NeoECU-Hardware/Architecture/IO_MODULES/ANALOG_INPUTS.md

Analogue and Temperature Inputs

Scope

This is the V1 front-end architecture for nine general 0--5 V channels and four NTC temperature channels. Exact protection parts, ADC timing, and sensor curves remain schematic-release validation items. All harness analogue signals use SENSOR_GND and are kept separate from ignition/injector return currents.

General 0--5 V channels

connector -- transient clamp -- R_TOP --+-- ADC-local C_HOLD -- ADC
                                        |
                                      R_BOTTOM
                                        |
                                    SENSOR_GND

R_TOP is the divider's upper resistor; do not add a separate series R_PROT in the normal passive channel. It is the element that limits current from the connector clamp into the ADC-side network, so select an appropriate pulse/voltage rating for the expected residual transient. R_TOP, the clamp, and R_BOTTOM are a system: select standoff, hot leakage, dynamic clamp voltage, pulse energy, negative-excursion path, and local return from the actual harness-fault specification. A nominal TVS voltage alone is not a sufficient selection. Microamp leakage can be a significant offset with a high-value divider, and neither clamp may feed an unpowered analogue rail or MCU protection diode.

For 0.5--4.5 V and 0--5 V active sensors, use a 0.55--0.60 divider with 1% parts or a matched network. The existing 0.55 target maps 5.25 V to 2.89 V. Choose impedance low enough that leakage and PCB contamination are negligible, but high enough not to load the sensor.

Place C_HOLD at the ADC after a small isolation resistor. It is both the low-pass/anti-alias capacitor and local sample-and-hold charge reservoir. Validate its value and ADC sample time together against worst-case divider Thevenin resistance, ADC sampling capacitance, channel-to-channel steps, and sample rate. A large capacitor does not by itself make arbitrary source impedance acceptable.

For ratiometric 5 V sensors, provide a corresponding divider/filter measurement of +5V_SENS and sample it close to the sensor channel.

NTC temperature channels

+5V_SENS -- R_PULLUP --+-- NTC -- SENSOR_GND
                       |
               attenuator / local filter --> ADC

The attenuator and ADC protection are required even if a normal temperature range appears below 3.3 V: an open thermistor raises the node to +5V_SENS. Calculate resistance from the thermistor and measured +5V_SENS ADC ratio, then use the selected sensor's R/T calibration rather than a nominal beta equation.

Fit one defined pull-up per channel, using 0.1% or better low-TCR resistance; provide alternate DNP footprints only where the sensor family is expected.

Sensor family / example	Initial R_PULLUP	Basis
Bosch Motorsport 2.5 kOhm at 20 C NTC	3.01 kOhm	Bosch states typical ECU pull-ups of 1 or 3 kOhm; 3 kOhm retains useful engine-temperature resolution with less self-heating than 1 kOhm.
10 kOhm at 25 C, beta about 3435--3976 K	10.0 kOhm	Centres resolution around ordinary ambient/medium temperatures.
30 kOhm at 25 C	30.1 kOhm	Only for a confirmed curve; reduces divider current and self-heating.

For Bosch's 2.5 kOhm curve, a 3.01 kOhm pull-up produces approximately 4.69 V at -40 C, 2.27 V at 20 C, and 0.29 V at 100 C. A 0.60 attenuator produces about 2.81 V, 1.36 V, and 0.18 V at the ADC; the open fault maps to 3.0 V. Make the attenuator high enough that it adds under 1% loading error at the coldest required NTC resistance, or include its loading in calibration. Its high Thevenin resistance is acceptable only with designed C_HOLD and settling/sample time.

Validate NTC self-heating in the actual mounting medium (worst case is normally hot), plus open/short detection, supply tolerance, pull-up TCR, cable resistance, clamp leakage, PCB contamination, and operation alongside ignition switching.