OP470
4. The test time to measure 0.1 Hz to 10 Hz noise should not ex-
ceed 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 elimi-
nate 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 via the amplifier supply pins.
100
The OP470 is a monolithic device with four identical amplifiers.
The noise voltage density of each individual amplifier will match,
giving:
e
OUT
=
101
Ê
ˆ
4e
n
2
= 101
(
2e
n
)
Ë
¯
NOISE MEASUREMENT—CURRENT NOISE DENSITY
The test circuit shown in Figure 10 can be used to measure
current noise density. The formula relating the voltage output to
current noise density is:
Ê
Á
Ë
nOUT
i
n
=
G
ˆ
˜ -
40nV /
¯
R
S
2
(
Hz
)
2
where:
80
G = gain of 10000
R
S
= 100 kW source resistance
R3
1.24k
GAIN – dB
60
40
R1
5
R2
100k
20
OP470
DUT
OP27E
R5
8.06k
0.1
1
FREQUENCY – Hz
10
100
e
n
OUT TO
SPECTRUM ANALYZER
0
0.01
R4
200
Figure 8. 0.1 Hz to 10 Hz Peak-to-Peak Voltage Noise Test
Circuit Frequency Response
NOISE MEASUREMENT—NOISE VOLTAGE DENSITY
GAIN = 50,000
V
S
= 5V
Figure 10. Current Noise Density Test Circuit
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
ˆ
e
2
+ e
nB
2
+
e
nC
2
+
e
nD
2
Ë
nA
¯
R1
100
R2
10k
1/4
OP470
1/4
OP470
1/4
OP470
1/4
OP470
e
OUT
TO SPECTRUM ANALYZER
e
OUT
(nV Hz) = 101(2e
n
)
V
S
= 15V
Figure 9. Noise Voltage Density Test Circuit
REV. B
–11–
AD [ ANALOG DEVICES ]
