LT1115

8

1115fa

Therefore, to realize truly low noise performance it is

important to understand the interaction between voltage

Total Noise vs Source Resistance

The total input referred noise of an op amp is given by

the two inputs

As a numerical example, consider the total noise at 1kHz

of the gain of 1000 amplifier shown below.

largest term—as in the example above—and the LT1115’s

increased, current noise becomes important. At 1kHz,

component is larger than the resistor noise. The Total

Noise vs Matched Source Resistance plot in the Typical

Performance Characteristics section, illustrates the above

calculations.

–

+

100Ω

100k

100Ω

LT1115

LT1115 • AI01

The plot also shows that current noise is more dominant

at low frequencies, such as 10Hz. This is because resistor

noise is flat with frequency, while the 1/f corner of current

current noise term will exceed the resistor noise.

When the source resistance is unmatched, the Total Noise

vs Unmatched Source Resistance plot should be con-

sulted. Note that total noise is lower at source resistances

below 1kΩ because the resistor noise contribution is less.

When Rs > 1kΩ total noise is not improved, however. This

is because bias current cancellation is used to reduce

input bias current. The cancellation circuitry injects two

correlated current noise components into the two inputs.

With matched source resistors the injected current noise

creates a common-mode voltage noise and gets rejected

by the amplifier. With source resistance in one input only,

the cancellation noise is added to the amplifier’s inherent

noise.

In summary, the LT1115 is the optimum amplifier for

noise performance—provided that the source resistance

is kept low. The following table depicts which op amp

manufactured by Linear Technology should be used to

minimize noise—as the source resistance is increased

beyond the LT1115’s level of usefulness.

Best Op Amp for Lowest Total Noise vs Source Resistance

Note 1: Source resistance is defined as matched or unmatched, e.g.,

other.

SOURCE RESISTANCE

BEST OP AMP

(NOTE 1)

AT LOW FREQ (10Hz)

WIDEBAND (1kHz)

0 to 400Ω

LT1028/1115

LT1028/1115

400Ω to 4kΩ

LT1007/1037

LT1028/1115

4kΩ to 40kΩ

LT1001*

LT1007/1037

40kΩ to 500kΩ

LT1012*

LT1001*

500kΩ to 5MΩ

LT1012* or LT1055

LT1012*

> 5M

LT1055

LT1055

*These op amps are best utilized in applications requiring less bandwidth

than audio.

APPLICATIO S I FOR ATIO