OP471 Datasheet, PDF (9/16 Pages) Analog Devices – High Speed, Low Noise Quad Operational Amplifier

OP471
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.
100
80
60
40
20
0
0.01
0.1
1
10
100
FREQUENCY – Hz
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_OUT
=
101
Ê
Ë
e_nA ²
+ e_nB²
+
e
2
nC
+
e
2
nD
ˆ
¯
The OP471 is a monolithic device with four identical amplifiers.
The noise voltage density of each individual amplifier will
match, giving:
( ) ^eOUT
=
101
Ê
Ë
4e
n
2
ˆ
¯
= 101 2e_n
1/4
OP471
1/4
OP471
R1
100⍀
R2
10k⍀
1/4
OP471
1/4
OP471
e_OUT
TO SPECTRUM ANALYZER
e_OUT ( nV Hz) = 101(2e_n)
V_S = ؎15V
Figure 9. Noise Voltage Density Test Circuit
REV. A
–9–