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LF400ACH Datenblatt(PDF) 9 Page - National Semiconductor (TI) |
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LF400ACH Datenblatt(HTML) 9 Page - National Semiconductor (TI) |
9 / 12 page Typical Performance Characteristics (Continued) CommonMode Voltage Transfer Characteristic TLH9414 – 19 TLH9414 – 20 Application Hints The LF400 is a high-speed low input bias current Bi-FET operational amplifier capable of settling to 001% of a 10V output swing in less than 400 ns The rugged JFET inputs allow differential input voltages as high as 32V without a large increase in input current However the inputs should never be driven to voltages lower than the negative supply as this can result in input currents large enough to damage the device To prevent this from occurring when power is first applied always turn the positive and negative power supplies on simultaneously or turn the negative supply on first Exceeding the common-mode input range will not damage the device as long as the Absolute Maximum Ratings are not violated but it will result in a high output voltage Latch- ing will not occur however and when the offending signal is removed the LF400 will recover quickly The nominal power supply voltage is g15V but the LF400 will operate satisfactorily from g10V to g16V The LF400 is functional down to g5V but performance will be degraded (See Typical Performance curves) Settling Time Considerations The settling performance of any high-speed operational am- plifier is highly dependent on the external components and circuit board layout Capacitance between the amplifier summing junction and ground affects the closed-loop trans- fer function and should be minimized The compensation capacitor CC between the output and the inverting input should be carefully chosen to counteract the effect of the input capacitance Since input capacitance is made up of several stray capacitances that are difficult to predict the compensation capacitor will generally have to be deter- mined empirically for best settling time A good starting point is around 10 pF for AV eb1 Settling time may be verified using a circuit similar to the one in Figure 1 The LF400 is connected for inverting opera- tion and the output voltage is summed with the input volt- age step When the LF400’s output voltage is equal to the input voltage the voltage on the gate of Q1 will be zero Any voltage appearing at this point will represent an error The FET source follower output is observed on an oscilloscope and the settling time is equal to the time required for the error signal displayed on the oscilloscope to decay to less than one-half the necessary accuracy (see oscilloscope photos of ‘‘Settling TimePositive Output Swing’’ and ‘‘Set- tling TimeNegative Output Swing’’) For a 10V input sig- nal settling time to 001% (1 mV) will occur when the dis- played error is less than mV Since settling time is strong- ly dependent on slew rate settling will be faster for smaller signal swings The LF400’s inverting slew rate is faster than its non-inverting slew rate so settling will be faster for in- verting applications as well It is important to note that the oscilloscope input amplifier will be overdriven during a settling time measurement so the oscilloscope must be capable of recovering from over- drive very quickly Very few oscilloscopes are suitable for this sort of measurement The signal generator used for set- 9 |
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