OP270GSZ2 datasheet(14/20 Pages) AD

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OP270

Rev. E | Page 14 of 20

NOISE MEASUREMENTS

Peak-to-Peak Voltage Noise

The circuit of Figure 35 is a test setup for measuring peak-to-

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

specification of the OP270 in the 0.1 Hz to 10 Hz range, the

following precautions must be observed:

•

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

As shown in the warm-up drift curve (see Figure 8), the

offset voltage typically changes 2 μV due to increasing chip

temperature after power-up. In the 10 sec measurement

interval, these temperature-induced effects can exceed tens

of nanovolts.

•

For similar reasons, the device has to be well shielded from

air currents. Shielding also minimizes thermocouple effects.

•

Sudden motion in the vicinity of the device can also feed

through to increase the observed noise.

•

The test time to measure noise of 0.1 Hz to 10 Hz should

not exceed 10 sec. As shown in the noise-tester frequency

response curve of Figure 36, the 0.1 Hz corner is defined by

only one pole. The test time of 10 sec acts as an additional

pole to eliminate noise contribution from the frequency

band below 0.1 Hz.

•

A noise voltage density test is recommended when measuring

noise on several units. A 10 Hz noise voltage density mea-

surement correlates well with a 0.1 Hz to 10 Hz peak-to-peak

noise reading because both results are determined by the

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

•

Power should be supplied to the test circuit by well bypassed

low noise supplies, such as batteries. Such supplies will min-

imize output noise introduced via the amplifier supply pins.

100

FREQUENCY (Hz)

0.01

0.1

100

Figure 36. 0.1 Hz to 10 Hz Peak-to-Peak Voltage Noise

Test Circuit Frequency Response

Noise Measurement—Noise Voltage Density

The circuit of Figure 37 shows a quick and reliable method for

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. Because the noise voltages of the amplifiers are

uncorrelated, they add in rms to yield

( ) ( )

(

101

OUT

The OP270 is a monolithic device with two identical amplifiers.

Therefore, the noise voltage densities of the amplifiers match,

giving

(

) ( )

OUT

101

eOUT (nV/√Hz) ≈ 101 (√2en)

VS = ±15V

TO SPECTRUM ANALYZER

eOUT

100Ω

10kΩ

1/2

OP270

1/2

OP270

Figure 37. Noise Voltage Density Test Circuit

Noise Measurement—Current Noise Density

The test circuit shown in Figure 38 can be used to measure current

noise density. The formula relating the voltage output to the current

noise density is

()

nOUT

−

⎟

⎠

⎞

⎜

⎝

⎛

where:

G is a gain of 10,000.

RS = 100 kΩ source resistance.

OP270

DUT

5Ω

100kΩ

1.24kΩ

OP27E

8.06kΩ

200Ω

enOUT

GAIN = 10,000

VS = ±15V

TO SPECTRUM ANALYZER

Figure 38. Current Noise Density Test Circuit

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