"one-off" implementation task (meaning a lot of work is necessary for a product with a single, specific use). Systematic design methods of analog and mixed-signal circuits are far more primitive than digital circuits. In general, analog circuit design cannot be automated to nearly the extent that digital circuit design can. Combining the two technologies multiplies this complication. Fast-changing digital signals send noise to sensitive analog inputs. One path for this noise is substrate coupling. A variety of techniques are used to attempt to block or cancel this noise coupling, such as fully differential amplifiers, P+ guard-rings, differential topology, on-chip decoupling, and triple-well isolation. Variations Mixed-signal devices are available as standard parts, but sometimes custom-designed application-specific integrated circuits (ASICs) are necessary. ASICs are designed for new applications, when new standards emerge, or when new energy source(s) are implemented in the system. Due to their specialization, ASICs are usually only developed when production volumes are estimated to be high. The availability of ready-and-tested analog- and mixed-signal IP blocks from foundries or dedicated design houses has lowered the gap to realize mixed-signal ASICs. There also exist mixed-signal field-programmable gate arrays (FPGAs) and microcontrollers. In these, the same chip that handles digital logic may contain mixed-signal structures like analog-to-digital and digital-to-analog converter(s), operational amplifiers, or wireless connectivity blocks. These mixed-signal FPGAs and microcontrollers are bridging the gap between standard mixed-signal devices, full-custom ASICs, and embedded software; they offer a solution during product development or when product volume is too low to justify an ASIC. However, they can have performance limitations, such as the resolution of the analog-to-digital converters, the speed of digital-to-analog conversion, or a limited number of inputs and outputs. Nevertheless, they can speed up the system architecture design, prototyping, and even production (at small and medium scales). Their usage also can be supported with development boards, development community, and possibly software support. History MOS switched-capacitor circuits The metal–oxide–semiconductor field-effect transistor (MOSFET, or MOS transistor) was invented by Mohamed M. Atalla and Dawon Kahng at Bell Telephone Laboratories in 1959, and the MOS integrated circuit (MOS IC) chip was proposed soon after. However, MOS technology was initially overlooked by Bell because they did not find it practical for analog telephone applications, before it was commercialized by Fairchild and RCA for digital electronics such as computers. MOS technology eventually became practical for telephony applications with the MOS mixed-signal integrated circuit, which combines analog and