#import "@preview/unify:0.8.1": num, qty, qtyrange, unit
#import "@preview/diverential:0.3.0": *
= Module design <module-design>
The preceding sections established what SAME is: its signal standards, precision
tiers, power distribution, and mechanical format. This section addresses how
SAME modules are designed to meet these specifications — particularly the
demanding requirements of the Metrologic tier.
The challenge is substantial. The Metrologic tier demands $qty(10, "ppm")$
precision, $qty(90, "dB")$ signal-to-noise ratio, and
$qty(0.014, "ppm per hour", per: "/")$ drift over a DC to $qty(20, "kHz")$ bandwidth.
These specifications, taken together, require that a $qty(20, "V")$ signal range
be resolved to $qty(200, "uV")$, that noise remain below $qty(632.455, "uVrms")$,
and that accumulated drift not exceed $qty(10, "ppm")$ over thirty days of
continuous operation. Achieving this with commodity through-hole components —
resistors, capacitors, and op-amps available from any electronics distributor —
would appear to be impossible. The tolerances of standard components are
measured in percent, not parts per million. The temperature coefficients of
common resistors would exhaust the entire drift budget in minutes, not months.
Yet it is possible. The gap between component specifications and system
requirements is bridged not by exotic parts but by topology: the deliberate
arrangement of ordinary components into configurations where their errors
cancel, track, or become irrelevant to the computation. A resistor's absolute
value may be uncertain to 1%, but its ratio to an adjacent resistor from the
same batch, at the same temperature, can be stable to $qty(10, "ppm")$. An
amplifier's offset voltage may drift by microvolts per degree, but a servo loop
can continuously measure and nullify that drift faster than it accumulates.
A multiplier's nonlinearity may be limited by transistor physics to
$qty(0.1, "percent")$, but encoding the multiplication in the time domain rather
than the amplitude domain sidesteps those physics entirely.
These techniques are not novel. They are the accumulated wisdom of precision
analog design, developed by metrologists and instrumentation engineers over
seven decades. What SAME contributes is their systematic application within
constraints that matter for sustainability and sovereignty: through-hole
construction that can be assembled and repaired with basic equipment, open
documentation that enables understanding rather than mere replication, and
component choices that will remain available as specific part numbers inevitably
become obsolete.
This section proceeds in six parts.
+ @module-design-terminology establishes terminology. Precision analog
design has a specialized vocabulary; ambiguous terms lead to ambiguous
analysis. We define our terms once, carefully, and use them consistently
throughout.
+ @module-design-modules-categories categorizes the three types of SAME
modules — Interface
| 