REV. A

OP37

–12–

FREQUENCY – Hz

140

10

100

1k

10k

100k

1M

120

100

80

60

40

AC TRIM @ 10kHz

1k

UNBALANCED

BALANCED

Figure 4b. TBD

Comments on Noise

The OP37 is a very low-noise monolithic op amp. The outstanding

input voltage noise characteristics of the OP37 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

±40 nA and 35 nA

respectively at 25

°C. This is particularly important when the input

has a high source resistance. In addition, many audio amplifier

previous designs have made direct coupling difficult, if not

impossible, to use.

10

50

10k

5

500

1k

5k

1

100

50

100

50k

OP07

5534

OP27/37

REGISTER

NOISE ONLY

OP08/108

1

2

Figure 5. Noise vs. Resistance (Including Resistor Noise

@ 1000 Hz)

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 OP37’s noise advantage disappears when high

source-resistors are used. Figures 5, 6, and 7 compare OP-37

observed total noise with the noise performance of other devices

in different circuit applications.

Total noise = [( Voltage noise)2 + (current noise

RS)2 +

(resistor noise_]1/2

Figure 5 shows noise versus source resistance at 1000 Hz. The

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

the vertical scale by the square-root of the bandwidth.

100

50

10k

50

500

1k

5k

10

1k

500

100

50k

OP07

5534

OP27/37

REGISTER

NOISE ONLY

OP08/108

1

2

Figure 6. Peak-to-Peak Noise (0.1 Hz to 10 Hz) vs. Source

Resistance (Includes Resistor Noise)

Ω key the OP37’s low voltage noise is maintained.

Ω, total noise increases, but is dominated by the

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

beyond Rs of 20kil 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 OP37 and OP07 and OP08 noise occurs

in the 15 k

Ω to 40 kΩ region.

10

50

10k

5

500

1k

5k

1

100

50

100

50k

OP07

5534

OP27/37

REGISTER

NOISE ONLY

OP08/108

1

2

Figure 7. !0 Hz Noise vs. Source resistance (Inlcludes

Resistor Noise)

Figure 6 shows the 0.1 Hz to 10 Hz peak-to-peak noise. Here

the picture is less favorable; resistor noise is negligible, current

noise becomes important because it is inversely proportional to

the square-root of frequency. The crossover with the OP-07

occurs in the 3 k

Ω to 5 kΩ range depending on whether bal-

anced or unbalanced source resistors are used (at 3 k

Therefore, for low-frequency applications, the OP07 is better

than the OP27/37 when Rs > 3 k

Ω. The only exception is when

gain error is important. Figure 3 illustrates the 10 Hz noise. As

expected, the results are between the previous two figures.

For reference, typical source resistances of some signal sources

are listed in Table I.