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Analogue designers continue to push the boundaries

Since 1954, the International Solid State Circuits Conference (ISSCC) has been the forum for presenting leading edge circuit designs. Whilst the focus over the years has shifted to digital designs, the conference continues to have significant analogue content, with sessions dedicated to such areas as data conversion and analogue techniques.

Axel Thomsen, a Silicon Laboratories Fellow, was chair of the analogue sub committee for this year's event. How did this year's presentations compare to previous events? "The analogue sessions, including one on analogue techniques, saw a mix of classic circuits, such as oscillators and amplifiers, but a growing focus over the last couple of years has been power management. Topics being addressed included energy harvesting and dc/dc conversion, with probably half of the papers dealing with switched capacitor converters; something I see as significant."

Thomsen said modern designs are all about power. "Not only power consumption, but also power handling. The most impressive papers were addressing low power issues."

He gave a couple of examples. "For instance, one paper described a crystal oscillator that runs on a single digit nA supply. Another was an ultra low power 32kHz RC oscillator.

In the data conversion session, we saw some really exciting developments, with successive approximation designs showing good figures of merit; not just resolution, but also what can be done with nW of power."

So how much of a challenge is power? "Integrated power management is definitely a challenge," he continued. "This is something which used to be done on the side with discrete devices. However, the use of capacitive techniques means more and more of that functionality can be integrated on chip using switched capacitor converters.

"In the end, what drives designs like these is cost reduction. If you can bring the power management circuitry on chip and use fewer external devices, that's good. It will provide the same functionality you would get with a classic analogue design approach."

He picked up on the 32kHz oscillator as an example. "The oscillator has the same job to do, but its power budget has been cut by a large amount because people worry about issues such as battery life. Our challenge as analogue designers is to use the power we have available efficiently."

As chair of the analogue sub committee, Thomsen is looking to select quality papers. "We don't have a problem filling three sessions with top notch contributions," he claimed, "and the analogue techniques session is typically the highest rated of all sessions at ISSCC."

Session 17 at this year's event focused on analogue techniques. What were the stand outs in Thomsen's opinion? "Because of their focus on low power, papers 17.7 and 17.8," he said. "But paper 17.9 from Columbia University showed real innovation, working with ultra low supply voltage."
Paper 17.9 describes a 0.6V 70MHz fourth order continuous time Butterworth filter. The circuit is said to have an SNR of 55.8dB and a THD of 60dB at an output signal power of +2.8dBm.

"The paper describes a filter in which an op amp is replaced with a PWM stage," he noted. "This brings stability when used with a low supply voltage, but also high efficiency. It's an innovative way to deal with the problem of designing op amps that run from such supplies and people have wondered what could be done to replace them. Techniques such as this will become the 'go to' solutions in the future."

Paper 17.7, presented by Keng-Jan Hsiao from MediaTek, described a 1.89nW/0.15V self charged crystal oscillator for real time clock generation (see fig 1). "The most outstanding thing," Thomsen claimed, "is the power number; 1.89nW is amazing."

The oscillator provides the 32.768kHz output used in a wide range of designs. But extending stand by time in devices such as mobile phones means these oscillators require ultra low power consumption.

The paper suggests a self charged crystal oscillator (SCXO) can reduce power consumption, supply voltage and chip area. The SCXO excites the crystal by charging power into it directly. Without the extra power consumed by load capacitors and the feedback resistor, says the author, power consumption is greatly reduced and the stable output clock provided by a conventional XO is still obtained.

Paper 17.8, presented by Texas Instruments, outlined a 190nW 33kHz RC oscillator with a temperature stability of ±0.21% and a long term stability of 4ppm. "While this design draws more power than 17.7, the numbers are still good."

The authors say low frequency crystal oscillators are a common choice for sleep timers, due to their excellent long term stability, frequency stability over temperature and very low power consumption. "However, the external crystal cost and board area are undesired. If an integrated oscillator is used as an alternative, the frequency variation must be minimised so sleep time can be maximised."

The proposed solution consists of an RC network, an inverting gain element from a resistor terminal to a capacitor terminal and another inverting gain element from the common resistor/capacitor terminal back to the resistor terminal.

Thomsen said three papers from the DC/DC conversion session were worthy of note. "Paper 4.7 is good because it sets a new benchmark in terms of power density. The authors have used a 32nm silicon on insulator process with a deep trench, the design has good capacitors and achieves excellent power density. In paper 4.8, the team decided switched capacitors would be better than the classic solution, with better potential for integration. But my favourite is paper 4.6, which overcomes potential high losses in certain applications."

The paper, presented by the University of California, describes an 85% efficiency fully integrated 15 ratio recursive switched capacitor DC/DC converter with an output voltage range of 0.1 to 2.2V (see fig 2).

"The traditional approach, for example, with 2:1 switched converter cells for buck or boost, is good," said Thomsen. "But if you want a ratio of 2.1:1, you can get high losses. This paper outlines how to generate these ratios automatically; something which was hard to do before."

Despite the impression that analogue design is a declining art, Thomsen said he remains 'amazed' by the innovation being seen in areas which have been around for a long time."Moore's Law hasn't driven new circuit ideas," he concluded, "these are being driven by applications; and applications have shifted the focus to power efficiency for analogue designs."

Graham Pitcher

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