LT6235IGN datasheet(18/24 Pages) LINER

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LT6233/LT6233-10/

LT6234/LT6235

623345f

APPLICATIO S I FOR ATIO

With RS + RG||RFB = 225Ω the total noise of the

amplifier is:

eN=√(1.9nV)2+(1.9nV)2 = 2.69nV/√Hz

Below this resistance value, the amplifier dominates the

noise, but in the region between 225

Ω and about 30k, the

noise is dominated by the resistor thermal noise. As the

total resistance is further increased beyond 30k, the

amplifier noise current multiplied by the total resistance

eventually dominates the noise.

The product of eN • √ISUPPLY is an interesting way to gauge

low noise amplifiers. Most low noise amplifiers with low

eN have high ISUPPLY current. In applications that require

low noise voltage with the lowest possible supply current,

this product can prove to be enlightening. The LT6233/

LT6234/LT6235 have an eN • √ISUPPLY product of only 2.1

per amplifier, yet it is common to see amplifiers with

similar noise specifications to have eN • √ISUPPLY as high

as 13.5.

For a complete discussion of amplifier noise, see the

LT1028 data sheet.

Enable Pin

The LT6233 and LT6233-10 include an ENABLE pin that

shuts down the amplifier to 10

µA maximum supply cur-

rent. The ENABLE pin must be driven high to within 0.35V

of V+ to shut down the supply current. This can be

accomplished with simple gate logic; however care must

be taken if the logic and the LT6233 operate from different

supplies. If this is the case, then open drain logic can be

used with a pull-up resistor to ensure that the amplifier

remains off. See Typical Characteristic Curves.

The output leakage current when disabled is very low;

however, current can flow into the input protection diodes

D1 and D2 if the output voltage exceeds the input voltage

by a diode drop.

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