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Two channels on the same resistor network can report different process values because they dissipate different power. Excitation duty, resistance, receiver loading and diagnostic pulses produce local heat; thermal coupling then shifts neighboring elements. The resulting offset may track scan order or controller mode instead of the physical process. Engineering review must calculate each channel’s dissipation, map thermal paths, measure actual temperature coefficients for the specified construction and validate timing. Complete loop accuracy and safe process action remain with the controls integrator.
System boundary
The boundary includes multichannel resistor network, substrate, mounting, ambient boundary, excitation, switches, receiver inputs, diagnostics, firmware scan order and process conversion. ChipSimple may review a drawing-defined network. Process accuracy, calibration, control action and safety remain customer-owned.
System integration decisions
- Inventory every normal and diagnostic excitation state.
- Calculate channel power using the loaded circuit.
- Measure thermal coupling and settling by channel sequence.
- Allocate correction only inside a controlled configuration.
List electrical states before assigning offset
Document continuous excitation, multiplex dwell, startup, diagnostic pulses, open-wire tests, fault pulls and sleep. Record source and receiver impedances. A channel described as off may still carry bias current. Include simultaneous states of neighboring elements. The power inventory must follow firmware timing, not only the schematic’s nominal measurement mode.
Calculate channel dissipation from terminal conditions
For a resistive element, power depends on the actual voltage after loading.
P_i=V_i^2/R_i=I_i^2 R_i; Delta T_i=sum_j(theta_ij P_j)
- P_i is dissipation in element i.
- theta_ij is temperature rise at i per watt dissipated at j.
- Delta T_i is the resulting local rise in the linear screening model.
Quasi-steady linear thermal coupling. Temperature-dependent resistance and transient storage require iteration or time-domain measurement.
Work an illustrative coupled-channel example
Let channel A dissipate 40 mW and B 10 mW. If self thermal coefficient theta_AA is 30 K/W and cross-coupling theta_BA is 8 K/W, A rises about 1.2 K from itself while B gains 0.32 K from A before its own heating. With an illustrative 100 ppm/K element coefficient, 1.2 K corresponds to 120 ppm resistance shift. Values are explanatory, not capability data.
Resolve warm-up and scan-order memory
Measure immediately after excitation and after controlled dwell. Reverse channel order and vary idle interval. If offset follows the previously powered channel, thermal memory or electrical settling is implicated. Record raw timestamps, switch command, excitation and ADC output. A stable final state does not prove a fast scanning sequence is unbiased. Avoid selecting a delay without evidence across temperature and mounting conditions.
Build a thermal coupling matrix
Energize one channel at an approved level while observing its resistance or local temperature and neighboring channels. Repeat by source location. Preserve mounting and airflow. The matrix can identify shared substrate paths and asymmetric cooling. Electrical cross-talk must be checked independently because a neighboring output change is not automatically thermal. Use source reversal or unpowered references where the circuit permits.
Control mounting and airflow around the network
Supports and connector copper can sink one edge. Enclosure airflow may cool outer channels more strongly. Record card orientation, standoffs, potting, cable and adjacent components. Compare fixture and final assembly only after their thermal boundaries are understood. A bare-board calibration is not automatically valid inside a warm cabinet.
Separate electrothermal and process evidence
Use comparisons that hold the physical process reference stable.
| Comparison | Observation | Interpretation owner |
|---|---|---|
| Low/high excitation | Offset versus calculated power | Electronics owner |
| Scan order reversed | Channel history dependence | Firmware owner |
| Single/neighbor powered | Cross-coupling response | Thermal owner |
| Fixture/final mounting | Boundary sensitivity | Mechanical owner |
| Network/reference standard | Process held constant | Metrology authority |
Use construction-specific temperature response
Temperature coefficient belongs to the specified resistor construction and relevant range; do not import a generic value. Measure or use controlled evidence with uncertainty. Separate reversible temperature response from permanent drift. Different values or geometries on one substrate may not share identical behavior. ChipSimple can review requested characteristics by drawing but does not create evidence for an unspecified network.
Apply compensation only after mechanism closure
A correction can use channel power, estimated temperature and timing, but it must be bounded and versioned. Validate residuals on assemblies beyond the calibration sample. Do not compensate an unstable contact, acquisition fault or unmodeled process change as self-heating. Retain an uncorrected diagnostic channel where practical so later drift is visible.
Validate the complete scan and process conversion
Use a stable independent input, representative network variations, supply states, scan modes, diagnostics, ambient and mounting. Record excitation, power, timing, raw output and reference temperature. Test startup and mode transitions. The customer owns loop accuracy, alarms and safe control action. ChipSimple review remains bounded to the passive network drawing.
Reopen coefficients after circuit or firmware changes
Changes to values, geometry, substrate, mounting, enclosure, excitation, input impedance, scan order, pulse width, filter or process scaling can alter self-heating. Bind coupling matrix and compensation to hardware and software revisions. A firmware optimization can change duty cycle without changing the schematic.
Keep offset claims within the measured configuration
This page does not promise TCR, accuracy, power capacity or process stability. Illustrative numbers explain a model only. Actual coefficients, limits and uncertainty require controlled evidence. If channel timing or mounting is undefined, self-heating cannot be separated confidently from process response.
Validate transient coupling and correction uncertainty
Transient thermal response can be represented by a coupling impulse or step test rather than a single steady coefficient. Apply a bounded power change to one channel, observe neighboring raw outputs and fit only the time range supported by evidence. Compare the identified time constant with firmware dwell and idle intervals. If conversion occurs before the network settles, either timing, excitation or compensation must be engineered explicitly. A coefficient measured at one ambient and mounting state should not be extrapolated silently to another enclosure.
Uncertainty should combine electrical reference accuracy, resistance measurement, channel repeatability, temperature observation and process-reference stability. Report corrected and uncorrected results together during development. If compensation improves one duty sequence but worsens another, the model is incomplete. Maintain guard bands for unmodeled coupling and define a diagnostic threshold that prompts investigation rather than allowing unlimited software correction.
Provide the complete multichannel excitation schedule
Network review needs geometry, loaded power, timing and thermal mounting.
- Network schematic, resistor values, geometry, terminals and requested behavior.
- Excitation, source impedance, receiver loading and diagnostic states.
- Firmware scan order, dwell, pulse widths, filters and conversion.
- Mounting, enclosure, airflow, ambient and adjacent heat sources.
- Accuracy budget, reference method, validation matrix and owners.
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