# 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

```text
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

```text
+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.