LT6014CDD datasheet(13/16 Pages) LINER

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LT6013/LT6014

60134fa

APPLICATIO S I FOR ATIO

Preserving Input Precision

Preserving the input accuracy of the LT6013 and LT6014

requires that the applications circuit and PC board layout

do not introduce errors comparable to or greater than the

µVtypicaloffsetoftheamplifiers.Temperaturedifferen-

tials across the input connections can generate thermo-

couple voltages of 10’s of microvolts so the connections

to the input leads should be short, close together and away

from heat dissipating components. Air currents across the

board can also generate temperature differentials.

The extremely low input bias currents allow high accuracy

to be maintained with high impedance sources and feed-

back resistors. The LT6013 and LT6014 low input bias cur-

rents are obtained by a cancellation circuit on-chip. This

causes the resulting IB+ and IB– to be uncorrelated, as

implied by the IOS specification being comparable to IB. Do

not try to balance the input resistances in each input lead;

instead keep the resistance at either input as low as pos-

sible for maximum accuracy.

Leakage currents on the PC board can be higher than the

input bias current. For example, 10G

Ωofleakagebetween

a 15V supply lead and an input lead will generate 1.5nA!

Surround the input leads with a guard ring driven to the

same potential as the input common mode to avoid exces-

sive leakage in high impedance applications.

Input Protection

The LT6013/LT6014 features on-chip back-to-back diodes

between the input devices, along with 500

Ω resistors in

series with either input. This internal protection limits the

input current to approximately 10mA (the maximum al-

lowed) for a 10V differential input voltage. Use additional

external series resistors to limit the input current to 10mA

in applications where differential inputs of more than 10V

are expected. For example, a 1k resistor in series with each

input provides protection against 30V differential voltage.

Input Common Mode Range

The LT6013/LT6014 output is able to swing close to each

power supply rail (rail-to-rail out), but the input stage is

limited to operating between V–+ 1V and V+– 1.2V. Exceed-

ing this common mode range will cause the gain to drop

to zero; however, no phase reversal will occur.

Total Input Noise

The LT6013 and LT6014 amplifiers contribute negligible

noise to the system when driven by sensors (sources) with

impedance between 10k

Ω and 1MΩ. Throughout this

range, total input noise is dominated by the 4kTRS noise

of the source. If the source impedance is less than 10k

Ω,

the input voltage noise of the amplifier starts to contribute

with a minimum noise of 9.5nV/

√Hzforverylowsourceim-

pedance. If the source impedance is more than 1M

Ω, the

input current noise of the amplifier, multiplied by this high

impedance, starts to contribute and eventually dominate.

Total input noise spectral density can be calculated as:

kTR

i R

n TOTAL

n S

()

where en = 9.5nV/√Hz , in = 0.15pA/√Hz and RS is the total

impedance at the input, including the source impedance.

Capacitive Loads

The LT6013 and LT6014 can drive capacitive loads up to

500pF at a gain of 5. The capacitive load driving capability

increases as the amplifier is used in higher gain configu-

rations. A small series resistance between the output and

the load further increases the amount of capacitance that

the amplifier can drive.

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