fix timing check of ADC delay (#235)

* fix timing check for adc delay

* split rf and adc case again and add comment

---------

Co-authored-by: Patrick Schuenke <[email protected]>

Author: mavel101

src/pypulseq/check_timing.py

@@ -75,10 +75,12 @@ def div_check(a: float, b: float, event: str, field: str, raster: str):
                 # For now this is only the case for arrays of extensions, but we cannot actually check extensions anyway...
                 continue
 
-            if hasattr(e, 'type') and e.type == 'adc':
-                raster = seq.system.adc_raster_time
-                raster_str = 'adc_raster_time'
-            elif hasattr(e, 'type') and e.type == 'rf':
+            if hasattr(e, 'type') and e.type == 'rf':
+                raster = seq.system.rf_raster_time
+                raster_str = 'rf_raster_time'
+            elif hasattr(e, 'type') and e.type == 'adc':
+                # note that ADC samples must be on ADC raster time, but the ADC start time must be on RF raster time!
+                # see https://github.com/pulseq/pulseq/blob/master/doc%2Fpulseq_shapes_and_times.pdf for details
                 raster = seq.system.rf_raster_time
                 raster_str = 'rf_raster_time'
             else: