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TABLE 2.2 Approximate —3-dB Frequencies Required for the Reproduction of Physiological Signals with Negligible and Acceptable Levels of Distortion

—3-dB Point Negligible Distortion (1%) Acceptable Distortion (5%)

„. , ,„ , „ 0.0016 „ 0.008 High-pass (Hz) <--<-

IC5 is powered via a Burr-Brown IS0107 isolation amplifier IC3, which generates isolated ±12V when powered from ±12V. L1, C6, C7, and C8 form a pi filter to clean the isolated +12-V power line generated by the IS0107 from switching noise. An identical network is used to filter the negative isolated supply rail. A pi filter formed by L3, C15, C16, and C17 is used to decouple the positive input power rail of IC3 so that switching noise within IC3 does not find its way into the postisolation amplifier stages (IC2 and IC4). R10 and C21 form a low-pass filter with — 3dB of approximately 7 kHz to eliminate any remaining trace of the carrier used to convey the signal across IC3's isolation barrier.

Op-amp IC2B is configured as a noninverting amplifier. The gain of this amplifier can be selected through potentiometers R16-R20 that are switched via SW3. These provide different levels of feedback to IC2B, depending on their setting. The output of this amplification stage is filtered via R9 and C20 with a — 3dB low-pass cutoff of approximately 7 kHz to eliminate any residual switching noise that may have coupled through the input or power supply and amplified by IC2B. The amplified signal is buffered and further low-pass filtered via fixed-gain noninverting amplifier IC4 before being presented to the output connector J1.

Since the biopotential amplifier is dc-coupled and there are occasions when the signal of interest may have a relatively large input dc offset, an automatic zero-offset circuit has been implemented through IC2A. Whenever offset nulling is desired, momentary pushbutton switch SW1 should be pressed. Doing so presents a sample of the output signal to the integrating sample-and-hold (S&H) circuit formed by IC2A, R5, and C9. The hold capacitor should be a low-leakage type, and the path between the capacitor and the inverting

Biopotential Amplifier

Figure 2.2 This wideband dc-coupled biopotential amplifier front end covers the complete frequency range of commonly recorded biopotentials. A Burr-Brown INA110AG ICIA is dc-coupled to the electrodes via current-limiting resistors R22 and R23 and IS-1-3.3DP fault-current limiters. Capacitors and diodes are used to protect the amplifier from high-frequency currents, such as those used in electrosurgery and ablation procedures as well as from high-voltage transients such as those that may be expected from defibrillation and electrostatic discharge.

Figure 2.2 This wideband dc-coupled biopotential amplifier front end covers the complete frequency range of commonly recorded biopotentials. A Burr-Brown INA110AG ICIA is dc-coupled to the electrodes via current-limiting resistors R22 and R23 and IS-1-3.3DP fault-current limiters. Capacitors and diodes are used to protect the amplifier from high-frequency currents, such as those used in electrosurgery and ablation procedures as well as from high-voltage transients such as those that may be expected from defibrillation and electrostatic discharge.

input of the op-amp should be shielded against stray and leakage currents through a guard ring on the circuit board. The output of the integrator/S&H is summed with the output of IC2B via R4. The output of IC2A is also attenuated via R6 and R8 and summed with the output of the isolation amplifier (IC3) via R7. This trick allows offsets that would otherwise saturate amplifier IC2B to be canceled in a very effective way.

A typical application for a dc-coupled wideband biopotential amplifier is the measurement of transmembrane potentials as well as for the detection of cardiac monophasic action potentials (MAPs). Dc coupling is important for these applications because they are usually related to measuring the timing and amplitude of shifts in potentials that have a dc offset.

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