STPM32 – settling time after reset (RMS, DC-removal HPF, ZCR)
Hello,
we plan to power the STPM32 only briefly at each measurement (every ~2 s),
so we need its settling time after power-up/reset, which is not in the
datasheet (DS10272 Rev 14).
On the EVALSTPM32 (12 V AC 50 Hz on L/N, default configuration after a
global reset) we measured:
- V1 RMS within 1% after ~400 ms, with or without the DC-removal HPF;
- with the HPF enabled, a DC of ~2 V (at the input) in V1 Data that decays
to < 0.1 V only after ~1.7 s, although the datasheet states -3 dB at 8 Hz;
- the same DC shifts the ZCR edges (rising and falling in opposite
directions) until it has decayed.
Questions:
1. After power-up (EN) or a global reset, how long should we wait before the
RMS values and the ZCR output are accurate? Is there a typical or
guaranteed value?
2. The DC-removal filter is specified at -3 dB at 8 Hz, but at start-up it
takes ~1.7 s to remove a ~2 V offset. Why is it so slow? Does the offset
come from the analog front end settling after reset? Is there a setting
to make it faster?
3. When the filter is bypassed (BHPFV1 = 1), does it keep running in the
background? In our tests, when we re-enabled it, the offset disappeared
immediately.
Thank you!