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LT1678IS8 Datasheet(PDF) 11 Page - Linear Technology |
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LT1678IS8 Datasheet(HTML) 11 Page - Linear Technology |
11 / 16 page LT1678/LT1679 11 sn16789 16789fs Rail-to-Rail Operation To take full advantage of an input range that can exceed the supply, the LT1678/LT1679 are designed to eliminate phase reversal. Referring to the photographs shown in Figure 1, the LT1678/LT1679 are operating in the fol- lower mode (AV = +1) at a single 3V supply. The output of the LT1678/LT1679 clips cleanly and recovers with no phase reversal. This has the benefit of preventing lock-up in servo systems and minimizing distortion components. input and a current, limited only by the output short-circuit protection, will be drawn by the signal generator. With RF ≥ 500Ω, the output is capable of handling the current requirements (IL ≤ 20mA at 10V) and the amplifier stays in its active mode and a smooth transition will occur. As with all operational amplifiers when RF > 2k, a pole will be created with RF and the amplifier’s input capacitance, creating additional phase shift and reducing the phase margin. A small capacitor (20pF to 50pF) in parallel with RF will eliminate this problem. APPLICATIO S I FOR ATIO Figure 1. Voltage Follower with Input Exceeding the Supply Voltage (VS = 3V) Input = –0.5V to 3.5V LT1678 Output Figure 2. Pulsed Operation Noise Testing The 0.1Hz to 10Hz peak-to-peak noise of the LT1678/ LT1679 are measured in the test circuit shown (Figure 3). The frequency response of this noise tester (Figure 4) indicates that the 0.1Hz corner is defined by only one zero. The test time to measure 0.1Hz to 10Hz noise should not exceed ten seconds, as this time limit acts as an additional zero to eliminate noise contributions from the frequency band below 0.1Hz. Measuring the typical 90nV peak-to-peak noise perfor- mance of the LT1678/LT1679 requires special test pre- cautions: 1. The device should be warmed up for at least five minutes. As the op amp warms up, its offset voltage changes typically 3µV due to its chip temperature increasing 10°C to 20°C from the moment the power supplies are turned on. In the ten-second measurement interval these temperature-induced effects can easily exceed tens of nanovolts. 2. For similar reasons, the device must be well shielded from air currents to eliminate the possibility of thermoelectric effects in excess of a few nanovolts, which would invalidate the measurements. 16789 F01a 3 2 1 –0.5 0 50µs/DIV 16789 F01b 3 2 1 –0.5 0 50µs/DIV 16789 F02 LT1678 RF OUTPUT 6V/µs Unity-Gain Buffer Application When RF ≤ 100Ω and the input is driven with a fast, large- signal pulse (>1V), the output waveform will look as shown in the pulsed operation diagram (Figure 2). During the fast feedthrough-like portion of the output, the input protection diodes effectively short the output to the |
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