Test & measurement
interference levels and more complicated EMC events, such as electrostatic discharge (ESD), electrical fast transients (EFT), radiated susceptibility (RS), conducted susceptibility (CS), and surges.
Additional discrete protection components are necessary to reduce the risk of damage to the downstream devices and improve the system’s robustness.
Figure 2. EMC-protected LTC2983 temperature measurement system.
RTD TEMPERATURE MEASUREMENT SOLUTIONS Take the LTC2983 temperature measurement AFE as an example. The system controller can read the calibrated temperature data from the LTC2983 directly via the SPI interface with an accuracy of 0.1°C and a resolution of 0.001°C. When a 4- wire RTD is connected, the excitation current rotation function can automatically eliminate the parasitic effect of the thermocouple and reduce the effect of the signal circuit leakage current. Based on these features, the LTC2983 can accelerate the design of multichannel precision temperature measurement systems and achieve high EMC performance without complex circuit design, giving you and your customers more confidence. Figure 2 shows the EMC-protected LTC2983 temperature measurement system block diagram. An RTD is undoubtedly the best choice for high accuracy temperature measurement and can measure temperature in the range of – 200°C to +800°C. 100Ω and 1000Ω platinum RTDs are the most common, but they can also be made of nickel or copper. The simplest RTD temperature measurement system is a 2-wire configuration, but the lead resistance introduces additional system temperature errors. A 3-wire configuration can eliminate the lead resistance errors by applying two matched current sources to the RTD, but the lead resistance should be equal. A Kelvin configuration or 4-wire configuration can remove the balanced or unbalanced lead resistance by measuring directly across the sensor using high impedance Kelvin sensing. However, the cost will be the main constraint for the 4-wire configuration as it needs more cables, especially for remote temperature measurement. Figure 3 shows the different RTD wire configurations. Considering real customer use
cases, this article chooses a 3-wire RTD configuration and tests its EMC performance. The 2-wire and 3-wire RTD sensors can also
use a Kelvin configuration on a PCB. When we need to add current limiting resistors and an RC filter to the signal link to protect the device’s analogue input pins, these additional resistances will introduce a large system offset. For example, replacing a 2-wire protection circuit with a 4- wire Kelvin configuration can help remove this offset, because the excitation current does not flow through these limiting resistors and RC filter, and the error caused by the protection resistance is negligible (see Figure 4). See the LTC2986 data sheet for more details.
The three elements of EMC events are the noise source, the coupling path, and the receiver. As shown in Figure 5, in this temperature measurement system, the noise source comes from the ambient environment. The coupling path is a sensor cable and the LTC2983 is the receiver. Industrial automation and railway applications always use long sensor cables to sense the temperature of remote devices. The length of the sensor cable may be several meters or even tens of meters. Longer cables will result in larger coupling paths, and the temperature measurement system will face more severe EMI challenges.
Figure 5. The three elements of a temperature measurement system’s EMI events.
Figure 4. A 4-wire configuration removes additional resistor errors.
WHAT ARE THE ROBUSTNESS CHALLENGES FOR TEMPERATURE MEASUREMENT SYSTEMS?
Like most temperature measurement ICs, the LTC2983 can tolerate over the 2kV HBM ESD level. But in industrial automation, railway, and other harsh electromagnetic environments, electronic devices need to face higher
Parameters Working peak reverse voltage Breakdown voltage Maximum clamping voltage
Figure 3. Different RTD wire configurations: (a) 2- wire, (b) 3-wire, and (c) 4-wire.
Instrumentation Monthly February 2024 Maximum reverse leakage
SYSTEM-LEVEL PROTECTION SOLUTION WITH A TVS Transient voltage suppressors (TVSs) and current limiting resistors are the most common protection components. Choosing the appropriate TVS and current limiting resistor can not only improve the system robustness but also maintains the high measurement performance of the system. Table 2 shows the key parameters of the TVS device, including the working peak reverse voltage, the breakdown voltage, the maximum clamping voltage, and the maximum reverse leakage. The working peak reverse voltage must be above the maximum sensor signal to ensure proper operation of the system. The breakdown voltage should not be much greater than the signal voltage to avoid creating wide, unprotected voltage ranges. The maximum clamping voltage determines the maximum interference signal voltage that the TVS can
TABLE 2. TVS KEY PARAMETERS Description
The voltage below which no significant conduction occurs
The voltage where the specified conduction is triggered
The maximum voltage across the device when conducting the specified maximum current
The leakage current when the maximum voltage is applied to the TVS before triggering conduction
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