Anyone who used a scope probe in Beaverton in the 1970s was sure to get a lesson in how to find the square-wave reference signal on the scope and in how to tweak that compensation screw on the probe. So, I understood Williams’ point about “breaking” the rules.
What I didn’t know was that Amdahl had understood that concept to the point where he used it on the backplanes of his minicomputers—a “renegotiation” of the laws of physics that saved Amdahl all sorts of interconnect headaches. IBM did something similar with disc drives.
DATA CONVERTERS AND AMPS National Semiconductor got considerable attention early in 2008 for bringing to market the first continuous-time deltasigma ADC, the 12-bit, 50-Msample/s ADC12EU050 (Fig. 1). Unlike a discretetime delta-sigma, a continuous-time converter moves the sampling operation from the input to the ADC to just after the forward loop filter in the modulator. But what’s so good about moving the sampling operation downstream?
In a discrete-time delta-sigma ADC, there is, as in most ADCs, a switched-capacitor filter. Designing one requires the creation of fast settling circuits and an input buffer to eliminate sample glitches. Also, switched-capacitor input filters set their poles and zeroes through capacitor ratios relative to the sampling clock frequency. Because capacitor thermal noise is inversely proportional to capacitance, the capacitors must be relatively large to obtain the best signal-to-noise ratio (SNR).
In addition, to acquire an accurate representation of the input signal on a hold capacitor, the input stages must settle to a finite level dictated by the accuracy limits of the system. During acquisition, settling time depends on the exponential time constant and slew rate of the system.
What’s good about discrete-time ADCs is that their input filter characteristics scale with clock frequency. Filter performance, therefore, always matches the sample clock rate. On the other hand, the higher the clock frequency, the more dynamic power is consumed.
In a continuous-time design, the filter characteristic depends on conventional active-filter design rules. If the sample rate is changed to match input-signal bandwidth, the continuous-time filter must be retuned. It becomes a real challenge to ensure that a single product platform can support a wide range of sample rates. A further challenge lies in achieving high linearity in high-resolution implementations, because the loop filter requires lots of gain.
Continuous-time design was so difficult, it essentially remained a laboratory curiosity for decades. But in 2005, Xignal, a fabless German company, made some breakthroughs. Then National bought Xignal and successfully transferred the laboratory technology to a production technology, which is no mean feat.
Some of the ADC12EU050’s special features take advantage of the oversampling architecture. One example is the chip’s integrated low-pass, brick-wall antialiasing filter. That means no external antialiasing filter is needed. The continuoustime architecture also means the IC has an easy-to-drive, purely resistive input stage that, of course, doesn’t require a sample-andhold amplifier.
National overcame the continuous- time architecture’s traditional susceptibility to clock jitter with an integrated phaselocked loop (PLL) and voltagecontrolled oscillator (VCO) for clock conditioning. The converters provide on-chip instantoverload recovery circuitry that recovers from saturation within one clock cycle if the input exceeds pre-determined limits.
LINEAR CREATIVITY Okay, let’s say you need to design a battery- powered instrument for measuring temperature, pressure, or some other sensor- related characteristic, or maybe even voltage, at a reasonably slow rate, say, up to 60 times a second. For battery-operated applications, Linear Technology and its 16-bit (guaranteed no missing codes) delta- sigma LTC2451 and LTC2452 ADCs offer a small footprint (3- by 2-mm dual flat no-lead package) and low (0.5-µA) shutdown current.
These devices operate from a single 2.7- to 5.5-V supply. They even have a built-in oscillator. The difference between them is that the LTC2451 communicates via I2C and can measure a single-ended input between 0 V and VCC, while the LTC2452 communicates via the serial peripheral interface (SPI) and can measure a differential input up to ±VCC.
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