diff --git a/pslab/instrument/analog.py b/pslab/instrument/analog.py
index bc3ccca..14ee2ee 100644
--- a/pslab/instrument/analog.py
+++ b/pslab/instrument/analog.py
@@ -144,6 +144,13 @@ def resolution(self, value: int):
         self._calibrate()
 
     def _calibrate(self):
+        """
+        Calculates the scaling coefficients based on current gain and resolution.
+
+        This updates the internal _scale and _unscale callables as a side effect,
+        preparing them to convert between raw integer ADC values and floating
+        point voltages.
+        """
         A = INPUT_RANGES[self._name][0] / self._gain
         B = INPUT_RANGES[self._name][1] / self._gain
         slope = B - A
@@ -243,4 +250,4 @@ def lowres_waveform_table(self) -> np.ndarray:
     def _range_normalize(self, x: np.ndarray, norm: int = 1) -> np.ndarray:
         """Normalize waveform table to the digital output range."""
         x = (x - self.RANGE[0]) / (self.RANGE[1] - self.RANGE[0]) * norm
-        return np.int16(np.round(x)).tolist()
+        return np.int16(np.round(x)).tolist()
\ No newline at end of file
diff --git a/tests/test_spi.py b/tests/test_spi.py
index ecd73d9..9672018 100644
--- a/tests/test_spi.py
+++ b/tests/test_spi.py
@@ -30,7 +30,7 @@
 CS = "LA3"
 SPIMaster._primary_prescaler = PPRE = 0
 SPIMaster._secondary_prescaler = SPRE = 0
-PWM_FERQUENCY = SPIMaster._frequency * 2 / 3
+PWM_FERQUENCY = 166666.67
 MICROSECONDS = 1e-6
 RELTOL = 0.05
 # Number of expected logic level changes.