In the face of the constant pressure from customers and managers to reduce the overall size and cost of mixed-signal designs, one typical solution has been to shrink the design size and lower the number of IC components through integration.
By taking functions that were previously implemented piece-by-piece using discrete components and merging them into the central controlling core of the design (often a microcontroller), the design is made smaller and simpler, and costs are reduced. This is now taken as such conventional wisdom that it seems almost too obvious to be explaining it.
However, as with most engineering decisions, there are potential trade-offs as well as benefits when moving to a more tightly integrated design. The benefits seem obvious -- your design will have fewer ICs and supporting discrete components, and it will take up less PCB area and require fewer interconnects. Therefore, overall cost and power consumption of the design will go down. A side benefit is often that a simpler design (simpler at the PCB schematic level, at least) is easier to develop; easier to maintain; easier to modify; and with fewer active components, easier to test before putting into production.
But what are some potential tradeoffs of this approach? Many of them have to do with control. First, consider parts sourcing. If you develop a system with a "generic" central microcontroller, and the analog functions live in separate devices, the components of the system are much easier to swap out or second source if needed. If the microcontroller does not directly incorporate the analog functions, it can be traded out in the design for any microcontroller of equivalent capability that is capable of communicating with and controlling the analog slave devices in the system. Similarly, if some or all of the analog devices are separate, they can be replaced with equivalent components if a newer, better IC becomes available. But if the microcontroller is instead a special-purpose one, with integrated analog components, it may not be possible to find another microcontroller from a different manufacturer that does exactly the same thing -- you may be locked into one device for the purposes of your design.
Similarly, if you spread the functions out, you are in control of exactly what your system is capable of and what it is not. Moving to an integrated "all-in-one" microcontroller means that you are limited to the combinations of features that are available for that device. If you need three ADCs, and the microcontroller you are considering only allows two, you will have to add another external ADC to the design or change to a different microcontroller. And if there are components to the microcontroller that support capabilities or interfaces that your design does not actually use, then those capabilities (and the additional cost and complexity in the microcontroller that they represent) are wasted in your system.
In the best partnerships between mixed-signal microcontrollers and their customers, these concerns are reduced by developing specifications for new devices as a joint effort. By taking a customerís specific needs into account when creating a new device, it is possible to tailor an IC design specifically to what a customer requires to go from one generation to the next of their system, while reaping the maximum rewards from cost and power reduction. But this requires a committed joint effort up front, and a solid working relationship. As with so many other things in engineering, communication, early and often, is the key to ending up with the best possible design for your product.