pre uc480 integration

adc_bug.md

@@ -0,0 +1,37 @@
+# ADC YOFF Sign Bug — sras_viewer.py
+
+## Status
+Fix applied, awaiting user testing.
+
+## What was wrong
+
+`DC_YOFF_ADC` in `sras_viewer.py` was `+87.04` instead of `-87.04`.
+
+The Tektronix scope stores CH3/CH4 waveform data as **signed int8** (−128 to +127), where ADC 0 = screen center. The scope's vertical position for CH3/CH4 is set to `−2.72 div` in `sc3_aui_app.py`, which places 0 V **below** center at ADC count `−2.72 × 32 = −87.04`. The comment in the code had the formula as `-position × (256/8)` (sign flipped), producing `+87.04` instead of the correct `−87.04`.
+
+## Effect of the bug
+
+- `adc_to_mv` was off by 272 mV in the negative direction
+  - ADC −87 (true 0 V signal) → −272 mV (should be ≈ 0 mV)
+  - ADC 0 (screen center, above ground) → −136 mV (should be +136 mV)
+- DC images for CH3/CH4 (Bias A/B) showed large negative voltages, physically impossible for DC bias signals
+- RF mask threshold (`mv_to_adc`) was also broken: threshold ADC value ~+87 was being compared against pixel means clustered around −87, so nearly every pixel would have been incorrectly masked
+
+## The fix
+
+`sras_viewer.py` line 48:
+```python
+# Before
+DC_YOFF_ADC  = 87.04    # ADC count that represents 0 V
+
+# After
+DC_YOFF_ADC  = -87.04   # ADC count that represents 0 V
+```
+Comment on line 47 also corrected from `-position × (256/8)` to `position × (256/8)`.
+
+## What to verify during testing
+
+1. CH3 and CH4 DC images show positive (or near-zero) voltages consistent with the bias signal levels
+2. RF (CH1) image is not excessively masked — pixels with a genuine bias signal above the threshold should appear
+3. `mv_to_adc(0.0)` should now return −87.04 (not +87.04)
+4. The default threshold of 0.125 mV should correspond to ADC ≈ −87.0, not +87.1