ZHCSIQ4B September 2018 – December 2018 OPA828
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). This concept is illustrated in Figure 45.
In a typical multiplexed application it is common that large transient voltages can be presented to the input of the op amp driving the ADC. Large input differential voltages are commonly seen during slewing or open-loop operation, which is especially common when switching from one MUX input to another. Traditional precision amplifiers often consist of a differential transistor pair that is protected from large differential transient input voltages with anti-parallel diodes between the inputs of the amplifier. These anti-parallel diodes are effective at limiting the voltage differential between the inputs to one or two forward diode voltage drops, which protects the precision input devices from damage. However, the anti-parallel diodes do have considerable drawbacks such as large inrush currents when they are turned on. lf passive filtering or high source impedance is present, large inrush current can disturb settling time, limiting the throughput of the system and degrading signal-chain precision. The OPA828 does not need anti-parallel diodes to protect the input JFET transistors and is free from large inrush currents even with differential input voltages as large as ±18 V. These concepts are illustrated in Figure 46: