REV. A

OP471

–7–

500

5k

1/4

OP471

50

50k

CHANNEL SEPARATION = 20 LOG

1/4

OP471

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 OP471 is a very low-noise quad op amp, exhibiting a typical

voltage noise of only 6.5

Hz @ 1 kHz. The low noise character-

istic of the OP471 is, 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 proportional to the square

root of the collector current. As a result, the outstanding voltage

noise performance of the OP471 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.

RS – SOURCE RESISTANCE –

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

RS – SOURCE RESISTANCE –

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

Figure 4 shows the relationship between total noise at 1 kHz

W the total noise is domi-

W,

total noise increases and is dominated by resistor noise rather

20 k

W, current noise of the OP471 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 pro-

portional 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 OP471, will provide lower total noise.