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OP284EP Datasheet(PDF) 11 Page - Analog Devices |
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OP284EP Datasheet(HTML) 11 Page - Analog Devices |
11 / 20 page REV. 0 –11– OP184/OP284/OP484 Thus, the saturation voltage of the output transistors sets the limit on the OP284’s maximum output voltage swing. Output short circuit current limiting is determined by the maximum signal current into the base of Q1 from the second gain stage. Under output short circuit conditions, this input current level is approximately 100 µA. With transistor current gains around 200, the short circuit current limits are typically 20 mA. The output stage also exhibits voltage gain. This is accomplished by use of common-emitter amplifiers, and as a result the voltage gain of the output stage (thus, the open-loop gain of the device) exhibits a dependence to the total load resistance at the output of the OP284. Input Overvoltage Protection As with any semiconductor device, if conditions exist where the applied input voltages to the device exceed either supply voltage, then the device’s input overvoltage I-V characteristic must be considered. When an overvoltage occurs, the amplifier could be damaged depending on the magnitude of the applied voltage and the magnitude of the fault current. Figure 43 illustrates the over voltage I-V characteristic of the OP284. This graph was generated with the supply pins connected to GND and a curve tracer’s collector output drive connected to the input. 5 4 3 2 1 0 –1 –2 –3 –4 –5 – 5 – 4 – 3 – 2 –1 0 12345 INPUT VOLTAGE – Volts Figure 43. Input Overvoltage I-V Characteristics of the OP284 As shown in the figure, internal p-n junctions to the OP284 en- ergize and permit current flow from the inputs to the supplies when the input is 1.8 V more positive and 0.6 V more negative than the respective supply rails. As illustrated in the simplified equivalent circuit shown in Figure 41, the OP284 does not have any internal current limiting resistors; thus, fault currents can quickly rise to damaging levels. This input current is not inherently damaging to the device, provided that it is limited to 5 mA or less. For the OP284, once the input exceeds the negative supply by 0.6 V, the input cur- rent quickly exceeds 5 mA. If this condition continues to exist, an external series resistor should be added at the expense of ad- ditional thermal noise. Figure 44 illustrates a typical noninvert- ing configuration for an overvoltage protected amplifier where the series resistance, RS, is chosen such that: RS = VIN (MAX ) –VSUPPLY 5 mA R1 R2 VIN VOUT 1/2 OP284 Figure 44. A Resistance in Series with an Input Limits Overvoltage Currents to Safe Values For example, a 1 k Ω resistor will protect the OP284 against input signals up to 5 V above and below the supplies. For other configurations where both inputs are used, then each input should be protected against abuse with a series resistor. Again, in order to ensure optimum dc and ac performance, it is recom- mended to balance source impedance levels. For more informa- tion on the general overvoltage characteristics of amplifiers, please refer to the 1993 System Applications Guide, Section 1, pages 56-69. This reference textbook is available from the Ana- log Devices Literature Center. Output Phase Reversal Some operational amplifiers designed for single-supply opera- tion exhibit an output voltage phase reversal when their inputs are driven beyond their useful common-mode range. Typically for single-supply bipolar op amps, the negative supply deter- mines the lower limit of their common-mode range. With these devices, external clamping diodes, with the anode connected to ground and the cathode to the inputs, prevent input signal ex- cursions from exceeding the device’s negative supply (i.e., GND), preventing a condition that could cause the output volt- age to change phase. JFET-input amplifiers may also exhibit phase reversal, and, if so, a series input resistor is usually re- quired to prevent it. The OP284 is free from reasonable input voltage range restric- tions provided that the input voltages no greater than the supply voltages are applied. Although the device’s output will not change phase, large currents can flow through the input protec- tion diodes, as was shown in Figure 43. Therefore, the technique recommended in the Input Overvoltage Protection section should be applied in those applications where the likelihood of input voltages exceeding the supply voltages is high. Designing Low Noise Circuits in Single Supply Applications In single supply applications, devices like the OP284 extend the dynamic range of the application through the use of rail-to-rail operation. In fact, the OP284 family is the first of its kind to combine single supply, rail-to-rail operation and low noise in one device. It is the first device in the industry to exhibit an input noise voltage spectral density of less than 4 nV/ √Hz at 1 kHz. It was also designed specifically for low-noise, single- supply applications, and as such some discussion on circuit noise concepts in single supply applications is appropriate. |
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