update where it enters for resistor thermal noise

1 files changed, 13 insertions, 3 deletions

diff --git a/module-design.typ b/module-design.typ
index 4d8c142..d3cca58 100644
--- a/module-design.typ
+++ b/module-design.typ
@@ -1217,9 +1217,19 @@ $
 
 ====== Where it enters <resistor-thermal-noise-where-it-enters>
 
-- Input resistors in summing amplifiers,
-- Feedback resistors (noise is gained at output),
-- High-impedance nodes (input buffers, integrators).
+/ Front-panel input nodes: The mandated $Z_"in" gt.eq qty(1, "mega ohm")$
+  impedance om white input jacks creates an unavoidable
+  $approx qty(18.2, "uVrms")$ noise floor right at the module boundary. This
+  necessitates dropping internal suming nodes down to $qty(1, "kilo ohm")$
+  post-buffering to prevent additive thermal noise accumulation,
+/ Feedback resistors: Thermal noise generated in feedback elements is scaled to
+  the output by the stage's noise gain ($G_N = 1 + R_f / R_"in"$).
+  Consequently, even attenuating configurations ($R_f lt R_"in"$) maintain
+  $G_N gt.eq 1$, meaning thermal noise is never attenuated along with the
+  signal,
+/ Input resistors: In summing junction amplifiers,
+/ High-impedance nodes: Within integrators, sample-an-holds, and
+  current-to-voltage converters.
 
 ====== Scaling law <resistor-thermal-noise-scaling-law>