Test & measurement
in Figure 10(b)), the RTD resistance RRTD2 is calculated by:
Where:
Vsense2 is the measured voltage value of sense resistor. VRTD2 is the measured voltage value of the
RTD in a reverse excitation flow cycle, as shown in Figure 10(b).
RRTD2 is the calculated value of the RTD in a reverse excitation flow cycle.
According to the TVS measured data, at 2V reverse voltage, the difference between the maximum leakage current and minimum leakage current is about 10 per cent on average. The position and matching degree of the four TVSs can cause systematic error to a significant extent. To show where the error is
the largest, we can assume that ITVS is the average leakage current and that ITVS1 = ITVS2 = 0.9 × ITVS, while ITVS3 = ITVS4 = 1.1 × ITVS.
Figure 11. System error vs. different hardware and software configurations.
If you are not using the excitation current rotation configuration, RRTD1 or RRTD2 will include the maximum TVS error contribution.
the system error, where:
RRTDROT is the final RTD resistance calculated result with the excitation current rotation.
or
Error(RRTDROT) is the TVS error contribution when using the excitation current rotation configuration, with units in °C.
are the error factors. When using the excitation current rotation
{Error(RRTD1), Error(RRTD2)}, then Error (RRTDROT) will be equal to Error (RRTD1) or Error(RRTDROT) will be equal to Error(RRTD2). According to Equation 13 through Equation 18,
configuration, the final calculated results are When the Error(RRTDROT) = min
when I = 6 × ITVS, the Error (RRTDROT) will be equal to min {Error RRTD1), Error RRTD2 }. When Iexc = 6 × ITVS, the accuracy of the system will be reduced by 16.7 per cent due to the TVS leakage current.
According to the configuration and test result, Iexc > 6 × ITVS, so
Usually, Iexc > 100 × ITVS. Figure 11 shows 48
Error(RRTD1) and Error(RRTD2) are the TVS error contribution when not using the rotation configuration, with units in °C. The derivation above tells us that the excitation current rotation configuration can reduce the TVS leakage current error contribution. The following test results confirm our assertion. Figure 11 shows the system errors in different excitation current modes and TVS configurations. As shown in the figure, the system accuracy is about the same for both rotating and nonrotating configurations when a TVS is not used. However, enabling excitation current rotation automatically eliminates the parasitic thermocouple effect, a more detailed description of which can be found in the LTC2983 data sheet. When using a TVS to protect the system, the total system error increases. But excitation current rotation configuration can significantly reduce the error impact of the TVS leakage current, thus helping to achieve a similar level of accuracy to non- TVS protected systems over most of the temperature measurement range. Compared to the system without a TVS, the additional error was contributed by the TVS device-to- device variation.
CONCLUSION Temperature measurement system design is not often considered to be a difficult task. However, for most system designers, developing a highly accurate and robust temperature measurement system is a challenge. The LTC2983 intelligent digital temperature sensor can help you overcome this challenge and create a product that can be brought to market quickly.
This protected LTC2983 temperature measurement system has ±0.4°C system accuracy. Measurement errors include the LTC2983 error, the TVS/ current limiting resistor error, and the PCB error contributions.
The LTC2983 rotation excitation current configuration can significantly reduce the protection components’ leakage current error effect.
The LTC2983 temperature measurement system provides high EMC performance despite the most common protection measures. See Table 3 for EMI test results.
This article presents the accuracy and EMC performance test results for some specific configurations. You can choose different TVS devices and current limiting resistors to obtain different measurement accuracy and EMC performance to meet your production requirements.
Analog Devices
www.analog.com February 2024 Instrumentation Monthly
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