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Dual-channel in-amp drives differentially

Norwood, Mass. — Analog Devices Inc. claims this is the only dual-channel instrumentation amplifier (in-amp) available in a 16-lead, 4 x 4-mm CSP, and the first to allow designers the use of a device specified for differential output operation.

The AD8222 in-amp offers designers twice the number of output channels and a common mode rejection ratio (CMRR) that is 40 dB higher than competing dual-channel in-amps, all in a package that is 60 percent smaller, said Steve Sockolov, product line director of ADI's Precision Linear Products. “The AD8222's combination of higher channel count, small size and superior noise rejection means manufacturers of factory automation systems, medical monitoring devices and other industrial and instrumentation equipment can significantly boost the precision, accuracy, channel density and response time of their products without increasing component count or board size,” he said.

The AD8222 in-amp can also be configured as a single differntial output instrumentation amplifier. This differential output capability, coupled with the AD8222's noise performance (8 nV/√ Hz input noise) allows designers to use smaller supply rails without sacrificing signal-to-noise (SNR) performance, according to Sockolov.

The ability of the new in-amp to drive differentially also allows systems designers to pair the AD8222 with high-performance differential input analog-to-digital (A/D) converters and affords them the flexibility to equip their products with single-supply operation or signal conditioning blocks to further improve noise immunity, Sockolov said. Additionally, the AD8222's single-chip differential output performance is specified.

The AD8222's CMRR provides an advantage for designers building electrocardiograms (ECGs), electroencephalograms (EEGs) and precision industrial control systems, Sockolov said. The AD8222 features a CMRR of 80 dB up to 10 kHz (G=1), which is 40 dB higher than the nearest competing dual-channel in amp and outperforms single-channel in amps by 6 dB (and more than 20 dB out to the full 10-kHz range), he said. The dual-channel in-amp also offers a 2 V/microsecond slew rate, 8 nV/√ maximum input voltage noise at 1 kHz, 0.25 microvolt peak to peak input noise and a wide power supply range of ±2.3 V to ± 18 V.

The AD8222 dual-channel in-amp is available in a 4 x 4-mm, 16-lead lead-frame chip-scale package (LFCSP).

 
 
See related diagram

The AD8222 is sampling now. Production is slated for May. The AD8222 is priced at $3.59 in 1,000-unit quantities. Click here for the AD8222 data sheet.

Analog Devices , 1-800-262-5643, www.analog.com.

Shortly after ADI started shipping its single in-amp in 2003 (AD2221), customers began asking them how to wire up two of these devices to create differential outputs. ADI claims the AD8222 is the first in-amp with specified differential output performance. This is achieved by wiring the dual channel in-amp into a single in-amp with a differential output.

Demand for differential outputs is a result of shrinking supply voltages. In order to realize the full dynamic range of differential input converters, the amps need to drive them differentially. “Customers can connect amplifiers and resistors themselves, but doing it that way could result in errors. We've done the integration for them, shrunk the package size, and we specify the performance. They just plug it in and go,” said Scott Pavlik, ADI's product marketing manager.

Essentially, ADI is removing variability as to how the solution will be arrived at — not just with the ICs, but with the resistors that are now on-chip and laser trimmed. “Otherwise, the customer would have to buy precision resistor networks,” Pavlik said.

Basically, differential range provides additional dynamic range. Besides the fact that supply voltages are shrinking, differential output is needed to enhance performance. “Driving a high precision converter single-endedly sacrifices dynamic range and SNR by 6 dB,” Pavlik said.

Industrial instrumentation is the primary market for these amplifiers. Customers are looking to boost channel density — sometimes while reducing cost, footprint and power, or they are looking to maintain or improve performance. Some customers have a fixed form factor like an industrial process control system or ECG and they are trying to get more channels in a given area, Pavlik said. “Whatever the customer desires, there is still a need for both dual and single-supply operation. There are plenty of applications that require a 30-V down to a single 5-V supply range,” he said.

In some instrumentation applications, such as patient monitoring, ADI is seeing a trend toward single-supply operation with differential input A/D converters.

Size is paramount nowadays in instrumentation, industrial control, and especially patient monitoring.

ADI is actually packing two in-amps in one LFCSP that measures 4 x 4 mm (body and leads) or 16-mm². This is the same size as one, single in-amp in an 8-lead MSOP (3 x 5 mm includes body and leads). Therefore, two in-amps in the MSOP would require 30-mm² space, versus 16-mm² for one of ADI's AD8222 in-amps.

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