AN-940 View Datasheet(PDF) - Analog Devices

Part Name

Description

Manufacturer

AN-940

Low Noise Amplifier Selection Guide for Optimal Noise Performance

Analog Devices 

AN-940

INPUT-REFERRED VOLTAGE NOISE

Input-referred voltage noise (en) is typically viewed as a noise

voltage source.

Voltage noise is the noise specification that is usually empha-

sized; however, if input impedance levels are high, current noise

is often the limiting factor in system noise performance. It is

analogous to offsets, where the input offset voltage often bears

the blame for output offset, when in reality the bias current

causes the output offset where input impedances are high.

Note the following points about input-referred voltage noise:

 Op amp voltage noise can be lower than 1 nV/√Hz for the

highest performance amplifiers.

 Although bipolar op amps traditionally have less voltage

noise than FET op amps, they also have substantially

greater current noise.

 Bipolar amplifier noise characteristics are dependent on

the quiescent current.

 Present day FET op amps are capable of obtaining both low

current noise and voltage noise similar to bipolar amplifier

performance, though not as low as the best bipolar input

amplifiers.

Application Note

INPUT-REFERRED CURRENT NOISE

Input-referred current noise (in) is typically seen as two noise

current sources pumping currents through the two differential

input terminals.

Shot noise (sometimes called Schottky noise) is current noise

due to random distribution of charge carriers in the current flow

through a potential barrier, such as a PN junction. The shot

noise current, in, is obtained from the formula

in  2I BqB

(4)

where:

IB is the bias current in ampere (A).

q is the electron charge in coulomb (1.6 × 10−19 C).

B is the bandwidth in hertz (Hz).

The current noise of a simple bipolar and JFET op amp is typically

within 1 dB or 2 dB of the shot noise of the input bias current.

This specification is not always listed on data sheets.

Note the following points regarding input-referred noise:

 The current noise of typical bipolar transistor op amps,

such as the OP27, is about 400 fA/√Hz, where IB is 10 nA,

and does not vary much with temperature except for bias,

current-compensated amplifiers.

 The current noise of JFET input op amps (such as the

AD8610: 5 fA/√Hz at IB = 10 pA) while lower, doubles

for every 20°C chip temperature increase, because JFET

op amp bias currents double for every 10°C increase.

 Traditional voltage feedback op amps with balanced inputs

usually have equal (correlated and uncorrelated) current

noise on both their inverting and noninverting inputs.

 Many amplifiers, especially those amps with input bias

current cancellation circuits, have considerably larger

correlated than uncorrelated noise components. Overall,

noise can be improved by adding an impedance-balancing

resistor (matching impedances on both positive and

negative input pins).

Rev. D | Page 4 of 12

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