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Custom Analog ICs: How to Avoid the Wrong Design

Many times when a custom analog IC design is undertaken, the design has already been realized on a PCB using off-the-shelf components. This is a good thing, but it can also be a bad thing if the ASIC design methodology is to simply transfer the same design from one platform (the PCB) to another (the ASIC).

In an earlier blog, I wrote about how the design of an analog IC is based on relative principles, rather than the absolute principles used on a PCB. For example, IC designers think about resistor dividers in terms of the ratio of one resistor to another, regardless of the absolute value of the resistor. This is because the absolute control of resistance is very limited on an IC, whereas lithography matching is very precise.

An even more common but wrong design principle is to assume that each circuit on the PCB can simply be translated over to the ASIC. This design methodology is not viable, because production yield statistics get in the way. As a result, the ASIC actually costs more, the performance will be worse, and the nonrecurring engineering will be higher.

Discrete analog functions available as standalone devices — for example, an op-amp or a voltage reference — have a specification based on the manufacturer's acceptable yield loss for that device. Also, with precision functions, manufacturers often grade the devices. For example, a voltage reference might be available with an actual voltage within a 0.5% tolerance or a 1% tolerance for a different price points. This same grading process can be applied to op-amps for specifications such as input offset voltage. To make matters worse, the process used to build a standalone function will be the optimum for performance, cost, or both.

Contrast all these issues with those of an ASIC. First, the ASIC is generally for one customer, so there is no opportunity to grade devices and sell the higher grades to other parties for more money. Second, multiple functions are collocated on a single process that is not optimum for each function on the original PCB design.

Unless the objective is simply to shrink the board design, a direct PCB-to-ASIC translation will almost always have a higher yield loss and be built on a more expensive wafer process. In extreme cases, one simply won't be able to execute the translation (i.e., include an 18-bit linear ADC on the ASIC). Finally, more functions on one IC means more opportunities for a single function to be out of specification, in which case the IC must be discarded. Consequently, the price for fully compliant parts must increase to make up for yield losses.

A simple way to see the claims above in action is to look at the performance specifications versus price for reconfigurable mixed signal devices, regardless of whether the configurability is via software or metal programming. If the devices don't cost more than an equivalent composite discrete solution — a highly unlikely situation — you can be assured the performance will be worse. Whereas it is almost impossible to find a discrete op-amp with offset voltages as bad as 7-9 mV and above, it is not uncommon to see op-amp and comparator offsets of 15 mV or more on feature-rich mixed signal devices.

The right way to design a custom analog ASIC is to write the design specification around the system-level objective. This enables ASIC circuit designers to utilize the advantages of an ASIC design methodology to arrive at a higher-performing and lower-cost solution. For example, most ASIC devices don't require a full-featured op-amp like a LM741. Many times a simple op-amp circuit with five or six transistors is acceptable and requires no internal compensation. Additionally, the ASIC supply voltage wouldn't have a range of 3-32 V, as with an LM324 op-amp. Usually it is one well-controlled voltage unique to the application.

A clever ASIC engineer can usually find a system methodology where the overall system precision is tied to a single statistical variable, rather than the composite of several discrete random variables. For example, functions such as auto-zeroed amplifiers with sub-millivolt offset voltages are relatively easy to implement on an ASIC for less than one-tenth of one cent — effectively free. Equivalent performance on a PCB would cost 100 times that.

As more and more standard product, system-level mixed signal ICs are introduced by traditional analog semiconductor manufacturers, the performance invariably improves, the price drops, and the time to market decreases. This is because those products are engineered based on the system objective. If one goes down the custom ASIC path, it is a good idea to mimic that proven approach. Those devices aren't simple amalgams of earlier functions.

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