REV. 0

–12–

OP184/OP284/OP484

Referring to the op amp noise model circuit configuration illus-

trated in Figure 45, the expression for an amplifier’s total

given by:

nOA

Hz

resistance,

power (1 Hz BW),

power (1 Hz BW),

(4kTR),

k = Boltzmann’s constant = 1.38

T = Ambient temperature of the circuit, in Kelvin, =

R

"NOISELESS"

R

"NOISELESS"

IDEAL

NOISELESS

OP AMP

Figure 45. Op Amp Noise Circuit Model Used to

Determine Total Circuit Equivalent Input Noise Voltage

and Noise Figure

As a design aid, Figure 46 illustrates the total equivalent input

noise of the OP284 and the total thermal noise of a resistor for

comparison. Note that for source resistance less than 1 k

Ω, the

equivalent input noise voltage of the OP284 is dominant.

100

100

1k

10k

100k

10

1

OP284 TOTAL

EQUIVALENT NOISE

RESISTOR THERMAL

NOISE ONLY

FREQUENCY = 1kHz

Figure 46. OP284 Total Noise vs. Source Resistance

Since circuit SNR is the critical parameter in the final analysis,

the noise behavior of a circuit is often expressed in terms of its

noise figure, NF. Noise figure is defined as the ratio of a

circuit’s output signal-to-noise to its input signal-to-noise. An

expression of a circuit’s NF in dB, and in terms of the opera-

tional amplifier’s voltage and current noise parameters defined

previously, is given by:

NF (dB)

= 10 log 1+

























where NF (dB) = Noise figure of the circuit, expressed in dB,

to amplifier,

Circuit noise figure is straightforward to calculate because the

signal level in the application is not required to determine it.

However, many designers using NF calculations as the basis for

achieving optimum SNR believe that low noise figure is equal to

low total noise. In fact, the opposite is true, as illustrated in

Figure 47. Here, the noise figure of the OP284 is expressed as a

function of the source resistance level. Note that the lowest

noise figure for the OP284 occurs at a source resistance level of

10 k

Ω. However, Figure 46 shows that this source resistance

level and the OP284 generate approximately 14 nV/

√Hz of total

equivalent circuit noise. Signal levels in the application would

invariably be increased to maximize circuit SNR—not an option

in low voltage, single supply applications.

6

100

1k

10k

100k

4

2

0

9

10

8

7

5

3

1

FREQUENCY = 1kHz

Figure 47. OP284 Noise Figure vs. Source Resistance

In single supply applications, therefore, it is recommended for

optimum circuit SNR to choose an operational amplifier with

the lowest equivalent input noise voltage and to choose source

resistance levels consistent in maintaining low total circuit noise.