A 4-wire PT100 RTD signal chain for a 4-20mA loop-powered process transmitter — worked excitation-current sizing against self-heating, instrumentation-amplifier gain sizing against a 12-bit ADC, and an anti-aliasing filter sized against sample rate, holding 0.5°C accuracy over 0-150°C.
The project brief: a process-control system needs a signal conditioning board for a 4-wire platinum RTD temperature sensor feeding a 12-bit ADC, powered entirely off a 4-20mA current loop, holding 0.5°C accuracy across a 0-150°C range. That accuracy target is tight enough that nothing in the signal chain can be sized by rule of thumb — the excitation current has to be checked against the RTD's self-heating limit, the instrumentation amplifier's gain has to be checked against both the ADC's full-scale range and its resolution, and the anti-aliasing filter's corner has to be checked against the ADC's sample rate.
This module walks the whole signal chain the way a real analog design engineer would: a worked self-heating calculation that sizes the excitation current, a ratiometric 3-op-amp instrumentation-amplifier gain derivation that sizes itself against the ADC's full-scale range and resolution, a single-pole anti-aliasing filter sized against the ADC's Nyquist frequency, the 4-wire Kelvin connection that makes any of this meaningful over a real cable run, and a full bill of materials. The complete worked numbers, error-budget breakdown, and finished design reasoning are part of the unlocked module below.