Issue link: https://resources.mouser.com/i/1442772
as the MUX switches from channel A to channel B, the noninverting input potential of the op-amp instantly changes to –10V. Since the op-amp output voltage does not change instantly, a large differential voltage appears at the inputs and the anti-parallel diodes begin to conduct current, causing a sharp increase in input bias current and drop in input impedance. Without the input anti-parallel diodes described earlier, this large differential voltage would have surpassed the breakdown voltage and thus permanently damaged the op-amp. With the input anti-parallel diodes, outlined in blue in Figure 2, the inputs are protected from large differential voltages—however, large inrush current flows through the diodes. If 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 time the op-amp takes to settle its output can be detrimental to high-speed or high-throughput applications. Most MUXs operate with nanosecond rise-times, far faster than most precision op-amps. If the slew rate of the op-amp cannot keep up with the slew rate of the MUX, a differential voltage develops and settling time worsens due to input current. When the MUX switches between channels, it will take an extended length of time for the output to respond to the input and the system performance may suffer. Some amplifiers attempt to solve this problem with high slew rate, but trade-off power consumption and stability. TI's Precision Amplifier team has developed a unique patented technology, which combines high slew rate with a diode-less front-end to achieve accurate signal processing without the tradeoffs of high-slew rate amplifiers. Its performance following a switched MUX is shown in Figure 3. Take note of the source- loading effects of a non-MUX-friendly amplifier (black) and a MUX-friendly amplifier (red). The top half of Figure 3 illustrates the inrush of current, which can reach several tens or hundreds of milliamperes depending on the amplifier's output current limit. The bottom half of Figure 3 illustrates the effects of settling time while slewing. Although the traditional amplifier output moves quickly due to the input diodes, the RC- network settling is disturbed and the system takes longer to settle. MUX-Friendly Op-Amps Along with junction gate field-effect transistor (JFET) input amplifiers, which are inherently MUX-friendly, TI has developed a new input circuitry for 36-V Complementary metal–oxide– semiconductor (CMOS) inputs, which does not require the anti- parallel diodes for device protection. These MUX-friendly amplifiers are still able to maintain the same level of protection and robustness while also improving settling time for switched systems. This patented input protection scheme uses a set of internal switches, which switch open and close to protect the 27 OPAx189 CMOS Precision Operational Amplifiers • Excellent DC Precision: CMRR: 168dB Open-Loop Gain: 170dB • Low Noise: en at 1kHz: 5.2nV/√Hz 0.1Hz to 10Hz Noise: 0.1µVPP • Excellent Dynamic Performance: Gain Bandwidth: 14MHz, Slew Rate: 20V/µs, Fast Settling: 10V step, 0.01% in 1.1µs Learn more Figure 2: MUX with Buffer-Configured Op-Amp Figure 3: Switching Timing Diagram