ZHCSK40D August 2019 – July 2020 OPA810
PRODUCTION DATA
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 9-16 shows one such implementation using an amplifier for the interface with each sensor, and driving into an ADC through a multiplexer. An alternate circuit, shown in Figure 9-17, can use a single higher GBWP and fast-settling amplifier at the output of the multiplexer. This architecture 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.
Figure 9-18 shows the output voltage and input differential voltage when a 8-V step is applied at the noninverting terminal of the OPA810 configured as a unity-gain buffer of Figure 9-17.
Because of the fast input transient, the amplifier is slew-limited and the inputs cease to track each other (a maximum VIN,Diff of 7 V is shown in Figure 9-18) until the output reaches its final value and the negative feedback loop is closed. For standard amplifiers with a 0.7-V to 1.5-V maximum VIN,Diff rating, current-limiting resistors must be used in series with the input pins to protect the device from irreversible damage, which also limits the device frequency response. The OPA810 has built-in input clamps that allow the application of as much as 7 V 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 OPA810 a good fit for multichannel sensor multiplexed systems.