OP270GSZ2 View Datasheet(PDF) - Analog Devices
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OP270GSZ2 Datasheet PDF : 20 Pages
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OP270
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
80
60
40
20
0
0.01
0.1
1
10
100
FREQUENCY (Hz)
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
( ) ( ) ( ) eOUT = 101 enA 2 + enB 2
The OP270 is a monolithic device with two identical amplifiers.
Therefore, the noise voltage densities of the amplifiers match,
giving
( ) ( ) eOUT = 101 2en2 = 101 2en
R1
100Ω
R2
10kΩ
1/2
OP270
1/2
OP270
eOUT
TO SPECTRUM ANALYZER
eOUT (nV/√Hz) ≈ 101 (√2en)
VS = ±15V
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
( ) ⎜⎛ enOUT ⎟⎞2 − 40 nV / Hz 2
in = ⎝ G ⎠ RS
where:
G is a gain of 10,000.
RS = 100 kΩ source resistance.
R3
1.24kΩ
R1 R2
5Ω 100kΩ
OP270
DUT
R4
200Ω
OP27E
R5
8.06kΩ
enOUT
TO SPECTRUM ANALYZER
GAIN = 10,000
VS = ±15V
Figure 38. Current Noise Density Test Circuit
Rev. E | Page 14 of 20
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