Chips that include both digital and analog elements have been around for some time, with functions such as internal timers, comparators, and input/output gates operating under firmware control. Analog functions that can be easily realised in a digital environment include D/A converters, A/D converters and integrators.
The implementation of these functions has been fairly primitive, so there have been performance penalties, but they often achieve enough in applications where the accuracy and fidelity of high-precision analog functions are not needed.
However, designers are now starting to demand that these shortcomings are eliminated. The established wisdom that precision analog cannot co-exist on the same chip with digital controllers or processors, is being challenged more and more. The creation of precision analog functions that are microprocessor/microcontroller-friendly requires three changes, which bring us very close to the system-on-chip (SoC) approach.
The most important change is to shift the fabrication process from bipolar to CMOS. Today, many more analog circuits are migrating from bipolar processes into the CMOS world. At the same time, the performance of these CMOS devices is improving. For example, operational amplifiers that were designed with a CMOS process used to have sub-substandard common-mode rejection, power-supply rejection and offset voltage. Now, these specifications have improved to a much more acceptable level. SAR A/D converters have migrated from an R-2R-ladder topology, to an input capacitive array arrangement. This reduced the silicon size and was synergistic with the CMOS process. The majority of the silicon in an A/D Delta-Sigma converter is dedicated to digital circuitry and has always been designed in CMOS processes. This high-precision device is a perfect candidate for the controller/processor chip. In conjunction with this migration of analog functions from bipolar to CMOS, the CMOS processes are tightening up and the IC designers are continuing to design with innovative improvements.
Another controller-friendly feature is the programmable-analog device. This is not the classical analog definition of programmability where a resistor is changed in the hardware. Instead, it is achieved with on-chip, non-volatile or volatile memory. With this change, the nichrome resistor and zener-zapping analog networks are abandoned in favour of digital RAM, EPROM or EEPROM cells. Permanent changes can be 'burnt' into non-volatile memory during the final manufacturing step. This reduces the overhead costs of laser-trim equipment and yield losses at the wafer level. Alternatively, changes can be implemented on-the-fly (volatile memory) during system operation, producing a device that works in a larger number of applications.
The third and most critical change is that digital specialist companies are now starting to include analog content in their established digital product lines. At first glance, this migration does not seem to be that difficult. However, when the differences between analog and digital disciplines are taken into account, there is a significant culture re-alignment on both sides.
The addition of a standard amplifier into a digital circuit is illustrated in Figure 1. In the days before the use of digital memory in analog circuits, the operational amplifier was designed in hardware to one bandwidth, one quiescent current, and one offset voltage. Although the operational amplifier is seen as being suitable for a variety of applications, the rigidity of these specifications lock a single amplifier into a limited set of applications. Now that the operational amplifier has come into its own in the single-supply CMOS process, these functions can be manipulated with a simple stroke of a key, ie, firmware programming.
Figure 1 also shows the ingenuity that is being applied to today's designs. This simple example of an operational amplifier uses the CMOS process in conjunction with non-volatile EPROM switches. These switches are used in the active load of the differential input pair of the amplifier. The offset voltage of the amplifier is adjusted by using the switches to steer current through one or the other side of the differential input pair. The switches can be electrically accessed in a test mode during final test. This approach produces higher yields, tighter specifications and on-the-fly programmability.
Until now, the analog approach has changed these currents by adjusting the load using nichrome laser-trimming or zener-zapping. These analog processes can damage the passivation area of the silicon chip and they are not synergistic with the CMOS digital process. EPROM switches do not compromise the integrity of the chip. Their reliability is established by the work done over the years with memory devices and controllers/processors. These types of switches are also used to implement amplifier bandwidth or quiescent current changes, to name just a few.
However much the world moves to digital solutions, it is still basically an analog environment, so it is important for complex digital circuits to co-exist with high-performance analog functions. One of the main advantages of the digital device has always been its versatility; with a simple adjustment of code, it can be applied to dramatically different markets. Now we are starting to see analog circuits that offer the same benefit. These devices must be economical, efficient, compact, and streamlined for multiple applications. Historically, microcontroller and microprocessor devices have catered for horizontal markets while analog devices have catered for vertical ones. These two domains are positioned to come together because of the power of digital programmability. The drive for analog in CMOS processes is starting to come to fruition. The combination of analog excellence and digital memory has been slow to arrive because analog houses have not had memory capability in their arsenal and the digital houses have not had the necessary analog expertise. These new hybrid chips have the same function as their analog predecessors, but now they are designed in the same process as the digital circuits. With these changes, the idea of system-on-chip looks more attractive. The only thing missing is to determine which analog functions are needed and when they will come together under one roof.
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