OP270GSZ datasheet(13/21 Pages) AD

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OP270

Data Sheet

Rev. F | Page 12 of 20

APPLICATIONS INFORMATION

VOLTAGE AND CURRENT NOISE

The OP270 is a very low noise dual op amp, exhibiting a typical

voltage noise density of only 3.2 nV/√Hz at 1 kHz. Because the

voltage noise is inversely proportional to the square root of the

collector current, the exceptionally low noise characteristic of

the OP270 is achieved in part by operating the input transistors

at high collector currents. Current noise, however, is directly

proportional to the square root of the collector current. As a

result, the outstanding voltage noise density performance of the

OP270 is gained at the expense of current noise performance,

which is normal for low noise amplifiers.

To obtain the best noise performance in a circuit, it is vital to

understand the relationships among voltage noise (en), current

noise (in), and resistor noise (et).

TOTAL NOISE AND SOURCE RESISTANCE

The total noise of an op amp can be calculated by

)

(

)

(

)

(





where:

En is the total input-referred noise.

en is the op amp voltage noise.

in is the op amp current noise.

et is the source resistance thermal noise.

RS is the source resistance.

The total noise is referred to the input and at the output is

amplified by the circuit gain.

Figure 32 shows the relationship between total noise at 1 kHz

and source resistance. When RS is less than 1 kΩ, the total noise

is dominated by the voltage noise of the OP270. As RS rises

above 1 kΩ, total noise increases and is dominated by resistor

noise rather than by the voltage or current noise of the OP270.

When RS exceeds 20 kΩ, the current noise of the OP270

becomes the major contributor to total noise.

100

10k

100k

SOURCE RESISTANCE (Ω)

RESISTOR

NOISE ONLY

OP200

OP270

Figure 32. Total Noise vs. Source Resistance

(Including Resistor Noise) at 1 kHz

Figure 33 also shows the relationship between total noise and

source resistance, but at 10 Hz. Total noise increases more

quickly than shown in Figure 32 because current noise is

inversely proportional to the square root of frequency. In

Figure 33, the current noise of the OP270 dominates the total

noise when RS is greater than 5 kΩ.

Figure 32 and Figure 33 show that to reduce total noise, source

resistance must be kept to a minimum. In applications with a

high source resistance, the OP200, with lower current noise

than the OP270, can provide lower total noise.

100

10k

100k

SOURCE RESISTANCE (Ω)

RESISTOR

NOISE ONLY

OP200

OP270

Figure 33. Total Noise vs. Source Resistance

(Including Resistor Noise) at 10 Hz

Figure 34 shows peak-to-peak noise vs. source resistance over

the 0.1 Hz to 10 Hz range. At low values of RS, the voltage noise

of the OP270 is the major contributor to peak-to-peak noise,

with current noise becoming the major contributor as RS

increases. The crossover point between the OP270 and the

OP200 for peak-to-peak noise is at a source resistance of 17 kΩ.

100

10k

100k

SOURCE RESISTANCE (Ω)

RESISTOR

NOISE ONLY

OP200

OP270

Figure 34. Peak-to-Peak Noise (0.1 Hz to 10 Hz) vs. Source Resistance

(Including Resistor Noise)

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