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issue: January 2008 APPLIANCE Magazine

Electronics Report
Extended Portable Power

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An operational amplifier consumes only 20 µA supply current while offering high linearity to maximize sensing accuracy in portable applications.

The AD8506 low-power CMOS dual operational amplifier, with rail-to-rail inputs and output, was designed for portable applications such as battery-powered patient monitors, remote sensors, handheld instrumentation, and other mobile devices that require precision measurement at low voltage levels with minimal power consumption.

Analog Devices Inc. (ADI; Norwood, MA, U.S.) designed the amplifier with novel circuit architecture to maintain high linearity by minimizing distortion through its power supplies and its inputs. Its 105-dB PSR minimizes errors caused by power supply voltage variations over a battery’s lifetime, making the amplifier suited for battery-powered applications.

“The input-stage architecture eliminates the input crossover notch that is so common with rail-to-rail input amplifiers,” explains Tony Ochoa, product marketing manager in ADI’s precision analog products group. “This results in much better distortion performance and eliminates dc offset shifts that occur when transitioning from one input pair to another. Furthermore, the amplifier offers low noise for its power class. This can be a big problem for low-power system designers since the noise of lower-power amplifiers is higher than their higher-power counterparts. The AD8506 also does not have spurious feedthrough that can affect system performance.”

The amplifier maintains 105-dB CMR, maximizing linearity by minimizing the common-mode induced distortion often found in amplifiers having rail-to-rail inputs. While maintaining high linearity, the unit consumes only 20 µA of supply current per amplifier—35% less than other operational amplifiers in its class.

The combination of low power and high linearity delivers extended battery life for portable applications regardless of external conditions, allowing accurate measurement of real-world phenomenon in portable devices.

“The patent-pending amplifier architecture solves the typical low CMRR specifications that plague previous untrimmed CMOS amplifiers,” says Paul Lee, product marketing engineer for ADI’s precision amplifiers products group. “But even-more-costly trimmed CMOS amplifiers have issues retaining CMRR versus temperature. The AD8506 retains 105 dB of CMRR over the full temperature range. This architecture also helps minimize power with its unique biasing arrangement.”

Ochoa says an ideal use for the amplifier would be a current sense application, where the amplifier can detect the end of battery life or charging of the battery for portable/battery powered devices. “It can be either high-side or low-side sensing as the amplifier features rail-to-rail inputs,” he explains. “Also, the distortion caused either through the input stages of the amplifier and/or the power supply is minimal as this amplifier features high CMRR and PSRR. In addition, the power consumption will be negligible compared with other components in the system.”

“Amplifiers can easily be integrated into product designs, as most designs require amplification and/or buffer of signals coming from sensors or other circuitry,” says Lee. “For portable applications, the current consumption of this amplifier is minimal compared with other circuit components in the system. Also, the distortion introduced—either through the inputs (CMR) or power supply (PSR)—to the signal by the amplifier is minimal during normal and end-of-battery-life operation.” 

Suppliers mentioned in this article:
Analog Devices Inc.

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