Medical Instrumentation

Functional Building Blocks

Generalized signal chain for biomedical measurement devices, compliant with IEC 60601 isolation. Click a stage to inspect its role, gain, and failure mode. Cross-link: Circuits II — cascaded transfer functions.

graph TD M["Measurand
e.g. Blood Pressure"] S["Sensor / Transducer
Sensitivity: K_sensor"] C["Signal Conditioner
Gain G_amp, Filter H_filter(s)"] ISO["Galvanic Isolation Barrier
Patient Safety (IEC 60601)"] ADC["Analog-to-Digital Converter
Resolution: K_ADC"] P["Microprocessor / DSP
Calibration and Algorithm"] D["Display / Storage
V_display"] M -->|Physical variable| S S -->|Analog voltage V_bio| C C -->|Conditioned analog| ISO ISO -->|Isolated signal| ADC ADC -->|Discrete binary| P P -->|Processed data| D classDef biological fill:#f9d0c4,stroke:#334155,color:#0f172a; classDef analog fill:#d4e1f9,stroke:#334155,color:#0f172a; classDef digital fill:#d4f9d4,stroke:#334155,color:#0f172a; classDef isolation fill:#fff3cd,stroke:#f59e0b,stroke-width:3px,stroke-dasharray: 6 4,color:#0f172a; class M biological; class S,C analog; class ISO isolation; class ADC,P,D digital;

System transfer function

Assuming a linear, time-invariant chain with impedance matching (no inter-stage loading), the displayed value is the product of the block gains:

\[ V_{\mathrm{display}} = K_{\mathrm{ADC}}\,H_{\mathrm{filter}}(s)\,G_{\mathrm{amp}}\,K_{\mathrm{sensor}}\,M \]

\(M\) — physiological measurand (pressure, biopotential, temperature).
\(K_{\mathrm{sensor}}\) — transducer sensitivity (physical units → volts).
\(G_{\mathrm{amp}}\) — linear voltage gain of the analog front end.
\(H_{\mathrm{filter}}(s)\) — filter transfer function; sets bandwidth \(\omega_c\).
\(K_{\mathrm{ADC}}\) — digitizer scale (volts → codes).

Research note — linearity is a textbook simplification

Many intro treatments assume \(K_{\mathrm{sensor}}\) is perfectly linear. In clinic, thermistors are exponential: \(R(T)=R_0\exp[\beta(1/T-1/T_0)]\). The microprocessor must invert that map (Steinhart–Hart lookup or Taylor expansion) before the display block. The companion signal-model dashboard shows this nonlinearity next to a linear strain gauge.