Figure 2.6 This bootstrapped design yields an ac-coupled differential amplifier that retains all of the superior performance inherent in decoupled instrumentation amplifiers. Ac voltages from the outputs of the ICIA's differential input stage are fed to the inverting inputs of their respective amplifiers via C3 and C4. This causes the ac voltage drop across R1 and R4 to be virtually zero. Ac current flow through resistors R1 and R4 is practically zero, while dc bias currents can flow freely to ground.
presented in the schematic circuits of Figures 2.7 through 2.12. In this design, biopotential signals are amplified by IC5, a Burr-Brown INA128U instrumentation amplifier. Its gain is fixed at 138 through resistor R7. The input of the amplifier is protected from high-voltage transients and electrosurgery currents by a network of resistors, capacitors, and diodes. Back-to-back zener diodes D2 and D4 clamp high-voltage transients induced into the electrodes by defibrillation currents to a level that can be handled by the rest of the protection network. C21 acts as a shunt for radio-frequency currents that may be induced into the electrodes and leads by sources of electromagnetic interference. This capacitor by itself has inherent filtering capability for high-frequency alternating current because capacitive reactance XC (in ohms) is inversely proportional to frequency:
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