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OP281GRU Arkusz danych(PDF) 10 Page - Analog Devices |
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OP281GRU Arkusz danych(HTML) 10 Page - Analog Devices |
10 / 16 page OP181/OP281/OP481 –10– REV. 0 APPLICATIONS THEORY OF OPERATION The OPx81 family of op amps is comprised of extremely low powered, rail-to-rail output amplifiers, requiring less than 4 µA of quiescent current per amplifier. Many other competitors’ devices may be advertised as low supply current amplifiers but draw significantly more current as the outputs of these devices are driven to a supply rail. The OPx81’s supply current remains under 4 µA even with the output driven to either supply rail. Supply currents should meet the specification as long as the inputs and outputs remain within the range of the power supplies. Figure 32 shows a simplified schematic of the OP181. A bipolar differential pair is used in the input stage. PNP transistors are used to allow the input stage to remain linear with the common- mode range extending to ground. This is an important consider- ation for single supply applications. The bipolar front end also contributes less noise than a MOS front end with only nano- amps of bias currents. The output of the op amp consists of a pair of CMOS transistors in a common source configuration. This setup allows the output of the amplifier to swing to within millivolts of either supply rail. The headroom required by the output stage is limited by the amount of current being driven into the load. The lower the output current, the closer the output can go to either supply rail. Figures 7, 8 and 9 show the output voltage headroom versus load current. This behavior is typical of rail-to-rail output amplifiers. +IN –IN VEE OUT VCC Figure 32. Simplified Schematic of the OP181 Input Overvoltage Protection The input stage to the OPx81 family of op amps consists of a PNP differential pair. If the base voltage of either of these input transistors drops to more than 0.6 V below the negative supply, the input ESD protection diodes will become forward biased, and large currents will begin to flow. In addition to possibly damaging the device, this will create a phase reversal effect at the output. To prevent these effects from happening, the input current should be limited to less than 0.5 mA. This can be done quite easily by placing a resistor in series with the input to the device. The size of the resistor should be pro- portional to the lowest possible input signal excursion and can be found using the following formula: R = VEE − VIN, MIN 0.5 ×10−3 where: VEE is the negative power supply for the amplifier, and VIN, MIN is the lowest input voltage excursion expected For example, an OP181 is to be used with a single supply volt- age of 5 V where the input signal could possibly go as low as –1.0 V. Because the amplifier is powered from a single supply, VEE is ground, so the necessary series resistance should be 2 kΩ. Input Offset Voltage Nulling The OPx81 family of op amps was designed for low offset voltages less than 1 mV. The single OP181 does provide two offset adjust terminals, should the user require greater precision. In general, these terminals should be used only to zero amplifier offsets and should not be used to adjust system offset voltages. A 20 k Ω potentiometer connected to the offset adjust terminals, with the wiper connected to VEE, can be used to reduce the offset voltage of the amplifier. The OP181 should be connected in the unity-gain configuration (as shown in Figure 33) or in a gain configuration. The potentiometer should be adjusted until VOUT is minimized. The wiper of the potentiometer must be connected to VEE; connecting it to the positive supply rail could damage the device. 6 5 7 4 1 2 3 +5V OP181 VOUT 20k Ω POT. VEE = –5V Figure 33. Offset Voltage Nulling Circuit Input Common-Mode Voltage Range The OPx81 is rated with an input common-mode voltage range from VEE to 1 volt under VCC. However, the op amp can still operate even with a common-mode voltage that is slightly less than VEE. Figure 34 shows an OP181 configured as a difference amplifier with a single supply voltage of +3 V. Negative dc voltages are applied at both input terminals creating a common- mode voltage that is less than ground. A 400 mV p-p input signal is then applied to the noninverting input. Figure 35 shows a picture of the input and output waves. Notice how the output of the amplifier also drops slightly negative without distortion. OP181 VOUT +3V 100k Ω 100k Ω 100k Ω 100k Ω VIN = 1kHz AT 400mV p-p –0.1V –0.27V Figure 34. OP181 Configured as a Difference Amplifier Operating at VCM < 0 V |
Podobny numer części - OP281GRU |
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Podobny opis - OP281GRU |
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