FEATURE PLCs, Industrial PCs & HMIs
Precision and robustness delivered
David Forde, Claire Croke and Jean McAdam of Analog Devices discuss precision solutions with protection and robustness for analogue outputs in process control
P
recision analogue output modules, known as programmable logic controllers (PLCs) or distributed control systems (DCS), produce
voltage or current outputs used to govern actuators that manage process parameters such as fl ow, temperature and pressure. These modules need to deliver robust, reliable and precise outputs in harsh industrial environments.
The combination of the AD5423 single- channel, 16-bit I/V output DAC and ADG5401F overvoltage protected SPST switch from Analog Devices are suited to these requirements for analogue output modules.
Precision and robustness Precision is a key feature of the AD5423. In voltage out mode, TUE is as low as ±0.01% at 25°C (±0.05% across temperature) with typical output drift of 0.35ppm FSR/°C. In current out mode, TUE is also ±0.01% at 25°C with typical output drift of 2ppm FSR/°C. Diff erential nonlinearity (DNL) in all output modes is ±1 LSB and guaranteed monotonic. The ADG5401F precision switch has an on- ) specifi cation of 6Ω and has an
resistance (RON
integrated secondary feedback channel that is used to connect the channel IOUT
/VOUT +VSENSE to the input on the AD5423; this removes
any error associated with the variation in on resistance of the ADG5401F. The maximum on leakage of the ADG5401F across temperature is 40 nA. This leakage accounts for less than 1 LSB on a 16-bit, 4-20mA current output DAC maintaining precision in the output signal chain.
Figure 1: AD5423 and ADG5401F configuration
Figure 3: AD5423
functional block diagram
The ADG5401F overvoltage protected SPST switch is used on the analogue output of the AD5423 DAC for both, powered and unpowered state. The ADG5401F can tolerate up to ±60V overvoltage on the source (S) and source feedback (SFB) pins. This will protect precision analogue output nodes that are vulnerable to damage caused by loss of system power, miswiring, power supply sequencing issues, etc; see Figure 1. The ADG5401F power supplies set the overvoltage fault threshold, so if the voltage on the source pins (S or SFB) goes above the ADG5401F supply voltage, this is considered a fault, and the main switch channel and the secondary feedback channel will automatically open. When the switch channels are open during a fault, any large fault currents are prevented from fl owing back into the DAC output and into the system power supply. With no large fault currents fl owing during an overvoltage event, the system power dissipation is no longer constrained by the power dissipation during a fault, thereby reducing the design eff ort required for the system power supplies. The ADG5401F allows the system to remove current- limiting resistors in the output signal path that can cause headroom issues in some applications. The ADG5401F includes an integrated open-loop prevention switch. If the VOUT IOUT
/ Figure 2: ADG5401F circuit diagram
node is subjected to an overvoltage signal up to ±60V, the ADG5401F will implement its overvoltage protection mode, and the main channel and secondary feedback channel switches will open. At the same time, the internal open-loop prevention switch, an internal connection between D and DFB, will close. This open- loop prevention switch keeps the DAC
22 February 2021 | Automation
output feedback loop intact and prevents the DAC from driving the output to the rails. To achieve protection from high voltage transients, such as IEC 61000-4-2 ESD, IEC 61000-4-4 electrical fast transient (EFT), and IEC 61000-4-5 surge, implement a circuit (Figure 2) by using discrete resistors and a transient voltage suppression (TVS) device. Place the resistors inside the feedback loop of the system so that the resistors do not add any error to the system output.
Diagnostics The AD5423 contains a 12-bit internal diagnostic ADC that provides diagnostic information on user-selectable inputs such as supplies, grounds, internal die temperatures, and references. On-board diagnostic registers contain fl ags that indicate various fault situations, as well as a FAULT pin that triggers for any fault. In voltage out mode, short-circuit detection is monitored while in current out mode, open- circuit detection is monitored. The AD5423 also off ers a cyclic redundancy check (CRC), which performs a check on the received data and triggers the FAULT pin if the current data package appears to be incorrect. Temperature monitoring is also available, which registers a fault if the die temperature exceeds a programmed limit. Together, the AD5423 and the ADG5401F
off er the precision and robustness required by modern analogue output modules, while the ADG5401F maintains that precision and provides robust protection against external stresses in harsh environments.
CONTACT:
Analog Devices
www.analog.com
automationmagazine.co.uk
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