Datasheet LT1469-2 (Analog Devices) - 9

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