diff --git a/iris/sg/_spectrograph.py b/iris/sg/_spectrograph.py
index d180813..e9c008a 100644
--- a/iris/sg/_spectrograph.py
+++ b/iris/sg/_spectrograph.py
@@ -59,7 +59,7 @@ class SpectrographObservation(
             spd=obs.outputs,
             wavelength=obs.inputs.wavelength.mean(obs.axis_time),
             axis=obs.axis_wavelength,
-            spd_min=0 * u.DN,
+            spd_min=0 * u.DN / u.s,
             spd_max=np.nanpercentile(
                 a=obs.outputs,
                 q=99,