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Operational amps reject EMI, RF interference

Posted: 28 Aug 2008 ?? ?Print Version ?Bookmark and Share

Keywords:operational amplifier? integrated EMI filter? high EMI rejection ratio?

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

The LMV831 single, LMV832 dual and LMV834 quad op amps feature CMOS inputs, a maximum input offset voltage of 1mV, 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 93dB. The LMV83x devices maintain stability for capacitive loads as large as 200pF and offer a temperature range of -40C to 125C.

The op amps also feature a 3MHz unity gain bandwidth while operating on only 240?A of supply current This yields a power-to-performance ratio of 80?A 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 1V of output offset, which would diminish a 10bit ADC's resolution (1,024 codes) to the equivalent of only 3bit, National Semiconductor said. Using a LMV83x op amp would only reduce resolution by 0.2bit.

To assist engineers with determining an op amp's EMI robustness, National developed the EMIRR specification, 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.

Samples of all three EMI-hardened op amps are available now and production quantities will be available in Q3 08.

- Gina Roos

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