Hello,
We are using the REF2025 in a test fixture that gets temperature cycled as part of the test. The test fixture voltage reference is measured with a high precision DMM, then the fixture is removed and temperature cycled some number of times, and the voltage reference is re-measured. This process may repeat many times. Our "thermal cycle" is defined as only 25C -> 50C -> 25C.
Measuring the Vref of a given fixture after 50, 60, and 70 cycles has shown a voltage offset of up to 200uV. This is right around the long-term stability and the thermal hysteresis numbers, but we have not run it for anywhere near 1000 hours, and our temp cycles are much more narrow than the datasheet reference. We are trying to determine if we are observing an expected behavior of the part ONLY, or an unexpected behavior, which then might be due to some other error in our fixture or measurements.
Looking at the datasheet for the REF2025, I am not certain what the defined behavior of a thermal cycle should be. Specifically:
One thermal cycle is defined as 25C -> -40C -> 125C -> 25C. The results of a single cycle are shown to be about 60ppm. In the characteristics section of the datasheet, "Cycle 2" is listed as having an offset of 35ppm.
Question 1: is that second thermal hysteresis defined with reference to the nominal 2.5V, or with reference to the 1st cycle voltage (i.e., 2.5V+35ppm or 2.50015V+35ppm)
Question 2: does thermal hysteresis eventually settle? So, after, say, 100 temp cycles, would the result of each cycle be independent of the last (i.e., still 2.5V +/- 60ppm regardless of last point), or would it settle around a particular offset (i.e., cycle 100 is +/- 1 ppm from cycle 99)?
Thank you very much for your time and consideration.