For almost 20 years, researchers and semiconductor manufacturers have been trying to develop a practical analog BIST (built-in self-test) for mixed-signal ICs. By enabling mixed-signal IC testing on digital testers and simpler multisite testing, this technique potentially reduces IC-test costs and time to market. Other anticipated benefits include faster test development and in-system self-test.
Most IC-design engineers understand the operation principles of digital BIST. It generates pseudorandom bit patterns with an LFSR (linear-feedback shift register) and applies them to a circuit under test using the flip-flops configured temporarily into serial shift registers. Digital BIST also captures responses using the same flip-flops, compresses the shifted-out result into a digital signature, and then bitwise-compares it to a correct signature.
Although the details of industrial-logic BIST are more complex, the basic principles still apply for accommodating multiple clock domains, multicycle delay paths, and power-rail noise, for example.
The principles of analog BIST, however, remain enigmas to most engineers, especially analog-circuit designers. This fact became evident at the 2009 Design Automation Conference when an experienced PLL (phase-locked-loop)-circuit designer asked, “If your BIST for PLLs is so accurate, why don’t you design PLLs?” To answer that question, you must first understand the principles of practical analog BIST.
How do you define “analog”?
An “analog” circuit means different things to different people. You can consider a PLL or a SERDES (serializer/deserializer) to be digital-, analog-, or mixed-signal. BIST tests for these units can be purely digital because these functions have only digital inputs and outputs. For example, some ICs measure their PLL’s output frequency with an on-chip frequency counter—which counts the number of oscillation cycles in a known number of cycles of a reference frequency—that fails the test if any bit in the count differs from what you expect.
Many ICs test their SERDES transceiver’s performance by looping back pseudorandom data and failing it if it detects a bit error. However, testing analog circuits, such as ADCs or DACs, clearly requires BIST circuitry that can generate or capture analog signals—signals whose instantaneous voltage is always relevant. Traditional analog circuits, such as filters and linear voltage regulators, have analog inputs and outputs, although many have digital-control signals or clocks. The purest analog circuits, such as RF circuits, may have no digital aspects at all.
In testing, an analog circuit has at least one signal whose instantaneous voltage is nondeterministic. Testing comprises checking that the signal, which might be a digital word, is between two voltages, digital values, or time thresholds; that a signal statistic is between limits; or that a mathematical computation involving the signal produces a value between limits. You should apply analog-test principles to all circuits that have any analog signals.
Responses from purely digital circuits are deterministic, so an acceptable output signal is one that you need to sample only once. If you look at digital-circuit signals in enough detail, such as millivolts or picoseconds, however, every circuit is analog. This consideration is nontrivial for nanometer CMOS processes in which power-rail noise, jitter, temperature, and parametric variations are significant effects relative to 1V power rails and subnanosecond clock periods. BIST circuitry that tests analog is subject to these effects, even if the BIST is almost entirely digital, which is the reason that many analog designers wonder how any analog BIST can be more accurate than an analog circuit under test on the same chip.
The challenge of designing analog BIST
Designing BIST for analog circuits involves more than accurately delivering and capturing analog signals. The variety of signals and parameters requiring testing is much larger than that of the logic zeros and ones that digital BIST handles. Analog stimuli and responses can range from dc voltages, linear ramps, and impulses to sine waves and frequency modulation. To complicate the challenge, the stimulus and response might belong to different domains. For example, a dc voltage input might generate a frequency output. The parameters requiring analysis further add to the challenge because they may range from amplitude, phase delay, and SNR (signal-to-noise ratio) to dc voltage, peak-to-peak jitter, and duty cycle.
Test equipment usually must be an order of magnitude more accurate than the circuit under test. So the most daunting challenge for analog BIST is how to economically achieve greater accuracy than the circuit under test, which presumably has achieved the best possible accuracy for its silicon area and technology. The range of signal amplitudes can be enormous. ADCs and DACs handle on-chip analog signals with dynamic ranges as high as 224, equivalent to eight orders of magnitude.
You can compare digital BIST to a student who is grading his own multiple-choice test. The student places a template over the answer sheet and counts the number of correct answers. Analog BIST, on the other hand, compares with a student who is grading his own essay answers. It’s not a simple, objective procedure. Now that you understand the magnitude of the challenge, consider the fundamental circuit principles that you must apply for analog BIST to be practical.