Datasheet OP471 (Analog Devices) - 10

OP471 Noise Measurements - Peak-to-Peak Voltage Noise 100
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-
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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
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shown in the warm-up drift curve, the offset voltage typically changes 13 mV due to increasing chip temperature after
GAIN – dB 40
power-up. In the 10-second measurement interval, these temperature-induced effects can exceed tens-of-nanovolts.
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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”
0 0.01 0.1 1 10 100
to increase the observed noise.
FREQUENCY – Hz
4. The test time to measure 0.1 Hz to 10 Hz noise should not exceed Figure 8. 0.1 Hz to 10 Hz Peak-to-Peak Voltage Noise 10 seconds. As shown in the noise-tester frequency-response curve Test Circuit Frequency Response 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 Measurement - Noise Voltage Density
noise contribution from the frequency band below 0.1 Hz. The circuit of Figure 9 shows a quick and reliable method of measuring the noise voltage density of quad op amps. Each 5. A noise voltage density test is recommended when measuring individual amplifier is series connected and is in unity-gain, save noise on a large number of units. A 10 Hz noise voltage density the final amplifier which is in a noninverting gain of 101. Since measurement will correlate well with a 0.1 Hz to 10 Hz the ac noise voltages of each amplifier are uncorrelated, they peak-to-peak noise reading, since both results are determined add in rms fashion to yield: by the white noise and the location of the 1/f corner frequency. Ê 2 2 2 2 ˆ 6. Power should be supplied to the test circuit by well bypassed, e = 101 e + e + e + e OUT nA nB nC nD Ë ¯ low noise supplies, e.g, batteries. These will minimize output noise introduced through the amplifier supply pins. The OP471 is a monolithic device with four identical amplifiers. The noise voltage density of each individual amplifier will match, giving: ˆ e 101 4e 2 = Ê = 101 2e OUT n n Ë ¯ ( )
R1 R2 100 10k 1/4 e OP471 OUT 1/4 OP471 TO SPECTRUM ANALYZER 1/4 OP471 1/4 OP471 eOUT (nV Hz) = 101(2en) VS = 15V
Figure 9. Noise Voltage Density Test Circuit REV. A –9–