REV. C

OP270

–10–

NOISE MEASUREMENTS

Peak-to-Peak Voltage Noise

The circuit of Figure 6 is a test setup for measuring peak-to-peak

voltage noise. To measure the 200 nV peak-to-peak noise specifica-

tion of the OP270 in the 0.1 Hz to 10 Hz range, the following

precautions must be observed:

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

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

changes 2

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 has to be well shielded from

air currents. Shielding also minimizes thermocouple effects.

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

through” to increase the observed noise.

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

exceed 10 seconds. As shown in the noise-tester frequency

response curve of Figure 7, 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.

FREQUENCY (Hz)

100

0.01

0.1

1.0

10

100

80

60

40

20

0

Figure 7. 0.1 Hz to 10 Hz Peak-to-Peak Voltage

Noise Test Circuit Frequency Response

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. They will minimize output

noise introduced via the amplifier supply pins.

Noise Measurement — Noise Voltage Density

The circuit of Figure 8 shows a quick and reliable method of

measuring the noise voltage density of dual op amps. The first

amplifier is in unity-gain, with the final amplifier in a noninverting

gain of 101. As noise voltages of each amplifier are uncorrelated,

they add in rms fashion to yield:

ee

e

OUT

nA

2

nB

2

=

Ê

ËÁ

ˆ

¯˜

101

The OP270 is a monolithic device with two identical amplifi-

ers. The noise voltage density of each individual amplifier will

match, giving:

ee

e

OUT

n

2

n

=

Ê

ËÁ

ˆ

¯˜ =

()

101

2

101

2

1/2

OP270

–

+

R1

100

1/2

OP270

–

+

R2

10k

TO SPECTRUM ANALYZER

Figure 8. Noise Voltage Density Test Circuit

OP270

DUT

–

+

R3

1.24k

R2

100k

R1

5

OP27E

–

+

R5

8.06k

TO SPECTRUM ANALYZER

R4

200

GAIN = 10,000

Figure 9. Current Noise Density Test Circuit

Noise Measurement — Current Noise Density

The test circuit shown in Figure 9 can be used to measure cur-

rent noise density. The formula relating the voltage output to

current noise density is:

i

e

G

nV

Hz

R

n

nOUT

S

=

Ê

ËÁ

ˆ

¯˜

-

()

2

2

40

/

where:

G = gain of 10,000

W source resistance