CN0229

The AD5750/AD5750-1/AD5750-2 are single-channel, low cost, precision voltage/current output drivers developed to meet the requirements of industrial process control applications. The output voltage range can be programmed for the 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. A 20% overrange setting is also provided for the standard ranges, 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, which is provided on a separate pin, can be programmed for the ranges of +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. The unipolar ranges have a 2% overrange setting.

Because the AD5750/AD5750-1/AD5750-2 current outputs can either source or sink current, they can interface to a wide variety of sensors or actuators. The voltage and current output pins can be connected together to configure the system as a single-channel output if desired.

Generally, the current output circuit needs at least one precision resistor for current sensing. The current accuracy and temperature drift characteristic of the circuit depends partly on the resistor as well as the reference. The AD5750/AD5750-1/AD5750-2 integrates the high precision, low drift resistor and still allows the use of an external resistor, if desired. As a method of improving the stability of the output current over temperature, an external low drift resistor can be connected to the REXT1 and REXT2 pins of the AD5750/AD5750-1/AD5750-2, instead of the internal resistor. The external resistor is selected via the input shift register. If the external resistor option is not used, leave the REXT1 and REXT2 pins floating.

The AD5686R is a quad 16-bit nanoDAC+ with rail-to-rail voltage-buffered outputs integrated with an on-chip 2 ppm/°C typical (5 ppm/°C maximum), 2.5 V reference. The on-chip reference can drive the reference input of all four AD5750-2s, has a low output impedance of 0.05 Ω, and can source and sink up to 5 mA. The AD5686R incorporates a power-on reset circuit to ensure that the DAC output powers up to 0 V and remains there until a valid write command takes place.

The interface between the AD5686R DAC and the AD5750-2 driver is simple and requires no external voltage reference or precision resistors. The output voltage range of the AD5686R is 0 V to 2.5 V, which matches the input range of the AD5750-2. In addition, the reference output voltage of the AD5686R is 2.5 V, which precisely matches the reference input requirement of the AD5750-2.

The ADuM1301 is a triple-channel digital isolator. The ADuM5400 is a quad-channel digital isolator with an integrated, isolated, dc-to-dc converter. They are both 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 an isolated 5 V power supply for the 5 V circuits in the secondary side.

Devices for PLC and DCS applications generally need ESD protection and overvoltage protection much higher than the formal recommend specifications. The AD5686R and the AD5750-2 have internal ESD protection diodes to each pin that can prevent damage from a 4 kV (AD5686R) and 3 kV (AD5750-2) transient (human body model). However, the industrial control environment can subject I/O circuits to much higher transients.

Use the external 54 V, 600 W transient voltage suppressors (TVSs) as the first stage of enhanced ESD protection. Place power Schottky diodes 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 polyswitch on the VOUT and IOUT pins. These protective circuits are included in the EVAL-CN0229-SDPZ circuit board to provide 50 V overvoltage protection and 50 mA overcurrent 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-CN0229-SDPZ-PADSSchematic pdf file) in the CN0229 Design Support package: http://www.analog.com/CN0229-DesignSupport.

Construct the circuit on a multilayer printed circuit board (PCB) board with a large area ground plane. Use proper layout, grounding, and decoupling techniques to achieve optimum system performance and low EMI (see Tutorial MT-031, Grounding Data Converters and 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 AD5750-2 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. The AD5750-2 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 for the first channel of the EVAL-CN0229-SDPZ board powered by an Agilent E3631A dc supply and measured with the Agilent 34401A digital multimeter.

Note that the output ranges 3.92 mA to 20.4 mA, 0 mA to 20.4 mA, and 0 mA to 24.5 mA are all designed to be trimmed by the customer to precisely 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 measurement in the 0 mA to 20.4 mA range includes the gain error, which is easily removed by customer calibration.

There is a lower dead band about 10 mV referenced to GND. All the linearity test results were calculated using a reduced code 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 lower dead band is larger than other standard ranges, and the measured code range is from 1000 to 65,535.