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ANALOG DESIGN: In-amp specifies differential output

Waltham, Mass. — Instrumentation amplifiers (in-amps) are specialized, precise-gain op amps with a challenging task: They take a low-level input signal, such as an EEG or strain gage, which is accompanied by a large amount of common-mode noise, and extract that signal. These amps increasingly have to provide differential-drive output to the next stage in the signal chain, typically an A/D converter.

Such differential signals are preferred because they provide increased immunity to on-board and inter-IC common-mode noise–a consequence of nearby digital signals aggravated by the reduction in supply voltages. Differential signals also provide 6-dB greater dynamic range and signal-to-noise ratio–both critical specs–compared with single-ended operation.

The in-amp specialists at Analog Devices Inc. repeatedly got “push-back” from designers, said product manager Scott Pavlik, who had to use two in-amps and configure the pair to provide a differential output, or use a single in-amp followed by a conventional, precision op amp for the single-ended-to-differential conversion. With either approach, however, final performance and common-mode rejection ratio (CMRR) are not specified by the vendor, but instead are a function of the design, layout and other factors. So why not produce an in-amp that is specifically designed and specified for differential output?

ADI considered doing a single-channel differential-output in-amp, said Pavlik, but “saw a better opportunity” in the AD8222, a dual-input in-amp with two independent channels. The two channels, housed in a package no larger than a single-channel device, can be configured by the user as a single-channel input, differential-output in-amp with guaranteed specs. The IC promises at least 80-dB CMRR out to 10 kHz and a 3-dB bandwidth of 825 kHz (both at gain of 1), which Pavlik said is 40 dB higher than competing solutions, but in a package that is 60 percent smaller.

By putting two devices, based on the established single-channel AD8221, on a single die, the dual device increases channel density. That makes it attractive for handheld systems, where package size is critical, and industrial applications, where channel counts are high.

It's not all about low-level signals, either. Many of the legacy and still-critical applications, such as industrial designs, use dual and relatively wide supply rails, such as ±15 volts dc, added Pavlik. So the in-amp must operate from those rails and also provide a wide output swing to the A/D converter, or else suffer loss in effective resolution. For this reason, the AD8222 is designed to operate from ±2.3-V to ±18-V supplies.

Housed in a 4 x 4-mm lead-free chip-scale package, the in-amp sells for $3.59 each in lots of 1,000. It is sampling now.

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