SCDA041 January 2022 TMUX7462F
The resistance temperature detector (RTD) is an excellent device to measure the signal transition of the temperature with extreme accuracy (more accurate than the thermocouple, and being able to handle higher temperature than the thermistor) helping reduce the external noise. There are various types that use two, three, or four-wire measuring systems depending upon the accuracy level desired and the expected cost. In all of the wire measuring systems, the voltage across RRTD is measured, which is converted to digital format using an analog-to-digital converter (ADC).
There are a number of fault cases that may occur that can be damaging to many of the integrated circuits in the industrial environment. Such fault conditions may include, but are not limited to, human error from miswiring connections, EOS (transient conditions above standard MUX rating), Latch-up failures, and so forth. Overvoltage protection is a desirable specification for an RTD module which provides protection to the entire signal conditioning circuit.
This article discusses the overvoltage protection solution for the RTD input systems using the fault-protected multiplexers and channel protectors.
The RTD sensor requires an excitation current to produce an output voltage. The excitation current is provided by the ADC containing the multiplexer, PGA, and internal reference. The two-wire system has one wire from each end and the three-wire system has one wire from each end and another wire from any end that goes to the measuring circuit.
Figure 1-2 shows the basic block diagram for the RTD input system with the ΔΣ ADC and protection using discrete components. As the image shows, the excitation current is provided by the IDAC1/2 for the RTD Sensors.
Generally, the discrete components are used in the arrangement in Figure 1-2 between the source or sense and the ADC. For overvoltage protection, various types of solutions are employed:
Figure 1-3 illustrates the RTD circuit configuration with the multiplexer.
The TMUX7462F is a four-channel protector with adjustable fault threshold, that can be placed on the front end of a signal path to protect analog input pins downstream from damages caused by overvoltage faults. Each of the four channels has an internal switch that is turned-off autonomously upon occurrence of an overvoltage fault without the need of external controls. This simplifies robust system-level protection designs by removing the need for control signals for each channel of the device. The overvoltage protection is available in powered and powered-off conditions, making the TMUX7462F suitable for applications with multiple channels that need to be protected simultaneously in an application. For a higher number of channels and multiplexing requirements, the TMUX7348F, an 8:1, 1-channel multiplexer with the adjustable fault threshold (secondary supplies) can also be used as an input multiplexer.
These multiplexers can accept input signals in the range of ±5 V to ±22 V and can protect the circuit against high voltage due to potential miswiring risk (outside the VFN, VFP range) up to the protection level of the MUX, that is, ±60 V.
VFP and VFN provide the much-needed protection to the ADC in the event that the input RTD is operating at a voltage level higher than the ADC and the high voltage gets exposed to the AIN pin during an overvoltage event. VFP and VFN can be configured to match the ADC supply voltage and can help clamp down the voltage to the supply voltage of the ADC.
Two secondary power supplies on the device, VFP and VFN, are the required secondary power supplies that set the level at which the overvoltage protection comes into play. VFP can be supplied from 3 V up to VDD and VFN can be supplied from VSS to 0 V.
Some of the ADC can have ±15 V (HV_AVDD, HV_AVSS) for the HV inputs. It has a +5/GND (AVDD, AGND) domain for the REF circuitry and the internal current sources. Introducing fault-protected multiplexers for the IDACs connecting the VFP, VFN to the AVDD/GND domain will work as protection to the IDAC pins and REF inputs.
The switch channels remain in the high-impedance state (regardless of switch input conditions) when the devices power supplies are floating, grounded, or at a level that is below the undervoltage (UV) threshold. If the signal level on any Sx pin exceeds the fault supply (VFP or VFN) by a threshold voltage (VT), then the Sx pin becomes high impedance and an output fault flag is asserted low to indicate a fault condition under normal operation. The drain pin (Dx) is either pulled to the fault supply voltage that was exceeded or left floating depending on the DR control logic.
The TMUX7412F and TMUX7462F is part of the fault-protected switches and multiplexers family of devices. The ability to protect downstream components from overvoltage events up to ±60 V and latch-up immunity features makes these switches and multiplexers suitable for harsh environments. Using analog switches and multiplexers in an RTD input system creates ease-of-use and have many advantages including making RTD modules secure from fault voltage, offering low noise and fast settling time.