40TH ANNIVERSARY
We’re still celebrating the 40th anniversary of Analog Devices, Inc. Such events create a temptation to wax historical, and this is no exception. If you’re interested in the high points of our history, the spread on pages 10-11 depicts the contents page of a timeline accessible on the Web (www.analog.com/timeline); in it you can click on any year to access a brief audiovisual clip reviewing that year’s major corporate events.

As part of the celebration, we’ve devoted this year’s four print issues to articles with principal technological focus on (1) digital, (2) conversion, (3) analog, and (4) sensors. This issue is devoted to articles discussing aspects of analog technology. As you will see, even digital phenomena are analog at the hardware level.

RFI AGAIN—GONE!
On multiple occasions, we’ve referred to—or had our attention called to—the dc offset problems caused by common-mode RF interference with low-frequency measurements, particularly as manifested by rectification in the input stages of instrumentation amplifiers.^1,2,3

Our purpose for bringing this subject up again is to mention the availability, since July 2005, of a digitally programmable instrumentation amplifier that contains an intrinsic solution to the problem. The AD8556,⁴ designed for automotive and industrial applications, features internal electromagnetic-interference (EMI) filtering; in addition, it has a very wide temperature range, low-offset voltage and drift, and open- and shorted-wire protection. It can provide a complete signal processing path from a bridge sensor to an A/D converter. Typical applications are with pressure sensors in anti-lock brake systems (ABS), occupant detection systems, fuel level sensors, transmission controls, and precision strain or pressure gauges.

The figure below shows where the on-chip EMI filters are applied to protect the device’s inputs: at the main differential inputs, at the clamp input, and at the input of the output amplifier. In brief, the problem that they solve is to filter out high frequencies before they reach the amplifier input junctions and create dc offsets through partial rectification.⁵

The effectiveness of this filtering scheme can be seen in the figures at the top of next column, comparing responses of the AD8556 and the AD8555, a similar device—but without internal EMI protection. The graph at left compares their dc responses to 200-mV common-mode high-frequency signals that drive both differential inputs (G = 70 mV/mV). The one at right measures dc output responses to 200-mV p-p of high-frequency sine waves driving VPOS, with VNEG grounded.

More about the AD8556: specified with 10-μV max offset voltage, 65-nV/°C max offset drift, and 112-dB typical common-mode rejection, it features digitally programmable gain (from 70 mV/mV to 1280 mV/mV) and output-offset voltage, open- and short-circuited-wire fault detection, low-pass filtering, EMI filtering, and output voltage clamping. Output offset voltage can be adjusted with 0.39% resolution. Gain and offset can be temporarily programmed by the user and evaluated in-circuit, then permanently programmed by blowing polysilicon fuse links. The AD8556 operates on a single 2.7-V to 5.5-V supply and consumes 2 mA. Specified from –40°C to +140°C, it is available in 8-lead SOIC and 16-lead, 3-mm × 3-mm LFCSP packages.

Dan Sheingold [dan.sheingold@analog.com]

_______________________________________________________________________________________

¹ “Ask The Applications Engineer—14” http://www.analog.com/library/analogdialogue/Anniversary/14.html

Ask The Applications Engineer, Analog Devices http://www.analog.com/library/analogdialogue/Anniversary/contents.html

² “A Reader Notes” http://www.analog.com/library/analogdialogue/Anniversary/Reader_Notes.html

³ H.R. Gelbach, “A Reader Notes: High-Frequency-Caused Errors in Millivolt Measurement Systems,” Analog Dialogue 37-3, 2003, pp. 2, 14. http://www.analog.com/library/analogdialogue/archives/issues/vol37n3.pdf

⁴ http://www.analog.com, Search <AD8556>

⁵ W. Jung, ed., “Op Amp Applications Handbook”, Elsevier-Newnes, 2005, pp. 719-726. Similar material can be found on-line in the Analog Devices seminar version, http://www.analog.com/library/analogdialogue/archives/39-05/Web_Ch7_final_J.pdf, pp. 7.122-7.129.

 

THE ANALOG WORLD
In mixed-signal systems, an analog-to-digital converter (ADC) translates real-world analog signals into the digital domain so that further signal processing can be implemented by the DSP or embedded processor. A digital-to-analog converter (DAC) then translates the digital signals back into the analog domain. At the start of the chain, and at the end of the day, the signals that we transmit, measure, and control are analog: audio, video, temperature, pressure, voltage, and current, for instance.

Some amount of analog signal conditioning is always required before the ADC and after the DAC. Even functions that may at first appear to be digital are often analog at their very heart. Take phased-lock loops (PLLs), for example. These include phase detectors, filters, and oscillators—all analog functions. Direct digital synthesis (DDS) devices, on the other hand, are mostly digital, but they provide output signals whose properties are measured using analog quantities such as phase, frequency, and amplitude.

When designing analog circuitry—especially where high speed, high resolution, or low noise is required—a good printed-circuit board layout becomes increasingly important. Careful attention to voltage levels and signal flow can minimize the need for an expensive, time-consuming redesign, and can maintain optimum performance throughout the signal path.
With all this in mind, we invite you to read the “analog” installment of Analog Dialogue’s tribute to ADI’s 40th anniversary. Here you will learn about adding stereo audio to a low-cost satellite set-top box; using DDS to generate waveforms for test, measurement, and communications; and avoiding the layout pitfalls inherent in high-speed amplifier designs. Enjoy!

Scott Wayne [scott.wayne@analog.com]