Datasheet LT1468-2 (Analog Devices) - 8

LT1468-2
applicaTions inForMaTion
The LT1468-2 may be inserted directly into many operational and minimize leakage (i.e., 1.5GΩ of leakage between an amplifier applications improving both DC and AC perfor- input and a 15V supply will generate 10nA—equal to the mance, provided that the nulling circuitry is removed. The maximum I – B specification.) suggested nulling circuit for the LT1468-2 is shown below. Board leakage can be minimized by encircling the input
Offset Nulling
circuitry with a guard ring operated at a potential close V+ to that of the inputs. For inverting configurations tie the 3 + 0.1µF 2.2µF ring to ground, in noninverting connections tie the ring 7 6 to the inverting input (note the input capacitance will LT1468-2 2 – 4 increase which may require a compensating capacitor as 5 discussed below.) 1 0.1µF 2.2µF 100k Microvolt level error voltages can also be generated in V– 14682 AI01 the external circuitry. Thermocouple effects caused by temperature gradients across dissimilar metals at the
Gain of 2 Stable
contacts to the inputs can exceed the inherent drift of The LT1468-2 is a decompensated version of the LT1468. the amplifier. Air currents over device leads should be The precision DC performance is identical, but the internal minimized, package leads should be short, and the two compensation capacitors have been reduced to a point input leads should be as close together as possible and where the op amp needs a gain of 2 or greater in order maintained at the same temperature. to be stable. Make no connection to Pin 8. This pin is used for factory In general, for applications where the gain around the op trim of the inverting input current. amp is ≥ 2, the decompensated version should be used, The parallel combination of the feedback resistor and gain because it will give the best AC performance. In applica- setting resistor on the inverting input can combine with the tions where the gain is < 2, the unity-gain stable version input capacitance to form a pole that can cause peaking should be used. or even oscillations. A feedback capacitor of the value: The appropriate way to define the ‘gain’ is as the inverse C of the feedback ratio from output to differential input, F = (RG)(CIN/RF) including all relevant parasitics. Moreover, as with all may be used to cancel the input pole and optimize dynamic feedback loops, the stability of the loop depends on the performance. For applications where the DC noise gain is value of that feedback ratio at frequencies where the total one, and a large feedback resistor is used, CF should be loop-gain would cross unity. Therefore, it is possible to less than or equal to one half of CIN. An example would have circuits in which the gain at DC is lower than the gain be a DAC I-to-V converter as shown on the front page of at high frequency, and these circuits can be stable even this data sheet where the DAC can have many tens of pF with a non unity-gain stable op amp. An example is many of output capacitance. current-output DAC buffer applications.
Nulling Input Capacitance Layout and Passive Components
RF The LT1468 requires attention to detail in board layout CF in order to maximize DC and AC performance. For best AC results (for example fast settling time) use a ground RG plane, short lead lengths, and RF-quality bypass capacitors – (0.01µF to 0.1µF) in parallel with low ESR bypass capaci- CIN LT1468-2 VOUT tors (1µF to 10µF tantalum). For best DC performance, use VIN + “star” grounding techniques, equalize input trace lengths 14682 AI02 14682fb 8 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Applications Information Simplified Schematic Package Description Revision History Typical Application Related Parts