Test & measurement TABLE 3. EMI TEST RESULTS IEC 61000-4 Transient RS CS ESD EFT Surge Protection Level
10V/m, 80MHz to ~1GHz, and 1.4GHz to ~2GHz
10V, 0.15MHz to ~80MHz ±8kV, conducted; ±15kV, air ±4kV, 5kHz ±4kV, 1.2/50 (8/20) s Max Temperature Fluctuation (°C) <0.5 <0.2 <0.15 <0.15 <0.2
suppress. The reverse leakage will contribute a lot to the measurement error of the system, so the TVS with the smallest possible reverse leakage should be selected.
Under normal operating conditions, the TVS device appears to have a high impedance to ground. When a transient voltage greater than the TVS breakdown voltage is applied to the input of the system, the voltage at the input is clamped as soon as the TVS breaks down and provides a low impedance path to ground, diverting the transient current from the input to ground. As Figure 2 shows, this is a 3-wire PT-1000 protected circuit. A 3-wire PT-1000 is connected to the LTC2983 by three adjacent channels, which are protected by an SMAJ5.0A TVS and a 100Ω current limiting resistor. The current limiting resistor and the downstream capacitor form a low-pass filter to remove as many RF components from the input lines as possible, to keep the AC signal balance between each line and ground, and to maintain a high enough input impedance over the measurement bandwidth to avoid loading the signal source. The differential-mode filter –3dB bandwidth is 7.9kHz, and the common-mode filter –3dB bandwidth is 1.6MHz.
This temperature measurement system was tested to IEC 61000-4-2, IEC 61000-4-3, IEC 61000-4-4, IEC 61000-4-5, and IEC 61000-4- 6 standards. Under these tests the system must work normally and provide precise temperature measurement. The sensor under test is a Class B 3-wire PT-1000, which uses a shielded wire of about 10m length. Table 3 lists the IEC 61000-4-x immunity test items, the test levels, and the temperature fluctuations when the system is disturbed by EMI events. Figure 6 displays the output temperature data curve when testing, which corresponds to the max temperature fluctuation in Table 3.
Figure 6. Output temperature data curve when testing. TABLE 4. ELECTRICAL CHARACTERISTICS OF THE LITTELFUSE SMAJ5.0A TVS
TEMPERATURE MEASUREMENT ACCURACY AFTER BEING PROTECTED The TVS and the current limiting resistor help protect the temperature measurement system from EMC. The TVS with the lowest possible clamp voltage can better protect the sensitive circuits. But they can in turn produce errors to the system. To counter this, we have to use a TVS with a higher breakdown voltage because the higher breakdown voltage means there is less leakage current at the normal operating voltage. The lower current leakage of a TVS results in fewer errors added to the system. Taking these considerations into account, we used a Littelfuse SMAJ5.0A TVS, which can be bought at most electronic component distributors, and a 100Ω current limiting resistor with ±0.1 per
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February 2024 Instrumentation Monthly
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