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OP279GRU Datasheet(PDF) 8 Page - Analog Devices |
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OP279GRU Datasheet(HTML) 8 Page - Analog Devices |
8 / 16 page OP179/OP279 –8– REV. G 7 2 0 0.01 0.100 10 1 5 1 3 4 6 CAPACITIVE LOAD – nF VS 5V RL 1k TA 25 C Figure 4. OP179/OP279 Bandwidth vs. Capacitive Load 1/2 OP279 RS 20V CS 1 F CL 10nF 5V VIN 100mV p-p VOUT Figure 5. Snubber Network Compensates for Capacitive Load The first step is to determine the value of the resistor, RS. A good starting value is 100 Ω (typically, the optimum value will be less than 100 Ω). This value is reduced until the small-signal transient response is optimized. Next, CS is determined—10 µF is a good starting point. This value is reduced to the smallest value for acceptable performance (typically, 1 µF). For the case of a 10 nF load capacitor on the OP179/OP279, the optimal snubber network is a 20 Ω in series with 1 µF. The benefit is immediately apparent as seen in the scope photo in Figure 6. The top trace was taken with a 10 nF load and the bottom trace with the 20 Ω, 1 µF snubber network in place. The amount of overshot and ringing is dramatically reduced. Table I illustrates a few sample snubber networks for large load capacitors. 90 100 10nF LOAD ONLY SNUBBER IN CIRCUIT 10 0% 50mV 2 s Figure 6. Overshoot and Ringing Are Reduced by Adding a “Snubber” Network in Parallel with the 10 nF Load Table I. Snubber Networks for Large Capacitive Loads Load Capacitance (CL) Snubber Network (RS, CS) 10 nF 20 Ω, 1 µF 100 nF 5 Ω, 10 µF 1 µF0 Ω, 10 µF Overload Recovery Time Overload, or overdrive, recovery time of an operational amplifier is the time required for the output voltage to recover to its linear region from a saturated condition. This recovery time is impor- tant in applications where the amplifier must recover after a large transient event. The circuit in Figure 7 was used to evaluate the OP179/OP279’s overload recovery time. The OP179/OP279 takes approximately 1 µs to recover from positive saturation and approximately 1.2 µs to recover from negative saturation. 1/2 OP279 RL 499 +5V VOUT –5V R3 10k R2 1k R1 909 2V p-p @ 100Hz Figure 7. Overload Recovery Time Test Circuit Output Transient Current Recovery In many applications, operational amplifiers are used to provide moderate levels of output current to drive the inputs of ADCs, small motors, transmission lines and current sources. It is in these applications that operational amplifiers must recover quickly to step changes in the load current while maintaining steady-state load current levels. Because of its high output current capability and low closed-loop output impedance, the OP179/OP279 is an excellent choice for these types of applications. For example, when sourcing or sinking a 25 mA steady-state load current, the OP179/OP279 exhibits a recovery time of less than 500 ns to 0.1% for a 10 mA (i.e., 25 mA to 35 mA and 35 mA to 25 mA) step change in load current. A Precision Negative Voltage Reference In many data acquisition applications, the need for a precision negative reference is required. In general, any positive voltage reference can be converted into a negative voltage reference through the use of an operational amplifier and a pair of matched resistors in an inverting configuration. The disadvantage to that approach is that the largest single source of error in the circuit is the relative matching of the resistors used. The circuit illustrated in Figure 8 avoids the need for tightly matched resistors with the use of an active integrator circuit. In this circuit, the output of the voltage reference provides the input drive for the integrator. The integrator, to maintain circuit equilibrium, adjusts its output to establish the proper relation- ship between the reference’s VOUT and GND. Thus, various negative output voltages can be chosen simply by substituting for the appropriate reference IC (see table). To speed up the |
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