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OP281GRU Folha de dados(PDF) 11 Page - Analog Devices |
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OP281GRU Folha de dados(HTML) 11 Page - Analog Devices |
11 / 16 page REV. B OP281/OP481 –11– Capacitive Loading Most low supply current amplifiers have difficulty driving capacitive loads due to the higher currents required from the output stage for such loads. Higher capacitance at the output will increase the amount of overshoot and ringing in the amplifier’s step response and could even affect the stability of the device. However, through careful design of the output stage and its high phase margin, the OPx81 family can tolerate some degree of capacitive loading. Figure 4 shows the step response of a single channel with a 10 nF capacitor connected at the output. Notice that the overshoot of the output does not exceed more than 10% with such a load, even with a supply voltage of only 3 V. 10 0% 100 90 Figure 4. Ringing and Overshoot of the Output of the Amplifier Micropower Reference Voltage Generator Many single-supply circuits are configured with the circuit biased to 1/2 of the supply voltage. In these cases, a false ground refer- ence can be created by using a voltage divider buffered by an amplifier. Figure 5 shows the schematic for such a circuit. The two 1 M W resistors generate the reference voltage while drawing only 1.5 mA of current from a 3 V supply. A capacitor connected from the inverting terminal to the output of the op amp provides compensation to allow for a bypass capacitor to be connected at the reference output. This bypass capacitor helps to establish an ac ground for the reference output. The entire reference generator draws less than 5 mA from a 3 V supply source. 1 8 4 2 3 OP281 10k 0.022 F VREF 1.5V TO 6V 1 F 1 F 1M 3V TO 12V 100 1M Figure 5. Single Channel Configured as a Micropower Bias Voltage Generator Window Comparator The extremely low power supply current demands of the OPx81 family make it ideal for use in long-life battery-powered applica- tions such as a monitoring system. Figure 6 shows a circuit that uses the OP281 as a window comparator. A1 R1 R2 3V OP281-A VIN 2k 5.1k 3V 3V VOUT Q1 5.1k VH D1 10k A2 R3 R4 3V 3V VL D2 OP281-B Figure 6. Using the OP281 as a Window Comparator The threshold limits for the window are set by VH and VL, pro- vided that VH > VL. The output of A1 will stay at the negative rail, in this case ground, as long as the input voltage is less than VH. Similarly, the output of A2 will stay at ground as long the input voltage is higher than VL. As long as VIN remains between VL and VH, the outputs of both op amps will be 0 V. With no current flowing in either D1 or D2, the base of Q1 will stay at ground, putting the transistor in cutoff and forcing VOUT to the positive supply rail. If the input voltage rises above VH, the output of A2 stays at ground, but the output of A1 will go to the positive rail, and D1 will conduct current. This creates a base voltage that will turn on Q1 and drive VOUT low. The same condition occurs if VIN falls below VL with A2’s output going high, and D2 conducting current. Therefore, VOUT will be high if the input voltage is between VL and VH, and VOUT will be low if the input voltage moves outside of that range. The R1 and R2 voltage divider sets the upper window voltage, and the R3 and R4 voltage divider sets the lower voltage for the window. For the window comparator to function properly, VH must be a greater voltage than VL. VH = R2 R1 + R2 VL = R4 R3 + R4 The 2 k W resistor connects the input voltage to the input terminals to the op amps. This protects the OP281 from possible excess current flowing into the input stages of the devices. D1 and D2 are small-signal switching diodes (1N4446 or equivalent), and Q1 is a 2N2222 or equivalent NPN transistor. |
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Descrição semelhante - OP281GRU |
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