Circuit functions and advantages

The circuit shown in Figure 1 is a full-featured, flexible, programmable analog output solution that meets most requirements for programmable logic controller (PLC) and distributed control system (DCS) applications. The AD5662 low-power (0.75 mW @ 5 V), rail-to-rail output, 16-bit nanoDAC® converter and AD5750 industrial current/voltage output driver have a reference voltage requirement that exactly matches the input and output voltages. The ADR444 features low drift (3 ppm/°C maximum for Grade B), high initial accuracy (0.04% maximum for Grade B), and low noise (1.8 μV pp typical, 0.1 Hz to 10 Hz) and is capable of operating both the AD5750 and AD5662 Provides a reference voltage to ensure ultra-low noise, high accuracy, and low temperature drift. This circuit features 16-bit resolution, no missing codes, 0.05% linearity, and less than 0.2% total output error across all typical voltage and current output ranges.

The circuit also has some important features that support industrial applications, such as on-chip output fault detection and protection (short circuit, undervoltage output, open circuit current output, and overtemperature), CRC check for packet error protection (PEC), and flexible Power-on options, etc., 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 AD5662 is a single-channel, low-cost, low-power, rail-to-rail voltage buffered output nanoDAC® device that guarantees ±1 LSB DNL over a wide reference voltage range from 0.75 V to VVD supplies, so the AD5750 and AD5662 can It is powered by a 4.096 V common reference voltage source provided by the ADR444. The entire system will benefit from the ADR444's ultra-low noise and low temperature coefficient. The ADR44x family of voltage references feature source and sink current capabilities, making them ideal for driving the reference or power input of a converter. The AD5662 also has a built-in power-on reset circuit to ensure that the DAC output powers up to mid-range or zero level and maintains that level until a valid write operation is performed.

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 has built-in ESD protection diodes to prevent 3 kV transients (human body model) from damaging the device. However, industrial control environments can subject I/O circuits to much higher transients. The EVAL-CN0202-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 CN0202 Design Support Package (EVAL-CN0202-SDPZ-SCH pdf file): www.analog.com/CN0202-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 Unraveling 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 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. Data was obtained in 0V to 5V voltage output mode at 25°C. 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-CN0202-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 1.54% FSR output error from 0 mA to +20.4 mA includes gain error, which is eliminated by customer calibration.