REV. B

OP470

–11–

4. The test time to measure 0.1 Hz to 10 Hz noise should not ex-

ceed 10 seconds. As shown in the noise-tester frequency-response

curve of Figure 8, the 0.1 Hz corner is defined by only one pole.

The test time of 10 seconds acts as an additional pole to elimi-

nate noise contribution from the frequency band below 0.1 Hz.

5. A noise-voltage-density test is recommended when measuring

noise on a large number of units. A 10 Hz noise voltage-density

measurement will correlate well with a 0.1 Hz to 10 Hz

peak-to-peak noise reading, since both results are determined

by the white noise and the location of the 1/f corner frequency.

6. Power should be supplied to the test circuit by well bypassed

low noise supplies, e.g. batteries. These will minimize output

noise introduced via the amplifier supply pins.

FREQUENCY – Hz

100

0.01

80

60

40

20

0

0.1

1

10

100

Figure 8. 0.1 Hz to 10 Hz Peak-to-Peak Voltage Noise Test

Circuit Frequency Response

NOISE MEASUREMENT—NOISE VOLTAGE DENSITY

The circuit of Figure 9 shows a quick and reliable method of

measuring the noise voltage density of quad op amps. Each

individual amplifier is series-connected and is in unity-gain, save

the final amplifier which is in a noninverting gain of 101. Since

the ac noise voltages of each amplifier are uncorrelated, they

add in rms fashion to yield:

e= 101

e

+ e

e

e

OUT

nA

nB

nC

nD

22

2

2

++

Ê

Ë

ˆ

¯

The OP470 is a monolithic device with four identical amplifiers.

The noise voltage density of each individual amplifier will match,

giving:

e

101

4e

= 101 2e

OUT

n

n

2

=

Ê

Ë

ˆ

¯

()

NOISE MEASUREMENT—CURRENT NOISE DENSITY

The test circuit shown in Figure 10 can be used to measure

current noise density. The formula relating the voltage output to

current noise density is:

i

G

40nV /

Hz

R

n

nOUT

S

=

Ê

ËÁ

ˆ

¯˜

-

()

2

2

where:

G = gain of 10000

W source resistance

R2

100k

R3

1.24k

OP470

DUT

R5

8.06k

OP27E

R4

200

SPECTRUM ANALYZER

R1

5

GAIN = 50,000

Figure 10. Current Noise Density Test Circuit

R2

10k

1/4

OP470

1/4

OP470

1/4

OP470

1/4

OP470

R1

100

TO SPECTRUM ANALYZER

Figure 9. Noise Voltage Density Test Circuit