REV. A

OP471

–9–

Noise Measurements - Peak-to-Peak Voltage Noise

The circuit of Figure 7 is a test setup for measuring peak-to-peak

voltage noise. To measure the 500 nV peak-to-peak noise speci-

fication of the OP471 in the 0.1 Hz to 10 Hz range, the following

precautions must be observed:

1. The device must be warmed up for at least five minutes. As

shown in the warm-up drift curve, the offset voltage typically

changes 13

mV due to increasing chip temperature after

power-up. In the 10-second measurement interval, these

temperature-induced effects can exceed tens-of-nanovolts.

2. For similar reasons, the device must be well-shielded from

air currents. Shielding also minimizes thermocouple effects.

3. Sudden motion in the vicinity of the device can also “feedthrough”

to increase the observed noise.

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

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 eliminate

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 through 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 OP471 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

¯

()

R2

10k

1/4

OP471

1/4

OP471

1/4

OP471

1/4

OP471

R1

100

TO SPECTRUM ANALYZER

Figure 9. Noise Voltage Density Test Circuit