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LT1469IS8-2-TRPBF Datasheet(PDF) 9 Page - Linear Technology |
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LT1469IS8-2-TRPBF Datasheet(HTML) 9 Page - Linear Technology |
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9 / 12 page  LT1469-2 9 14692f APPLICATIONS INFORMATION Gain of 2 Stable The LT1469-2 is a decompensated version of the LT1469. The DC precision performance is identical, but the internal compensation capacitors have been reduced to a point where the op amp needs a gain of 2 or greater in order to be stable. In general, for applications where the gain around the op amp is ≥ 2, the decompensated version should be used, because it will give the best AC performance. In applica- tions where the gain is <2, the unity-gain stable version should be used. The appropriate way to define the ‘gain’ is as the inverse of the feedback ratio from output to differential input, including all relevant parasitics. Moreover, as with all feedback loops, the stability of the loop depends on the value of that feedback ratio at frequencies where the total loop-gain would cross unity. Therefore, it is possible to have circuits in which the gain at DC is lower than the gain at high frequency, and these circuits can be stable even with a non unity-gain stable op amp. An example is many current-output DAC buffer applications. Layout and Passive Components The LT1469 requires attention to detail in board layout in order to maximize DC and AC performance. For best AC results (for example, fast settling time) use a ground plane, short lead lengths and RF quality bypass capacitors (0.01μF to 0.1μF) in parallel with low ESR bypass capaci- tors (1μF to 10μF tantalum). For best DC performance, use “star” grounding techniques, equalize input trace lengths and minimize leakage (e.g., 1.5GΩ of leakage between an input and a 15V supply will generate 10nA—equal to the maximum IB– specification). Board leakage can be minimized by encircling the input circuitry with a guard ring operated at a potential close to that of the inputs: for inverting configurations tie the ring to ground, in noninverting connections tie the ring to the inverting input (note the input capacitance will increase which may require a compensating capacitor as discussed below). Microvolt level error voltages can also be generated in the external circuitry. Thermocouple effects caused by temperature gradients across dissimilar metals at the contacts to the inputs can exceed the inherent drift of the amplifier. Air currents over device leads should be minimized, package leads should be short and the two input leads should be as close together as possible and maintained at the same temperature. The parallel combination of the feedback resistor and gain setting resistor on the inverting input can combine with the input capacitance to form a pole which can cause peak- ing or even oscillations. A feedback capacitor of value CF = RG • CIN/RF may be used to cancel the input pole and optimize dynamic performance. For applications where the DC noise gain is one, and a large feedback resistor is used, CF should be less than or equal to one half of CIN. An example would be a DAC I-to-V converter as shown on the front page of the data sheet where the DAC can have many tens of picofarads of output capacitance. Figure 1. Nulling Input Capacitance Figure 2. Input Stage Protection + – VOUT RF CF CIN 1/2 LT1469-2 VIN 14692 F01 RG Q1 +IN 14692 F02 R1 100Ω Q2 –IN R1 100Ω V– V+ |
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