LT1468-2

8

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APPLICATIONS INFORMATION

The LT1468-2 may be inserted directly into many operational

amplifier applications improving both DC and AC perfor-

mance, provided that the nulling circuitry is removed. The

suggested nulling circuit for the LT1468-2 is shown below.

Offset Nulling

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LT1468-2

1

5

100k

4

2.2μF

0.1μF

2.2μF

0.1μF

7 6

3

2

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and minimize leakage (i.e., 1.5GΩ of leakage between an

input and a 15V supply will generate 10nA—equal to the

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.

Make no connection to Pin 8. This pin is used for factory

trim of the inverting input current.

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 that can cause peaking or

even oscillations. A feedback capacitor of the value:

may be used to cancel the input pole and optimize dynamic

performance. For applications where the DC noise gain is

be a DAC I-to-V converter as shown on the front page of

this data sheet where the DAC can have many tens of pF

of output capacitance.

Gain of 2 Stable

The LT1468-2 is a decompensated version of the LT1468.

The precision DC 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 LT1468 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

Nulling Input Capacitance

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LT1468-2

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