CGM scenario and system-level partitioning

Continuous glucose monitoring (CGM) converts glucose changes in interstitial fluid into a continuous electrochemical current. A small patch module and wireless link deliver trend curves and threshold alerts to a phone or receiver, enabling higher reading frequency and earlier warnings compared to traditional finger-stick glucose meters.

• Conventional finger-stick meters depend on manual sampling at a few fixed times per day and only provide sparse, point-in-time results that can miss rapid excursions and overnight trends.
• CGM uses a subcutaneous probe to sample every few minutes, generating a continuous trend curve and enabling early high and low glucose alerts instead of isolated readings.
• Higher reading frequency and 24/7 monitoring introduce system constraints on power budget, sensor drift management and long-term reliability.

• Subcutaneous probe: a 2- or 3-electrode glucose sensor with a nearby temperature sensing element, operating in interstitial fluid and generating current proportional to glucose concentration.
• Disposable or reusable patch electronics: a compact module that integrates the electrochemical AFE, temperature measurement and compensation, ultra-low-power MCU, wireless interface and power management, responsible for sampling, compensation and data packaging.
• Phone or dedicated receiver app: receives data over BLE or proprietary radio, renders trends, logs events and synchronizes with cloud services if required.

• Most CGM patches target a 7–14 day wear period with a very small battery, so electrochemical AFE, MCU and wireless radio must achieve extremely low average current.
• Disposable patches are cost-sensitive and push designs toward high integration and minimal external components while still maintaining necessary accuracy and robustness.
• Long continuous operation requires monitoring of sensor drift, electrode aging and connector integrity so that trend errors do not accumulate silently over the wear period.

A typical CGM signal chain starts with an electrochemical sensor generating a current related to glucose concentration. The patch AFE biases the electrodes, converts the nanoamp–microamp current into a clean voltage and digitizes it. The ultra-low-power MCU applies temperature compensation and basic filtering, then wraps the measurements into BLE advertisements or connection-mode packets. The phone or receiver consumes these packets to build trend plots, store history and trigger alerts locally or in the cloud.

Subcutaneous electrochemical sensor basics and error sources

Subcutaneous glucose sensors typically use a 2- or 3-electrode electrochemical structure. In interstitial fluid, a controlled working potential converts glucose concentration into a nanoamp–microamp current. The AFE must bias the electrodes correctly, measure the tiny current and control error sources so that later temperature compensation, calibration and trend algorithms have a stable foundation.

• A 3-electrode sensor uses a working electrode, reference electrode and counter electrode. The reference electrode is held at a stable potential, the working electrode is biased at a specified voltage, and the counter electrode completes the current loop.
• Sensor datasheets usually specify a recommended working potential window. The AFE must provide a low-noise, stable bias source and enough drive capability to keep the electrodes within this window under varying conditions.
• Electrode geometry and materials determine sensitivity and noise; AFE input range and noise performance should be sized against the actual sensor characteristics.

• In the target glucose range, typical CGM sensors generate current on the order of nanoamps to microamps, so the front-end must resolve very small signals without saturating during high-glucose or transient conditions.
• Required dynamic range often spans tens of times variation in current, driving designs toward high-value feedback resistors or multi-range TIA structures with smooth transitions between ranges.
• Design margins for TIA feedback resistance, ADC full-scale and safety current limits should be derived from sensor sensitivity, maximum expected concentration and worst-case operating conditions.

• Temperature effects: temperature changes enzyme reaction rates and diffusion properties, so the same blood glucose level can produce different sensor currents. A nearby temperature sensor and compensation model are required to correct this effect.
• Aging and contamination: enzyme activity loss, protein fouling and tissue response gradually reduce sensitivity, so the slope of the current–glucose curve drifts over the wear period and must be managed with calibration, trend monitoring and end-of-life handling.
• Interstitial fluid lag: glucose changes in blood appear later in interstitial fluid, so CGM readings have a systematic delay relative to blood measurements. This is a system-level property rather than AFE noise and must be considered in interpretation and algorithms.
• Connection and contact issues: poor contact or variable impedance between sensor and electronics introduces extra offsets and noise. Mechanical design and open-/short-circuit detection help limit this error source.

Sensor vendors usually provide polarization curves, sensitivity and linearity ranges, temperature coefficients and recommended operating conditions. These curves define boundaries for selecting TIA feedback resistors, setting bias voltages, sizing ADC dynamic range and building temperature compensation models. Based on these parameters, later sections can define suitable TIA structures and compensation chains so that the CGM system maintains acceptable accuracy and stability over the full wear period.

Potentiostat AFE and ultra-low current measurement architecture

The potentiostat AFE keeps the electrochemical sensor biased at the correct working potential while converting nanoamp–microamp currents into a clean voltage for the ADC. The loop must maintain a stable reference electrode voltage, support high-value feedback resistors and provide diagnostic hooks for sensor open, short and leakage conditions.

• A three-electrode configuration uses a working, reference and counter electrode. The potentiostat loop senses the reference electrode potential and drives the counter electrode to keep the reference electrode at the commanded working voltage.
• The working electrode current flows through the transimpedance amplifier (TIA), which produces a proportional voltage while the loop maintains the reference potential within a narrow window.
• Loop stability and low noise are essential, because reference potential excursions directly disturb the current and degrade glucose reading repeatability.

• Feedback resistors in CGM TIAs often range from megaohms to gigaohms to resolve nanoamp-level currents. Higher resistance increases gain and thermal noise, and raises sensitivity to leakage and parasitic capacitance.
• Output swing must leave margin to avoid saturation at high glucose or transient conditions while still giving adequate resolution at low currents.
• A small feedback capacitor shapes bandwidth and ensures phase margin. The target bandwidth is low enough to suppress noise but high enough to track physiologically relevant changes and system diagnostics.

• CGM sensors show wide variation in current levels across patients, sensors and wear stages, so a single fixed TIA gain can either saturate at the high end or lose resolution at the low end.
• Multi-range TIAs use selectable feedback resistors or programmable gain stages to cover the full current range. Range-change thresholds and hysteresis prevent frequent toggling.
• Range transitions can be smoothed with overlapping ranges, short averaging intervals and firmware logic to avoid visible steps in the reported glucose trend.

• A low-speed, high-resolution delta-sigma ADC offers excellent noise shaping and integrated digital filtering, matching the low bandwidth of CGM signals with high effective resolution.
• A medium-speed SAR ADC with oversampling and averaging is attractive when integrated in an MCU or mixed-signal AFE, providing flexibility and good power efficiency.
• Input range, INL/DNL, reference stability and digital averaging strategy jointly determine the effective resolution in nanoamp units over the intended current range.

• Open-circuit conditions can be detected by combining very low measured current with abnormal polarization voltage at the reference or working electrode.
• Shorted electrodes often force the potentiostat output and TIA node toward rail values; monitoring loop drive and electrode potentials can flag these faults.
• Leakage paths appear as unexpected bias currents or drift under test bias. Periodic health checks with modified bias conditions help identify contamination or connector degradation.

Temperature measurement and compensation chain

Glucose sensor sensitivity depends strongly on temperature, so every CGM front-end needs a reliable way to measure local temperature and compensate the electrochemical current. The compensation chain starts with a properly placed temperature sensor and continues through calibration data, runtime lookup or polynomial evaluation and trend filtering.

• A small NTC thermistor placed close to the probe provides low-cost, fast temperature measurement but needs its own AFE path and linearization.
• An integrated temperature sensor inside the AFE or MCU simplifies hardware, but the reading may lag or differ from the actual subcutaneous environment depending on package geometry.
• Co-packaged or on-die temperature sensing with the electrochemical chip offers the closest thermal tracking, at the cost of tighter process integration and vendor dependence.

• The current produced by the glucose sensor at a given concentration typically follows a first-order trend with temperature, with second-order effects at the extremes.
• Practical implementations store calibration curves or lookup tables that map raw current and temperature to compensated glucose values instead of relying on a single global linear coefficient.
• The chosen model must balance accuracy, memory footprint and computation cost on the ultra-low-power MCU.

• Factory calibration typically measures the sensor at multiple temperatures and multiple reference glucose concentrations to build a two-dimensional response map.
• The resulting data are compressed into lookup tables or polynomial coefficients and stored in MCU flash or AFE registers, either per sensor, per batch or using a hybrid scheme.
• Calibration planning must consider production test time, memory limits and the expected spread of sensor behavior over lifetime.

• During operation, the MCU periodically samples both sensor current and temperature, then feeds them into the compensation block that applies the calibration model and produces a temperature-compensated glucose value.
• Temperature readings are usually low-pass filtered to remove short-term jitter so that the compensation does not convert small thermal noise into visible glucose fluctuations.
• Rapid temperature transients, such as hot showers, call for special handling with rate limiting or reduced compensation weight to avoid false glucose trends and alarms.

Ultra-low-power supply architecture and measurement cadence

A CGM patch must deliver continuous trend information over 7–14 days from a very small energy source. The power architecture and measurement cadence are designed together so that the AFE, ADC, MCU and BLE radio only wake briefly to measure and advertise, then return to deep sleep to keep average current within the battery budget.

• Many CGM patches rely on a primary coin cell, while others use a small rechargeable cell combined with contact or inductive charging during manufacturing or docking.
• A 7–14 day wear period forces the average current into the low microamp range, after adding up sensing, processing, radio activity and leakage.
• Power budgeting starts from battery capacity and required lifetime, then allocates current to AFE biasing, ADC conversions, MCU run time and BLE transmissions.

• The electrochemical AFE and potentiostat introduce a static bias current, so device selection and configuration must minimize bias while preserving accuracy and loop stability.
• The ADC and MCU consume higher current only during short measurement windows; deep-sleep modes reduce their average to microamp or sub-microamp levels.
• BLE radio bursts can reach milliamp peaks during advertising, but very short duty cycles keep the average contribution compatible with the overall budget.
• Average current is the weighted sum of each module’s active current multiplied by its duty cycle, plus always-on leakage from regulators, references and protection circuits.

• CGM patches typically measure every few minutes, with a short active interval for polarization, sampling, processing and radio activity followed by a long sleep period.
• A typical cycle is: deep sleep → wake → stabilize electrode polarization → take multiple ADC samples → execute compensation and trend checks → advertise → return to sleep.
• Measurement frequency, averaging length and advertising strategy are tuned together so that clinical trend requirements are met while leaving margin in the power budget.

• A compact PMIC provides regulated rails for AFE, MCU and radio using low-quiescent-current LDO or DC-DC stages tailored for light loads.
• Battery voltage monitoring and low-battery flags feed into firmware so that alerts and end-of-life handling can be triggered in advance.
• Integrated protection functions such as overcurrent shutdown, short-circuit protection and thermal limits safeguard the cell and the patch in abnormal conditions.

ULP MCU roles: scheduling, filtering and data packaging

The ultra-low-power MCU coordinates the CGM patch measurement cadence, applies basic digital processing and packages data for wireless transmission. Firmware state machines manage startup, warm-up, normal operation and fault or end-of-life conditions while balancing power, accuracy and robustness.

• Schedule AFE and ADC operation, control sleep and wake-up timing and ensure that polarization and measurement windows follow the power budget.
• Execute temperature compensation, light digital filtering and trend calculations on the sampled sensor current.
• Build compact packets carrying glucose values, status bits and optional event markers for BLE advertising or connections.

• A simple state machine governs how the MCU configures the AFE, ADC and BLE radio across Init, Warm-up, Normal and Fault or End-of-life phases.
• During warm-up, measurement cadence and thresholds may differ from normal operation while the electrochemical sensor stabilizes.
• Fault or end-of-life states respond to open or shorted sensors, excessive drift, low battery or expired wear time and adapt measurement and alert behavior accordingly.

• Simple averaging and low-pass filters smooth sensor noise without adding excessive lag to the glucose trend.
• Temperature-compensated glucose values are produced by combining raw current and local temperature with stored calibration tables or polynomial coefficients.
• Data reduction schemes such as sending fewer updates when readings are stable help reduce radio duty cycle and extend battery life.

• A small ring buffer holds recent glucose samples so that several hours of data can be retained if the receiver is temporarily out of range.
• Event logs track high or low glucose alerts, sensor faults and other important status transitions together with timestamps.
• Storage structures are sized to match flash and RAM limits while preserving key clinical and diagnostic context.

• MCU firmware reserves hooks for secure boot, firmware integrity checks and encrypted communication as defined by the broader security architecture.
• Over-the-air update mechanisms and version management interfaces are planned so that fielded patches can receive firmware improvements where permitted.
• Detailed cryptography and compliance topics are handled in the Security & Compliance subsystem, while this CGM MCU focuses on providing the right integration points.

BLE broadcasting and NFC: pairing, configuration and user interaction

Wireless links between the CGM patch and a smartphone or dedicated receiver are built around low-duty-cycle BLE advertisements, optional BLE connections and NFC for close-range pairing and diagnostics. The goal is to deliver timely glucose trends and alerts while preserving battery life and keeping a clear separation between sensor data, configuration and service functions.

• BLE advertisements provide lightweight, periodic updates that carry current glucose information and key status bits without maintaining a continuous connection.
• BLE connections are used when a smartphone or receiver needs richer interaction, such as reading history, adjusting configuration or synchronizing logs.
• The patch prioritizes advertisement-driven operation to minimize radio duty cycle, reserving connection mode for short, intentional sessions initiated by the user or clinician.

• A typical CGM advertisement contains a temperature-compensated glucose value or encoded band, trend direction, alert flags and basic patch state such as sensor age and battery level.
• The smartphone or receiver merges incoming advertisements into a local trend curve, applies alarm policies and forwards data to higher-level connectivity stacks when required.
• Detailed history, calibration logs and configuration settings are retrieved via BLE connections and are covered by the Medical Gateway & Connectivity subsystem.

• NFC tap can be used for first-time pairing, allowing the reader to obtain device identifiers and security tokens before establishing a BLE link.
• Configuration options such as calibration parameters, patch mode settings or clinician-controlled limits can be read or written during an NFC session in a controlled environment.
• Passive NFC access supports basic diagnostics and data extraction even when the battery is depleted or BLE is disabled, which is valuable for returns analysis and service.

• Broadcast interval and transmit power are tuned to satisfy latency and range targets while meeting the CGM patch average current budget.
• Connection sessions are time-limited and drop back to advertisement-only operation when the user interface becomes idle or the link is no longer needed.
• During rapid glucose changes or critical alerts, the firmware may temporarily increase advertisement frequency, with subsequent return to normal cadence once conditions stabilize.

Safety, reliability and fault detection for CGM sensors

Continuous glucose monitoring introduces sensor-specific safety and reliability concerns that extend beyond general electrical safety. The system must detect physical probe issues, abnormal electrochemical behavior and end-of-life conditions, then map them to clear sensor-fault actions that remain distinct from glucose trend alerts.

• Probe breakage, partial detachment or full detachment can lead to misleading readings or sudden loss of valid signals while the user still believes the patch is operating normally.
• Electrode open or short circuits, connector intermittency and flex tail damage change the effective cell impedance and can distort bias conditions.
• Contamination, bubbles or chemical degradation may introduce drift or loss of sensitivity, gradually reducing correlation between measured current and true glucose.
• Excessive bias voltage or current can damage the electrochemical interface, accelerate aging or introduce unwanted electrolysis.

• Polarization voltage and TIA output swing are monitored to detect open or shorted electrodes, abnormal loading and saturation behavior.
• Bias and leakage currents are evaluated under controlled test conditions to reveal contamination or unwanted parallel paths that grow over time.
• Periodic self-test sequences adjust bias or inject small test steps while the MCU evaluates the response against expected envelopes.
• A dedicated health state machine on the MCU classifies sensor status as normal, degraded, faulted or expired and records transitions in local logs.

• A runtime counter tracks wear time from insertion or activation, aligned with the specified patch lifetime and safety margins.
• Approaching the end of the allowed wear window, the system issues early replacement reminders while still providing valid readings.
• Once the defined end-of-life limit is reached, the patch may reduce measurement activity, flag data as unreliable or require replacement before further use.

• Glucose alarms indicate clinically relevant levels and rates of change, whereas sensor fault alarms indicate that the underlying measurement cannot be trusted.
• Sensor fault conditions use dedicated flags or channels so that user interfaces and connected systems can distinguish them clearly from metabolic events.
• In severe fault states the system can suppress new glucose trend updates, display strong sensor-error warnings and instruct the user to replace the patch.

• The CGM module exposes status lines and data fields that allow higher-level safety controllers to react to sensor faults and end-of-life conditions.
• System-level leakage current, insulation and EMC requirements are handled in the EMC / Patient Safety Subsystem; this CGM AFE and MCU are designed to integrate with those protections.
• Consistent reporting of sensor health enables downstream systems to enforce safe behavior and meet regulatory expectations for continuous monitoring devices.

Recommended topics you might also need

• IVD Electrochem Reader
• Point-of-Care Test (POCT)
• Bio-impedance (BIA)
• Temperature Sensing
• Medical Gateway & Connectivity
• Security & Compliance

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