/voltage output driver, combine to provide all typical Current and voltage output range, 16-bit resolution with no missing codes, 0.05% linearity, and less than 0.1% output error. 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 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 standard output ranges for 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, an overrange setting of 20% is provided, giving 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 output is provided on a separate pin 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. Unipolar ranges have a 2% overrange setting. Because the current output of the AD5750/AD5750-1 can either source or sink current, 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-channel, low-cost, low-power, rail-to-rail voltage buffered output nanoDAC with an on-chip integrated 1.25V, 5 ppm/°C voltage reference. The AD5660-1 contains a power-on reset circuit that ensures that the DAC output powers up to 0 V and remains there until a valid write operation 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. Additionally, the AD5660-1's reference output voltage is 1.25 V, which exactly matches the AD5750-1's reference input requirements.

Devices used in PLC and DCS applications generally require much more ESD protection and overvoltage protection than are formally recommended. Each pin of the AD5750-1 has built-in ESD protection diodes to prevent 3 kV transients from damaging the device (human body model). However, industrial control environments can subject I/O circuits to much higher transients. The EVAL-CN0203-SDPZ circuit board has built-in external 30 V/600 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 30 V overvoltage protection. The optional external protection circuit is not shown in the schematic diagram of Figure 1, but can be found in the detailed schematic of the CN0203 Design Support Package (EVAL-CN0203-SDPZ-SCH pdf file): www.analog.com/CN0203-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 AD5750-1 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 AD5750-1 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 the EVAL-CN0203-SDPZ circuit board at 25°C, powered by an Agilent E3631A DC power supply, and 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 0.24% FSR output error from 0 mA to +24.5 mA includes gain error, which is eliminated by customer calibration.