Circuit functions and advantages

The circuit shown in Figure 1 is a multi-channel, flexible analog output solution using only two analog devices. It meets the needs of multi-channel I/O cards, programmable logic controllers (PLCs) and distributed control systems (DCS). most requirements of the application. Quad-channel, 16-bit nanoDAC+ AD5686R with rail-to-rail buffered outputs, used with four industrial current/voltage output drivers AD5750-2 , provides outputs in all typical current and voltage ranges with 16-bit resolution and no missing codes , 0.05% linearity and less than 0.1% output error.

The AD5686R integrates a 2.5 V reference voltage source with high drive capability (up to ±5 mA) and ultra-low drift (2 ppm/°C, typical). It can provide the reference voltage for both the AD5686R and the AD5750-2 to ensure the circuit's reliability. Low noise, high precision, low temperature drift.

The ADuM1301 and ADuM5400 provide 2500 V rms isolation of all necessary signals between the power and analog signal chains and the host controller.

For multi-channel I/O card applications requiring more than 4 channels, multiple AD5686Rs can be connected in a daisy chain without requiring additional external digital I/O circuitry. This keeps costs to a minimum, especially for high channel count isolation applications.

The circuit also has some key features for industrial applications such as on-chip output fault detection, packet error checking (PEC) via CRC, flexible power-up options, and ESD protection (4 kV for AD5686R, Human Body Model, AD5750-2 3 kV, human body model), 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 /AD5750-2 are single-channel, low-cost, precision voltage/current output drivers designed to meet the needs of industrial process control applications. The output voltage 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 available, 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 + 24mA. Unipolar ranges have a 2% overrange setting.

Because the current output of the AD5750/AD5750-1/AD5750-2 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 connected together to configure the system as a single channel output.

Generally speaking, current output circuits require at least one precision resistor for current sensing. The circuit's current accuracy and temperature drift characteristics depend in part on the resistor and reference voltage. The AD5750/AD5750-1/AD5750-2 integrate high-precision, low-drift resistors, and external resistors can be used if desired. To improve the stability of the output current over the full temperature range, one method is to connect an external low-drift resistor to the REXT1 and REXT2 pins of the AD5750/AD5750-1/AD5750-2 to replace the internal resistor. The external resistor is selected through the input shift register. If the external resistor option is not used, the REXT1 and REXT2 pins should be left floating.

The AD5686R is a quad-channel, 16-bit, rail-to-rail voltage buffered output nanoDAC+ with an integrated 2.5 V reference of 2 ppm/°C typical (5 ppm/°C maximum). The on-chip voltage reference drives all four AD5750-2 reference inputs, has a low output impedance of 0.05 Ω, and can source and sink currents up to 5 mA. The AD5686R has a built-in 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 simple interface between the AD5686R DAC and the AD5750-2 driver requires no external reference or precision resistors. The output voltage range of the AD5686R is 0 V to 2.5 V, matching the input range of the AD5750-2. In addition, the AD5686R's reference output voltage is 2.5 V, which exactly matches the reference input requirements of the AD5750-2.

The ADuM1301 is a three-channel digital isolator. The ADuM5400 is a four-channel digital isolator that integrates an 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 ADuM5400 provides 5 V isolated power for the 5 V circuitry on the secondary side.

Devices used in PLC and DCS applications generally require much higher levels of ESD protection and overvoltage protection than are typically recommended. The AD5686R and AD5750-2 have built-in ESD protection diodes on each pin to prevent 4 kV (AD5686R) and 3 kV (AD5750-2) voltage transients (human body model) from damaging the device. However, industrial control environments can subject I/O circuits to much higher voltage transients.

Use an external 54 V, 600 W transient voltage suppressor (TVS) as the first stage of enhanced ESD protection. Place a Schottky power diode in series with a 1 kΩ, 0.5 W resistor on the VSENSE+ and VSENSE− pins of the AD5750-2, and place a 50 mA, 30 V resettable fuse on the VOUT and IOUT pins. These protection circuits are placed in the EVAL-CN0229-SDPZ circuit board to provide 50 V overvoltage protection and 50 mA overcurrent 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 CN0229 Design Support Package ( EVAL-CN0229-SDPZ-PADSSchematic pdf file): http://www.analog.com/CN0229 -DesignSupport

This circuit must be built on a multilayer circuit board (PCB) with a large ground plane. For optimal system performance and low EMI, use proper layout, grounding, and decoupling techniques (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 AD5750-2 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 voltage output mode at 25°C. The AD5750-2 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 obtained using the first channel of the EVAL-CN0229-SDPZ circuit board at 25°C, powered by an Agilent E3631A DC power supply, and measured using an Agilent 34401A digital multimeter.

Note that the customer needs to adjust the output ranges 3.92 mA to 20.4 mA, 0 mA to 20.4 mA, and 0 mA to 24.5 mA to exactly match the 4 mA to 20 mA, 0 mA to 20 mA, and 0 mA to 24 mA ranges. The 1.86% FSR output error from 0 mA to 20.4 mA includes gain error, which is easily eliminated by customer calibration.

Referenced to GND, there is a low dead zone of about 10 mV. All linearity test results are calculated using a reduced data range of 256 to 65,535. For 3.92 mA to 20.4 mA, 0 mA to 20.4 mA, and 0 mA to 24.5 mA, the low deadband is larger than other standard ranges, and the measured data range is 1000 to 65,535.