JAJSFO4B August 2017 – December 2018 OPA2810
High-Z input amplifiers are particularly useful when interfaced with sensors that have relatively high output impedance. Such multichannel systems usually interface these sensors with the signal chain through a multiplexer. Figure 77 shows one such implementation using an amplifier for interface with each sensor, and driving into an ADC through a multiplexer. An alternate circuit, shown in Figure 78, may use a single higher GBWP and fast-settling amplifier at the output of the multiplexer. This gives rise to large signal transients when switching between channels, where the settling performance of the amplifier and maximum allowed differential input voltage limits signal chain performance and amplifier reliability, respectively.
Because of the fast input transient, the amplifier is slew-limited and the inputs cease to track each other (a maximum VIN,Diff of 7V is seen in Figure 79) until the output reaches its final value and the negative feedback loop is closed. For standard amplifiers with a 0.7-1.5V maximum VIN,Diff rating, it is required to use current-limiting resistors in series with the input pins to protect from irreversible damage, which also limits the device frequency response. The OPA2810 has built-in input clamps that allow the application of as much as 7V of VIN,Diff, with no external resistors required and no damage to the device or a shift in performance specifications. Such an input-stage architecture coupled, with its fast settling performance, makes the OPA2810 a good fit for multichannel sensor multiplexed systems.