Test & measurement


cent accuracy to protect the system and avoid inserting any remarkable measurement errors. To achieve high measurement accuracy, we use a precision resistance matrix to replace the PT-1000 sensor and simulate changes in temperature. This precision resistance matrix has been calibrated with a Keysight Technologies 3458A multimeter. To ease the difficulty of removing matched lead resistance errors, we use a 4-wire configuration to evaluate the system’s accuracy performance. This is more conducive to eliminating sensor errors. To calculate the system error more accurately, we need to convert the resistance values to temperature using the same standard as the LTC2983. The temperature lookup table published by the sensor manufacturer is the most accurate conversion method. However, it would be unwise to write every temperature point into the processor’s memory. Therefore, we use the following formula to compute the temperature results.


When T > 0°C, the equation is:


Calculate the temperature corresponding to the resistance value:


TVS ERROR CONTRIBUTION AND OPTIMISED CONFIGURATION You can find the I-V curve characteristics of a TVS from the device’s data sheet. However, most TVS manufacturers only provide typical values for the device’s parameters - not all the I-V data you may need to calculate the error contribution of the TVS at a particular voltage, especially the leakage current error.


A Littelfuse SMAJ5.0A TVS is used in this application. After testing some samples, we


found the leakage current to be about 1µA at 1V reverse voltage, far less than the TVS data sheet maximum reverse leakage. This leakage current contributes significant error to the system. But if the excitation current rotation of the LTC2983 is enabled, the leakage error effect will be greatly reduced. Figure 10 shows the excitation current rotation configuration and TVS leakage current flow.


When the current flowing through Rsense is the same as the excitation current flowing


through the RTD, the resistance of the RTD, RT, can be expressed by:


When using the excitation current rotation configuration for forward excitation flow (shown


in Figure 10(a)), the RTD resistance RRTD1 is calculated by:


Figure 7. System error vs. temperature. resistor


When T ≤ 0°C, the equation is: The temperature is obtained by polynomial


fitting:


where: T is the RTD temperature, °C.


RRTD(T) is the RTD resistance, Ω. R0 is the RTD resistance at 0°C, R0 = 1000Ω. A = 3.9083 × 10–3


B = –5.775 × 10–7 C = –4.183 × 10–12 Figure 7 shows that the total system error does not exceed ±0.4°C over the temperature range of –134°C to +607°C. Compared to Figure 9, which shows the error contribution of the LTC2983 to the RTD temperature measurement, the additional protection component adds approximately ±0.3°C to the system error, especially the TVS leakage current. We can see that as the temperature rises, the system error increases. This is where the TVS’s I-V curve characteristics come in.


System error can be calculated by:


where: Terror is the total LTC2983 temperature measurement system output error, °C.


Tcal is the computed temperatures by precision resistor, which has been calibrated with


a Keysight Technologies 3458A, °C.


TLTC2983 is the LTC2983 output temperature, °C. Figure 8 tells us that the total system peak-to- peak noise does not exceed ±0.01°C. This result corresponds with the data sheet.


Instrumentation Monthly February 2024


Where: Rsense is the real resistance value of sense


RRTD is the real resistance value of RTD in measurement cycle


Vsense1 is the measured voltage value at sense resistor


VRTD1 is the measured voltage value at RTD in forward excitation flow cycle, as Figure 10(a) shows.


RRTD1 is the calculated value of RTD in forward excitation flow cycle


Figure 8. System peak-to-peak noise vs. temperature.


When using the excitation current rotation configuration for reverse excitation flow (shown


Figure 9. LTC2983 error contribution to RTD temperature measurement.


Figure 10. Excitation current rotation configurations: (a) the forward excitation flow and (b) the reverse excitation flow.


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