LT6010

10

6010f

Preserving Input Precision

Preserving the input accuracy of the LT6010 requires that

the applications circuit and PC board layout do not intro-

duce errors comparable to or greater than the 20

µV typical

offset of the amplifier. Temperature differentials across

the input connections can generate thermocouple volt-

ages 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 (20pA typical) allow

high accuracy to be maintained with high impedance

sources and feedback resistors. The LT6010 low input

bias currents are obtained by a cancellation circuit on-

chip. The input bias currents are permanently trimmed at

wafer testing to a low level. Do not try to balance the input

resistances in each input lead; instead, keep the resistance

at either input as low as possible for maximum accuracy.

Leakage currents on the PC board can be higher than the

LT6010’s input bias current. For example, 10G

Ω of leak-

age between a 15V supply lead and an input lead will gen-

erate 1.5nA! Surround the input leads by a guard ring, driven

to the same potential as the input common mode, to avoid

excessive leakage in high impedance applications.

Input Protection

The LT6010 features on-chip back-to-back diodes be-

tween 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

allowed) 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 LT6010 output is able to swing nearly to each power

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

common mode range will cause the gain to drop to zero,

however no phase reversal will occur.

Total Input Noise

The LT6010 amplifier contributes negligible noise to the

system when driven by sensors (sources) with impedance

between 20k

Ω and 1MΩ. Throughout this range, total

If the source impedance is less than 20k

Ω, the input

voltage noise of the amplifier starts to contribute with a

minimum noise of 14nV/

√Hz for very low source imped-

ance. If the source impedance is more than 1M

Ω,theinput

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:

v

e

kTR

i R

n TOTAL

n

S

n S

()

()

=+

+

2

2

4

impedance at the input, including the source impedance.

APPLICATIO S I FOR ATIO