Accurate and stable, how would you rate this combination?
Industrial processes require precise and robust control of actuators to manage process parameters such as flow, temperature, and pressure. Precision analog output modules, also known as programmable logic controllers (PLCs) or distributed control systems (DCSs), can generate voltage or current outputs for controlling such actuators. These modules need to provide stable, reliable, and accurate outputs in harsh industrial environments.
The combination of ADI's single-channel 16-bit I/V output DAC AD5423 and overvoltage protection SPST switch ADG5401F meets this type of control needs and can meet the requirements of analog output modules.
Precision is a key feature of the AD5423. In voltage output mode, TUE is as low as ±0.01% at 25°C (±0.05% over temperature), with a typical output drift of 0.35 ppm FSR/°C. In current output mode, TUE is also ±0.01% at 25°C, with a typical output drift of 2 ppm FSR/°C. Differential nonlinearity (DNL) is ±1 LSB in all output modes, and monotonicity is guaranteed.
The ADG5401F precision switch has an on-resistance (R O ) of 6Ω and integrates an auxiliary feedback channel that connects channel I OUT /V OUT to the +V SENSE input of the AD5423. This eliminates any errors associated with variations in the on-resistance of the ADG5401F. The maximum on-leakage current of the ADG5401F over the entire temperature range is 40 nA. This leakage current is less than 1 LSB for a 16-bit 4 mA to 20 mA current output DAC, which does not affect the accuracy of the output signal chain and maximizes the dynamic range.
The ADG5401F overvoltage protection SPST switches are used on the analog outputs of the AD5423 DAC to provide overvoltage protection in both the powered and unpowered states. The source (S) and source feedback (SFB) pins of the ADG5401F can withstand overvoltages up to ±60 V. This protects the precision analog output nodes from damage due to system power loss, wiring errors, power sequencing, and other issues. Figure 1 shows in detail how the AD5423 and ADG5401F should be connected in the analog output module.
Figure 1. AD5423 and ADG5401F configuration.
The ADG5401F power supply sets the overvoltage fault threshold. If the voltage on the source pin (S or SFB) exceeds the ADG5401F power supply voltage, a fault is considered to have occurred and the main switch channel and the auxiliary feedback channel will be automatically disconnected.
When the switch channel is open during a fault, any large fault current is prohibited from flowing back to the DAC output and the system power supply. No large fault current flows during an overvoltage event, so the system power consumption is no longer constrained by the fault power consumption, and the design effort required for the system power supply is reduced. The ADG5401F allows the system to remove the current limiting resistor in the output signal path, which may cause load margin issues in some applications.
The ADG5401F integrates an anti-open-loop switch. If the V OUT /I OUT node is subjected to an overvoltage signal, the ADG5401F will start the overvoltage protection mode, and both the main channel and the auxiliary feedback channel switches will be opened. At the same time, the internal anti-open-loop switch (internal connection between D and DFB) will be closed. This anti-open-loop switch keeps the DAC output feedback loop intact and prevents the DAC from clamping the output to the power supply rail.
To implement high voltage transient protection, such as IEC 61000-4-2 ESD, IEC 61000-4-4 electrical fast transient (EFT), and IEC 61000-4-5 surge, discrete resistors and transient voltage suppression (TVS) devices should be used to implement a circuit similar to that shown in Figure 2. Place the resistor within the feedback loop of the system so that the resistor does not add any error to the system output.
Figure 2. ADG5401F circuit diagram.
Figure 3. AD5423 functional block diagram.
Table 1. High Voltage Transient Protection
The AD5423 contains a 12-bit internal diagnostic ADC that provides diagnostic information on user-selectable inputs such as power supply, ground, internal die temperature, and reference voltage.
On-chip diagnostic registers contain flags to indicate various fault conditions, as well as a FAULT pin that is triggered by any fault. Short-circuit detection is monitored in voltage output mode and open-circuit detection is monitored in current output mode. The AD5423 also provides a cyclic redundancy check (CRC) that verifies the received data and triggers the FAULT pin if the current packet does not appear to be correct. It also provides temperature monitoring, and a fault is logged if the chip temperature exceeds a set limit.
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