REV. B

OP470

–9–

500

5k

1/4

OP470

50

50k

CHANNEL SEPARATION = 20 LOG

1/4

OP470

Figure 2. Channel Separation Test Circuit

7

6

5

1

2

3

+1V

+18V

4

–18V

11

A

+1V

B

D

14

13

12

–1V

C

8

9

10

–1V

Figure 3. Burn-In Circuit

APPLICATIONS INFORMATION

Voltage and Current Noise

The OP470 is a very low-noise quad op amp, exhibiting a typi-

cal voltage noise of only 3.2 nV

÷Hz @ 1 kHz. The exceptionally

low-noise characteristics of the OP470 are in part achieved by

operating the input transistors at high collector currents since

the voltage noise is inversely proportional to the square root of

the collector current. Current noise, however, is directly propor-

tional to the square root of the collector current. As a result, the

outstanding voltage noise performance of the OP470 is gained

at the expense of current noise performance, which is typical for

low noise amplifiers.

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

TOTAL NOISE AND SOURCE RESISTANCE

The total noise of an op amp can be calculated by:

Ee

i R

e

nn

n

S

t

=

22

2

where:

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

be amplified by the circuit gain. Figure 4 shows the relationship

W

the total noise is dominated by the voltage noise of the OP470.

W, total noise increases and is dominated

by resistor noise rather than by voltage or current noise of the

W, current noise of the OP470

becomes the major contributor to total noise.

Figure 5 also shows the relationship between total noise and

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

quickly than shown in Figure 4 because current noise is inversely

proportional to the square root of frequency. In Figure 5, current

W.

From Figures 4 and 5 it can be seen that to reduce total noise,

source resistance must be kept to a minimum. In applications

with a high source resistance, the OP400, with lower current

noise than the OP470, will provide lower total noise.

100

1

100

100k

10

10k

1k

OP11

OP400

OP471

OP470

RESISTOR

NOISE ONLY

Figure 4. Total Noise vs. Source Resistance (Including

Resistor Noise) at 1 kHz

100

1

100

100k

10

10k

1k

OP11

OP400

OP471

OP470

RESISTOR

NOISE ONLY

Figure 5. Total Noise vs. Source Resistance (Including

Resistor Noise) at 10 Hz