[Technology Report]
Programmable Analog Functions Are Going Mainstream
System-level analog function blocks and software for top-level design move forward.
We have always had programmable analog circuits. Any circuit that makes outputs change as a function of inputs is in some way programmable. Examples include gain change as a function of voltage or current (automatic gain control, or AGC) and frequency as a function of voltage or current (a voltage-controlled oscillator). With appropriate signal conditioning, a clever designer can make parameters vary with time (an ultrasound time-varying gain "s" curve amplifier), or any other base parameter.
As we look at the spaces for analog functions that need to change parameters while in operation, the designs are becoming increasingly complex and unwieldy. Improvements in all electronic components that make them faster and more accurate while the devices operate at lower power levels per function mean that analog control functions also are becoming more precise. That precision comes at the cost of resources—more design time or more complex components.
Analog is always custom. Every application varies, which is why there are so many different analog parts out there. The measurements and controls in analog systems require many specialized components to fulfill the diverse requirements. Due to this demand for unique combinations of functions and parameters, many parts fill the analog world, each representing a wide variety of performance. The proliferation of products shows that one size or generic function doesn't fulfill the needs of all circuit designers. Instrumentation amplifiers, for in-stance, come in 20 to 30 variants that address all possible combinations of input and output parameters.
As the systems become more refined, the controls must become more responsive and flexible, but at a reasonable cost. Therefore, a designer may not want an external signal to control the function. In a data-acquisition system, you might want to get away from the full converter chain per signal, so all of the signals go into a multiplexer. The signals get adjusted in the common portions of the signal-processing chain. But an AGC may distort the critical parts of the signal if signal changes are of the same time scale as the AGC loop.
Enter Programmable: Depending on where the part is used in the system, programmable analog could mean different things. Instead of complex feedback loops and critical component values, designers want to migrate toward simpler architectures. Designers want to give the task to programmers because the crusty analog guru is getting crotchety and incorrigible, not to mention nearing retirement.
Mick Britchfield, product line manager for precision converters at Analog Devices, notes the trend of programmability in analog design. One issue is the need to create hardware and software that enable a high level of programmability without requiring the designer to become a DSP programmer.
The integration of more analog components into the systems requires more interfaces to functions like analog-to-digital converters (ADCs) and other mixed-signal functions. Vendors need to offer the right combination of analog functions to solve the problems in the analog spaces. This is both an integration issue and a technology issue. Silicon no longer limits the number of components and structure complexity.
Silicon solutions cover a wide range of programmable functions. Continuous functions can be controlled by an analog switch that changes some parameter in an on-off basis, or a circuit could contain a digital potentiometer that adjusts its operating performance.
At the other end of the range, the analog subsystems can include ADCs and digital-to-analog converters (DACs), plus some DSP to calculate the signal-processing algorithms. In between these extremes, the discrete programmable components and the data acquisition and conversion systems, are analog equivalents to programmable logic devices (PLDs)—arrays of programmable switches and some fixed functions that can be configured to make up medium- to high-complexity circuits on a single programmable chip (Fig. 1).
Due to the nature of analog signals, all systems need to work with fairly high dynamic ranges, and all of the system implementations still require continuous gain stages before much of the actual signal processing can take place. Analog designers face the challenge of finding the best mix of components and function blocks and putting them together in a stable, robust, and cost-effective system.
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