DESIGNIDEAS - READERS SOLVE DESIGN PROBLEMS: High-impedance buffer amplifier's input includes ESD protection

摘要

Certain measurement applications, such as for pH (acidity) and bio-potentials, require a high-impedance buffer amplifier. Although several semiconductor manufacturers offer amplifier ICs featuring low bias and offset-input currents, attaching a sensor cable to an amplifier circuit can inflict damage from ESD (electrostatic discharge). Figure 1 shows one unsatisfactory approach to ESD protection. Resistor R{sub}1 limits an ESD event's discharge current, and diodes D{sub}(1A) and D{sub}(1B) clamp amplifier IC{sub}1's input to its power-supply rails. Unfortunately, when shunting a pH sensor's 400-MΩ input impedance, even low-leakage diodes, such as Fair child Semiconductor's (www.fairchildsemi.com) MMBD-1503A, introduce significant offset voltages.