Data Sheet

OP27

Rev. H | Page 15 of 21

COMMENTS ON NOISE

The OP27 is a very low noise, monolithic op amp. The out-

standing input voltage noise characteristics of the OP27

are achieved mainly by operating the input stage at a high

quiescent current. The input bias and offset currents, which

would normally increase, are held to reasonable values by the

input bias current cancellation circuit. The OP27A/OP27E has

IB and IOS of only ±40 nA and 35 nA at 25°C respectively. This

is particularly important when the input has a high source

resistance. In addition, many audio amplifier designers prefer

to use direct coupling. The high IB, VOS, and TCVOS of previous

designs have made direct coupling difficult, if not impossible,

to use.

Voltage noise is inversely proportional to the square root of bias

current, but current noise is proportional to the square root of

bias current. The noise advantage of the OP27 disappears when

high source resistors are used. Figure 38, Figure 39, Figure 40

compare the observed total noise of the OP27 with the noise

performance of other devices in different circuit applications.

2

/

1

2

2

2

)

(

)

(

)

(

Noise

Resistor

R

Noise

Current

Noise

Voltage

Noise

Total

S

Figure 38 shows noise vs. source resistance at 1000 Hz. The

same plot applies to wideband noise. To use this plot, multiply

the vertical scale by the square root of the bandwidth.

RS—SOURCE RESISTANCE ( )

10

50

10k

5

500

1k

5k

1

100

50

100

50k

RS1

RS2

OP07

5534

OP27/37

REGISTER

NOISE ONLY

OP08/108

1

2

1 RS UNMATCHED

e.g. RS = RS1 = 10k , R S2 = 0

2 RS MATCHED

e.g. RS = 10k , R S1 = RS2 = 5k

Figure 38. Noise vs. Source Resistance (Including Resistor Noise) at 1000 Hz

At RS < 1 kΩ, the low voltage noise of the OP27 is maintained.

With RS < 1 kΩ, total noise increases but is dominated by the

resistor noise rather than current or voltage noise. lt is only

beyond RS of 20 kΩ that current noise starts to dominate. The

argument can be made that current noise is not important for

applications with low-to-moderate source resistances. The

crossover between the OP27 and OP07 noise occurs in the 15 kΩ

to 40 kΩ region.

Figure 39 shows the 0.1 Hz to 10 Hz p-p noise. Here the picture

is less favorable; resistor noise is negligible and current noise

becomes important because it is inversely proportional to the

square root of frequency. The crossover with the OP07 occurs

in the 3 kΩ to 5 kΩ range depending on whether balanced or

unbalanced source resistors are used (at 3 kΩ the IB and IOS

error also can be 3× the VOS spec).

RS—SOURCE RESISTANCE ( )

100

50

10k

50

500

1k

5k

10

1k

500

100

50k

RS1

RS2

1 RS UNMATCHED

e.g. RS = RS1 = 10k , R S2 = 0

2 RS MATCHED

e.g. RS = 10k , R S1 = RS2 = 5k

OP07

5534

OP27/37

REGISTER

NOISE ONLY

OP08/108

1

2

Figure 39. Peak-to-Peak Noise (0.1 Hz to 10 Hz) as Source Resistance

(Includes Resistor Noise)

For low frequency applications, the OP07 is better than the

OP27/OP37 when RS > 3 kΩ. The only exception is when gain

error is important.

Figure 40 illustrates the 10 Hz noise. As expected, the results are

between the previous two figures.

10

50

10k

5

500

1k

5k

1

100

50

100

50k

OP07

5534

OP27/37

REGISTER

NOISE ONLY

OP08/108

RS1

RS2

1 RS UNMATCHED

e.g. RS = RS1 = 10k , R S2 = 0

2 RS MATCHED

e.g. RS = 10k , R S1 = RS2 = 5k

1

2

RS—SOURCE RESISTANCE ( )

Figure 40. 10 Hz Noise vs. Source Resistance (Includes Resistor Noise)

Audio Applications