REV. C

OP270

–8–

1/2

OP270

5k

500

5k

50

1/2

OP270

CHANNEL SEPARATION = 20 log

V

1

V

2

/1000

Figure 1. Channel Separation Test Circuit

1/2

OP270

1/2

OP270

7

1

8

5

6

3

2

100k

100k

200k

4

+18V

–18V

Figure 2. Burn-In Circuit

APPLICATIONS INFORMATION

VOLTAGE AND CURRENT NOISE

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

voltage noise of only 3.2 nV/

÷÷÷÷÷Hz @ 1 kHz. The exceptionally

low noise characteristic of the OP270 is achieved in part 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 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

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 3 shows the relationship between total noise at 1 kHz

W the total noise is dominated

W, total

noise increases and is dominated by resistor noise rather than by

20 k

W, current noise of the OP270 becomes the major contributor

to total noise.

100

100

1k

10k

100k

1

10

OP270

RESISTOR

NOISE ONLY

OP200

Figure 3. Total Noise vs. Source Resistance

(Including Resistor Noise) at 1 kHz

Figure 4 also shows the relationship between total noise and

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

quickly than shown in Figure 3 because current noise is inversely

proportional to the square root of frequency. In Figure 4, current

W.

Figures 3 and 4 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,

will provide lower total noise.