OP270
NOISE MEASUREMENTS
Peak-to-Peak Voltage Noise
Noise Measurement — Noise Voltage Density
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.
100
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:
Ê
e
OUT
=
101
Á
Ë
(
e
nA
)
+
(
e
nB
)
2
2
ˆ
˜
¯
The OP270 is a monolithic device with two identical amplifi-
ers. The noise voltage density of each individual amplifier will
match, giving:
2
ˆ
Ê
e
OUT
=
101
Á
2
e
n
˜ =
101 2
e
n
Ë
¯
( )
R1
100
R2
10k
–
–
1/2
OP270
+
1/2
e
OUT
OP270
+
TO SPECTRUM ANALYZER
e
OUT
(nV/
Hz)
=
101 (
2
e
n
)
V
S
= 15V
80
Figure 8. Noise Voltage Density Test Circuit
R3
1.24k
GAIN (dB)
60
R1
5
R2
100k
–
OP270
DUT
+
40
–
OP27E
+
e
nOUT
TO SPECTRUM ANALYZER
20
R4
200
R5
8.06k
GAIN = 10,000
V
S
= 15V
0
0.01
0.1
1.0
FREQUENCY (Hz)
10
100
Figure 9. Current Noise Density Test Circuit
Noise Measurement — Current Noise Density
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.
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:
Ê
e
nOUT
ˆ
Á
G
˜ -
40
nV
/
Hz
¯
Ë
R
S
2
i
n
=
(
)
2
where:
G
= gain of 10,000
R
S
= 100 kW source resistance
–10–
REV. C
AD [ ANALOG DEVICES ]
