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26 MUX-Friendly Precision Operational Amplifiers By Tamara Alani, Richard Barthel, Texas Instruments Multiplexing is a frequently used technique to perform data acquisition in multi-channel systems with minimal signal-chain requirements. In this context, the role of the multiplexer (MUX) in an acquisition system is to switch between channels and send each signal as fast as possible to a single data converter—maximizing system throughput and minimizing delay. To ensure accurate processing, a precision amplifier is placed downstream from the multiplexer to precisely drive the analog-to- digital converter (ADC). Traditional Amplifier Architecture Traditional CMOS-input amplifier architectures consist of a differential transistor pair with the metal-oxide semiconductor field-effect transistor (MOSFET) sources connected together and then taken to ground via an active current source, shown in Figure 1. Modern transistor manufacturing techniques attempt to maximize MOSFET transconductance (gm) by reducing oxide thickness (tox)—however, this trade-off results in breakdown voltages of approximately 5V from the gate to source. Large gate to source voltages typically stem from large input differential signals, which are commonly seen during slewing or open-loop operation. To protect the input from permanent damage, amplifiers have two robust anti- parallel diodes between the inputs of the amplifier with a clamp voltage of typically, ±0.5V to ±1.5V. This will limit the voltage swing across the inputs to one or two forward diode voltage drops, which is well below the breakdown voltage. While these inputs provide a level of protection, they do have considerable drawbacks. Large Differential Inputs with Traditional Op-Amps Figure 2 shows a MUX with two channels: Channel A and Channel B. When the output of the MUX is connected to Channel A, 10V is present at the non- inverting input of the op-amp. Since the amplifier is operating linearly, the potential across the inputs is 0V (neglecting offset voltage). As soon Figure 1: Transistor Differential Input Pair