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Op amps integrate EMI filters

Santa Clara, Calif. — National Semiconductor Corp. has introduced three operational amplifiers (op amps) with integrated electromagnetic interference (EMI) filters that maintain the accuracy of analog systems by reducing the effects of radio frequency (RF) interference. The LMV83x op amps are reported to deliver the industry's highest EMI rejection ratio (EMIRR) of 120 dB, thereby eliminating EMI-induced errors.

The LMV831 single, LMV832 dual and LMV834 quad op amps feature CMOS inputs, a maximum input offset voltage of 1 mV, a rail-to-rail output stage and an input common-mode voltage range that includes ground. These devices provide a common-mode rejection ratio (CMRR) and a power supply rejection ratio (PSRR) of 93 dB. The LMV83x devices maintain stability for capacitive loads as large as 200 pF and offer a temperature range of -40°C to 125°C.

The op amps also feature a 3-MHz unity gain bandwidth while operating on only 240 uA of supply current This yields a power-to-performance ratio of 80 uA per MHz, which places these devices among National's PowerWise family of energy-efficient products.

In addition, the LMV83x op amps reduce development time and board size by minimizing the need for metal shielding, filters and extra components. They are well-suited for use in phone accessories, medical instruments, precision weigh scales and other industrial electronic equipment that are sensitive to electromagnetic disturbances in noisy environments. Applications include filters/buffers, photodiode preamp and piezoelectric sensors.

For example, an injected RF signal in a weigh scale can result in as much as 1 V of output offset, which would diminish a 10-bit analog-to-digital converter's (ADC's) resolution (1024 codes) to the equivalent of only 3-bits, National Semiconductor said. Using a LMV83x op amp would only reduce resolution by 0.2-bits.

To assist engineers with determining an op amp's EMI robustness, National developed the EMIRR specification (app note: AN-1698 www.national.com/an/AN/AN-1698.pdf), which quantifies how well various pins of the op amp reject RF interference. Evaluation boards are also available.

The LMV831 is offered in a 5-pin SC70 package, while the LMV832 is offered in an 8-pin MSOP package and the LMV834 is offered in a 14-pin TSSOP package.

Pricing: The LMV831 is priced at 55 cents each, the LMV832 at 79 cents each, and the LMV834 at $1.10 each, all in quantities of 1,000.
Availability: Samples of all three EMI-hardened op amps are available now and production quantities will be available in third quarter 2008.
Product information: LMV831, LMV832, LMV834, WEBENCH, designing with amplifiers, and amplifiers

National Semiconductor , www.national.com

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