70 lines
3.6 KiB
Markdown
70 lines
3.6 KiB
Markdown
# Analogue and Temperature Inputs
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## Scope
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This is the V1 front-end architecture for nine general 0--5 V channels and
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four NTC temperature channels. Exact protection parts, ADC timing, and sensor
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curves remain schematic-release validation items. All harness analogue signals
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use `SENSOR_GND` and are kept separate from ignition/injector return currents.
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## General 0--5 V channels
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`R_TOP` is the divider's upper resistor; do not add a separate series
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`R_PROT` in the normal passive channel. It is the element that limits current
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from the connector clamp into the ADC-side network, so select an appropriate
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pulse/voltage rating for the expected residual transient. `R_TOP`, the clamp,
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and `R_BOTTOM` are a system: select standoff, hot leakage, dynamic clamp
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voltage, pulse energy, negative-excursion path, and local return from the
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actual harness-fault specification. A nominal TVS voltage alone is not a
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sufficient selection. Microamp leakage can be a significant offset with a
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high-value divider, and neither clamp may feed an unpowered analogue rail or
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MCU protection diode.
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For 0.5--4.5 V and 0--5 V active sensors, use a 0.55--0.60 divider with 1%
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parts or a matched network. The existing 0.55 target maps 5.25 V to 2.89 V.
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Choose impedance low enough that leakage and PCB contamination are negligible,
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but high enough not to load the sensor.
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Place `C_HOLD` at the ADC after a small isolation resistor. It is both the
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low-pass/anti-alias capacitor and local sample-and-hold charge reservoir.
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Validate its value and ADC sample time together against worst-case divider
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Thevenin resistance, ADC sampling capacitance, channel-to-channel steps, and
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sample rate. A large capacitor does not by itself make arbitrary source
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impedance acceptable.
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For ratiometric 5 V sensors, provide a corresponding divider/filter measurement
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of `+5V_SENS` and sample it close to the sensor channel.
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## NTC temperature channels
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The attenuator and ADC protection are required even if a normal temperature
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range appears below 3.3 V: an open thermistor raises the node to `+5V_SENS`.
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Calculate resistance from the thermistor and measured `+5V_SENS` ADC ratio,
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then use the selected sensor's R/T calibration rather than a nominal beta
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equation.
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Fit one defined pull-up per channel, using 0.1% or better low-TCR resistance;
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provide alternate DNP footprints only where the sensor family is expected.
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| Sensor family / example | Initial `R_PULLUP` | Basis |
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| --- | ---: | --- |
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| 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. |
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| 10 kOhm at 25 C, beta about 3435--3976 K | 10.0 kOhm | Centres resolution around ordinary ambient/medium temperatures. |
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| 30 kOhm at 25 C | 30.1 kOhm | Only for a confirmed curve; reduces divider current and self-heating. |
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For Bosch's 2.5 kOhm curve, a 3.01 kOhm pull-up produces approximately 4.69 V
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at -40 C, 2.27 V at 20 C, and 0.29 V at 100 C. A 0.60 attenuator produces
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about 2.81 V, 1.36 V, and 0.18 V at the ADC; the open fault maps to 3.0 V.
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Make the attenuator high enough that it adds under 1% loading error at the
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coldest required NTC resistance, or include its loading in calibration. Its
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high Thevenin resistance is acceptable only with designed `C_HOLD` and
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settling/sample time.
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Validate NTC self-heating in the actual mounting medium (worst case is normally
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hot), plus open/short detection, supply tolerance, pull-up TCR, cable resistance,
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clamp leakage, PCB contamination, and operation alongside ignition switching.
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