Electronics Guide

On-Die Temperature Sensors

Modern integrated circuits incorporate on-die temperature sensors that measure the actual junction temperature of the silicon. These sensors provide the most accurate indication of the thermal conditions affecting the transistors, which may differ significantly from package or ambient temperature due to thermal resistance and power dissipation.

Diode-based sensors exploit the temperature dependence of the forward voltage of a pn junction. When a constant current flows through a diode, the forward voltage decreases approximately 2 millivolts per degree Celsius increase in temperature. By measuring this voltage with an analog-to-digital converter, the die temperature can be determined with typical accuracy of plus or minus 1 to 3 degrees Celsius.

Bipolar transistor sensors extend the diode concept using the difference in base-emitter voltages of a transistor operated at two different current densities. This proportional-to-absolute-temperature (PTAT) approach provides better accuracy because it eliminates dependence on the absolute forward voltage, which varies with process parameters.

Ring oscillator sensors measure temperature through its effect on transistor switching speed. A ring oscillator's frequency decreases with increasing temperature as carrier mobility drops. Digital counting of oscillator cycles provides temperature information without requiring analog circuitry, making this approach attractive for digital-only processes.

Large processors may incorporate multiple temperature sensors distributed across the die to capture thermal gradients. Hot spots near high-activity regions can exceed the average die temperature by tens of degrees, making localized sensing essential for effective thermal management.

External Temperature Sensors

External temperature sensors measure conditions at various points in the system beyond the semiconductor die, including package surfaces, heat sinks, circuit boards, enclosures, and the ambient environment.

Thermistors are resistive elements whose resistance changes with temperature. Negative temperature coefficient (NTC) thermistors decrease in resistance as temperature increases, providing high sensitivity that enables detection of small temperature changes. Positive temperature coefficient (PTC) thermistors increase in resistance and are often used for over-temperature protection. Thermistors offer low cost and high sensitivity but require linearization for accurate measurement over wide temperature ranges.

Resistance temperature detectors (RTDs) use the temperature dependence of metal resistance, typically platinum. RTDs provide excellent accuracy, stability, and linearity but cost more than thermistors and require excitation current that can cause self-heating errors. The platinum PT100 and PT1000 are industry standards with well-characterized behavior.

Integrated temperature sensors combine sensing elements with signal conditioning and digital interfaces in a single package. Devices are available with I2C, SPI, or single-wire interfaces that report temperature directly in digital format. These sensors simplify system design by eliminating the need for external analog signal conditioning.

Thermocouples generate a voltage proportional to the temperature difference between their measurement junction and a reference junction. Different metal combinations provide different sensitivity and temperature ranges. Thermocouples can measure extremely high temperatures but require cold junction compensation and have lower accuracy than other sensor types for moderate temperature ranges.

Infrared temperature sensors measure surface temperature without physical contact by detecting thermal radiation. These sensors are valuable for measuring the temperature of moving parts, inaccessible locations, or surfaces where contact sensors would disturb the measurement.

Temperature Monitoring Applications

Temperature monitoring enables multiple protective and optimization functions in electronic systems:

• Thermal throttling reduces processor clock frequency or voltage when temperature approaches critical limits, trading performance for reliability.
• Fan speed control adjusts cooling system operation based on measured temperatures, balancing thermal performance against acoustic noise and power consumption.
• Emergency shutdown powers down the system if temperature exceeds safe limits despite other protective measures.
• Temperature logging records thermal history for warranty analysis, failure investigation, and reliability studies.
• Calibration compensation adjusts circuit parameters to maintain accuracy as temperature varies.

Supply Voltage Supervisors

Voltage detector ICs monitor one or more supply rails and provide digital output signals indicating whether voltages are within acceptable limits. These devices typically include threshold comparators with built-in hysteresis to prevent oscillation near trip points. Output signals can trigger interrupts, enable reset circuits, or indicate status through LEDs.

Power-on reset generators hold processors and other logic in reset until supply voltages have stabilized within operating range. This function prevents erratic behavior during power sequencing when supplies may be at intermediate levels. The reset signal is released after a delay following voltage reaching the threshold, ensuring adequate stabilization time.

Brownout detectors monitor for supply voltage dips that may not trigger power-on reset but could cause operational errors. When voltage drops below the brownout threshold, these circuits can trigger corrective actions such as saving volatile data, suspending non-critical operations, or initiating a controlled shutdown.

Overvoltage detectors protect against supply voltage exceeding maximum ratings, which can occur during hot-swap events, power supply failures, or transient disturbances. Detection of overvoltage typically triggers immediate protective action such as disconnecting the supply or crowbar clamping.

Precision Voltage Measurement

Many applications require not just detection of out-of-range conditions but precise measurement of supply voltage values. This capability supports power management optimization, battery state-of-charge estimation, and diagnostic data collection.

Resistor dividers scale high voltages to levels compatible with analog-to-digital converter inputs. For accurate measurement, precision resistors with low temperature coefficients are required. The divider ratio must account for the full range of expected voltages while maintaining adequate resolution.

Dedicated voltage monitoring ICs integrate multiple measurement channels, scaling networks, and analog-to-digital converters in a single device. These devices often include programmable alert thresholds, averaging functions, and digital interfaces for reporting measurements to a host processor.

Power management ICs in complex systems typically include voltage monitoring for all rails they generate. This integrated monitoring provides consistent measurement capability across the power system and enables coordinated fault response.

Measurement accuracy depends on reference voltage stability, analog-to-digital converter resolution, and scaling network precision. For critical applications, periodic calibration against known standards may be required to maintain measurement accuracy over temperature and time.

Voltage Sequencing Monitors

Complex systems with multiple supply rails require specific power-up and power-down sequences to prevent damage or latch-up conditions. Voltage sequencing monitors verify that rails reach their targets in the correct order and timing.

Sequencing supervisors monitor multiple voltage rails and verify that they reach specified thresholds in the programmed sequence. If a rail fails to reach its target or comes up out of order, the supervisor can hold the system in reset, signal a fault, or trigger a shutdown.

Power tracking requirements in some systems mandate that certain supplies rise and fall together, maintaining a fixed ratio during transitions. Monitoring systems verify this tracking relationship and flag violations that could stress circuits.

Shunt-Based Current Sensing

Shunt resistors are precision low-value resistors inserted in the current path. The voltage developed across the shunt, according to Ohm's law, is proportional to current. This voltage is then measured with appropriate amplification and analog-to-digital conversion.

Shunt selection involves trade-offs between measurement resolution and power dissipation. Lower resistance values minimize power loss but produce smaller signals that are more susceptible to noise and offset errors. Typical shunt values range from milliohms for high-current applications to ohms for low-current circuits.

High-side sensing places the shunt between the supply and the load, allowing measurement of total load current including ground faults. High-side sensing requires differential amplifiers or specialized current-sense amplifiers that can reject the common-mode voltage of the supply rail.

Low-side sensing places the shunt between the load and ground, simplifying signal conditioning because the measurement voltage is referenced to ground. However, low-side sensing cannot detect ground faults and introduces a voltage drop in the ground path that may affect sensitive circuits.

Current-sense amplifiers are specialized ICs designed for high-side or low-side current measurement. These devices integrate precision gain, common-mode rejection, and temperature compensation in a compact package. Many include digital interfaces for direct reporting of current measurements.

Magnetic Current Sensing

Magnetic current sensors measure the field generated by current flow without requiring a series element in the current path. This approach provides galvanic isolation and eliminates shunt power dissipation.

Hall effect sensors detect the magnetic field surrounding a current-carrying conductor. The sensor produces a voltage proportional to field strength, which correlates to current. Hall sensors can measure DC and AC currents but have limited bandwidth and accuracy compared to shunt-based methods.

Current transformers couple AC current magnetically to a secondary winding where it can be measured with isolation from the primary circuit. Current transformers provide excellent isolation and can handle high currents safely but cannot measure DC and have bandwidth limitations at low frequencies.

Rogowski coils are air-core current transformers that produce an output proportional to the rate of change of current. Integration of this signal recovers the current waveform. Rogowski coils offer wide bandwidth, no saturation, and easy installation around existing conductors.

Current Monitoring Applications

Current monitoring data supports numerous system functions:

• Power measurement combines current and voltage measurements to calculate instantaneous and average power consumption for energy management and billing applications.
• Overcurrent protection detects excessive current that may indicate short circuits, component failures, or overload conditions, triggering circuit breakers or electronic disconnects.
• Battery fuel gauging integrates current over time to track battery state of charge, enabling accurate remaining capacity estimates.
• Hot-swap control monitors inrush current during board insertion and limits current to prevent supply droops or connector damage.
• Workload characterization correlates current consumption with system activity to understand power profiles and optimize energy efficiency.

Single-Event Effect Detection

Single-event upsets (SEUs) occur when ionizing particles deposit enough charge in a memory cell or flip-flop to change its state. While not permanently damaging, SEUs can corrupt data or cause system malfunctions. Detection mechanisms compare redundant copies of data or use error-detection codes to identify bit flips.

Error-detecting and correcting codes (EDAC) add redundant bits to stored data that enable detection and correction of bit errors. Single-bit error correction with double-bit error detection (SECDED) is common in memory systems. The error rate reported by EDAC circuitry provides a measure of radiation exposure.

Triple modular redundancy (TMR) replicates logic circuits three times and votes on outputs. When radiation causes an error in one replica, the other two provide the correct result. TMR implementations can log voting corrections as a measure of single-event effects.

Single-event latchup (SEL) detectors monitor for the sudden current increase that indicates a parasitic thyristor has been triggered by ionizing radiation. Rapid detection enables power cycling to clear the latchup before thermal damage occurs.

Total Ionizing Dose Monitoring

Total ionizing dose (TID) accumulates over time and causes gradual degradation of transistor parameters, primarily through charge trapping in oxide layers. Monitoring cumulative dose enables prediction of remaining useful life and scheduling of component replacement before parametric failures occur.

Radiation-sensitive field-effect transistors (RADFETs) are specialized MOSFETs designed to exhibit measurable threshold voltage shifts in response to ionizing radiation. The threshold voltage change correlates to cumulative dose, providing a direct measurement of TID exposure. RADFETs can be read periodically to track dose accumulation.

PIN diodes operated under forward bias show changes in forward voltage with accumulated radiation dose. These devices provide simple, low-cost dosimetry for applications where high accuracy is not required.

Thermoluminescent dosimeters (TLDs) store energy from radiation exposure that is released as light when heated. TLDs provide passive dose measurement that can be read out periodically but do not support real-time monitoring.

Active Radiation Monitors

Active radiation detectors measure radiation flux in real time, enabling immediate protective responses to changing radiation environments.

Solid-state particle detectors use silicon diodes to detect ionizing particles through the charge they generate in the depletion region. The detector output indicates both the rate and energy of incident particles. These detectors are commonly used in space systems to monitor the radiation environment.

Geiger-Mueller tubes detect ionizing radiation through gas ionization avalanches. While not providing energy information, GM tubes are sensitive detectors suitable for area monitoring and personnel dosimetry applications.

Scintillator detectors convert radiation energy to visible light pulses that are detected by photomultipliers or photodiodes. Different scintillator materials optimize detection of gamma rays, neutrons, or charged particles.

Relative Humidity Sensors

Capacitive humidity sensors use a moisture-sensitive dielectric material between capacitor plates. As humidity changes, the dielectric absorbs or releases water vapor, changing its permittivity and thus the capacitance. Capacitive sensors offer good accuracy, wide measurement range, and low hysteresis.

Resistive humidity sensors measure the resistance change of hygroscopic materials with moisture content. These sensors are simpler and less expensive than capacitive types but may have larger hysteresis and slower response time.

Thermal conductivity sensors detect humidity through its effect on the thermal conductivity of air. Two heated elements, one sealed in dry air and one exposed to ambient humidity, show differential temperature that correlates to moisture content.

Integrated humidity sensors combine sensing elements with signal conditioning and calibration data in a single package. Many provide digital outputs with factory-trimmed accuracy, simplifying system integration. Temperature compensation is typically included because humidity measurement is temperature-dependent.

Dew Point Detection

Condensation occurs when temperature falls below the dew point determined by absolute moisture content. Monitoring for conditions approaching condensation enables protective actions before moisture damages electronics.

Dew point calculation from relative humidity and temperature measurements determines the temperature at which condensation will occur. If system temperatures approach this value, warnings or environmental control activation can prevent condensation.

Chilled mirror hygrometers directly measure dew point by cooling a reflective surface until condensation forms, detected as a change in reflectivity. These instruments provide high accuracy but are more complex and expensive than relative humidity sensors.

Condensation sensors directly detect the presence of liquid water through changes in impedance or capacitance when moisture bridges sensing electrodes. These sensors provide immediate warning when condensation occurs but do not predict approaching conditions.

Humidity Control Integration

Humidity monitoring often integrates with environmental control systems to maintain conditions within acceptable ranges:

• HVAC integration uses humidity data to control humidification and dehumidification equipment, maintaining data center or equipment room conditions.
• Desiccant monitoring tracks the moisture absorption capacity of desiccant materials in sealed enclosures, indicating when replacement is needed.
• Conformal coating verification assesses whether protective coatings adequately protect circuits by monitoring internal versus external humidity levels.
• Sealed enclosure integrity detects seal failures through unexpected humidity changes inside nominally hermetic packages.

Pressure Sensor Technologies

Piezoresistive pressure sensors use strain gauges on a deflecting diaphragm to measure pressure. The resistance change of the strain elements correlates to applied pressure. These sensors offer good accuracy and wide pressure ranges in compact packages.

Capacitive pressure sensors measure the capacitance change as pressure deflects one plate of a capacitor toward another. These sensors typically offer high sensitivity and low power consumption, making them suitable for portable and battery-powered applications.

MEMS pressure sensors integrate sensing elements and signal conditioning on a single silicon die using micromachining techniques. MEMS sensors provide excellent performance in small, low-cost packages and are widely used in consumer and industrial applications.

Piezoelectric sensors generate charge in response to pressure changes, making them suitable for dynamic pressure measurement. However, they cannot measure static pressure and are more commonly used for vibration and acoustic sensing.

Altitude and Environmental Pressure

Barometric pressure measurement enables altitude determination and weather monitoring. For electronic systems, this data supports:

• Thermal derating calculations adjust maximum power dissipation and operating frequency for reduced air density at altitude.
• High-voltage derating reduces operating voltages when reduced air pressure lowers dielectric breakdown strength.
• Sealed enclosure stress monitoring tracks differential pressure across enclosure walls, which can cause mechanical failure at extreme altitudes.
• Altitude logging records operating altitude for warranty and reliability analysis of aerospace and portable equipment.

Differential and Gauge Pressure

Beyond absolute atmospheric pressure, electronic systems may monitor differential pressure across filters, enclosures, or cooling systems:

Filter monitoring measures pressure drop across air filters to detect clogging that reduces cooling airflow. Increasing differential pressure indicates filter replacement is needed.

Enclosure pressurization maintains positive pressure inside equipment housings to exclude dust and moisture. Pressure sensors verify that pressurization systems maintain the required differential.

Liquid cooling systems monitor pressure at various points to detect leaks, pump failures, or blockages that could compromise cooling performance.

Data Acquisition

Environmental monitoring telemetry begins with systematic data collection from distributed sensors:

Sensor interfaces connect temperature, voltage, current, and other sensors to data acquisition systems. Common interface types include analog voltages, digital buses such as I2C and SPI, and industrial protocols such as 4-20 mA current loops.

Multiplexing enables a single analog-to-digital converter to measure multiple sensor channels sequentially. This approach reduces cost but limits simultaneous measurement capability and introduces channel-to-channel timing skew.

Sampling rates must be appropriate for each measured parameter. Temperature typically changes slowly and may be sampled every few seconds, while current measurements may require kilohertz sampling to capture transients.

Calibration data converts raw sensor readings to engineering units. Factory calibration coefficients stored in non-volatile memory enable accurate measurement without per-system adjustment.

Communication Protocols

Telemetry data must be communicated reliably from sensors to monitoring systems using appropriate protocols:

IPMI (Intelligent Platform Management Interface) is an industry standard for server and system management that includes environmental monitoring. IPMI baseboard management controllers collect sensor data and make it available through standardized interfaces to management software.

Modbus is a widely used industrial protocol for connecting sensors, instruments, and controllers. Both serial (RS-485) and TCP/IP versions enable integration of environmental monitoring into industrial automation systems.

SNMP (Simple Network Management Protocol) enables network-based access to monitoring data through standardized management information bases (MIBs). Environmental monitoring equipment often provides SNMP interfaces for integration with network management systems.

Wireless telemetry using protocols such as Zigbee, LoRa, or Bluetooth Low Energy enables monitoring of distributed sensors without wiring. This approach is valuable for large facilities, remote locations, or retrofitting monitoring to existing equipment.

Data Management and Analysis

Raw telemetry data requires processing, storage, and analysis to provide actionable information:

Threshold monitoring compares measured values against configured limits and generates alerts when thresholds are exceeded. Multi-level thresholds enable warning and critical alerts at different severity levels.

Trend analysis examines historical data to identify gradual changes that may indicate developing problems. A slowly rising temperature over weeks may indicate fan degradation before an over-temperature condition occurs.

Data logging stores historical sensor readings for later analysis. Log data supports failure investigation, warranty determination, and reliability studies. Retention periods and storage granularity balance data value against storage costs.

Predictive analytics applies statistical methods and machine learning to environmental monitoring data to predict future conditions and potential failures. These techniques enable proactive maintenance before problems cause unplanned downtime.

Visualization dashboards present environmental monitoring data in graphical formats that enable rapid assessment of system status. Color-coded status indicators, trend graphs, and spatial representations help operators understand current conditions and identify anomalies.

System Management Controllers

Baseboard management controllers (BMCs) in servers and enterprise equipment integrate environmental monitoring with system management functions. BMCs typically monitor temperatures at multiple points, voltages on all power rails, fan speeds, and chassis intrusion. Operating independently of the main processor, BMCs continue monitoring even when the system is powered down or hung.

Microcontroller-based monitors in embedded systems collect environmental data, evaluate thresholds, and take protective actions without host processor involvement. This approach ensures monitoring continues even if software hangs or the main processor fails.

FPGA-based monitoring provides high-speed environmental monitoring with deterministic response times for safety-critical applications. Hardware implementation ensures that protective functions cannot be delayed by software execution.

Environmental Monitoring ICs

Specialized integrated circuits combine multiple environmental monitoring functions in single devices:

• Hardware monitors integrate temperature, voltage, and fan speed monitoring with digital interfaces and alert outputs. These devices simplify board design by consolidating monitoring functions.
• Power management ICs often include environmental monitoring for the supplies they regulate, providing integrated power control and monitoring.
• Smart sensors combine sensing elements with processing capability, performing local threshold evaluation and generating alerts without host processor involvement.

Facility-Level Monitoring

Data centers and equipment facilities extend environmental monitoring beyond individual systems to encompass the entire operating environment:

Distributed sensor networks place temperature, humidity, and airflow sensors throughout the facility to map environmental conditions. This data guides cooling system operation and identifies hot spots or cooling deficiencies.

Power monitoring tracks energy consumption at facility, row, rack, and individual system levels, enabling efficiency optimization and capacity planning.

Environmental management systems integrate data from all monitoring sources to provide comprehensive visibility into facility conditions. Automated responses to environmental changes maintain optimal operating conditions while minimizing energy consumption.