OP470
Figure 6 shows peak-to-peak noise versus source resistance over
the 0.1 Hz to 10 Hz range. Once again, at low values of R
S
, the
voltage noise of the OP470 is the major contributor to peak-to-peak
noise with current noise the major contributor as R
S
increases.
The crossover point between the OP470 and the OP400 for
peak-to-peak noise is at R
S
= 17 kW.
The OP471 is a higher speed version of the OP470, with a slew
rate of 8 V/ms. Noise of the OP471 is only slightly higher than
the OP470. Like the OP470, the OP471 is unity-gain stable.
1000
OP11
TABLE I.
Source
Strain gauge
Magnetic
tapehead
Device
Impedance
<500
W
<1500
W
Comments
Typically used in
low frequency applications.
Low I
B
very important to reduce
self-magnetization problems
when direct coupling is used.
OP470 I
B
can be neglected.
Similar need for low I
B
in direct
coupled applications. OP470
will not introduce any self-
magnetization problem.
Used in rugged servo-feedback
applications. Bandwidth of
interest is 400 Hz to 5 kHz.
PEAK-TO-PEAK NOISE – nV/ Hz
OP400
Magnetic
phonograph
cartridges
<1500
W
OP471
100
OP470
RESISTOR
NOISE ONLY
Linear variable <1500
W
differential
transformer
For further information regarding noise calculations, see “Minimization of Noise
in Op Amp Applications,” Application Note AN-15.
NOISE MEASUREMENTS—
PEAK-TO-PEAK VOLTAGE NOISE
10
100
1k
10k
RS – SOURCE RESISTANCE –
100k
Figure 6. Peak-To-Peak Noise (0.1 Hz to 10 Hz) vs. Source
Resistance (Includes Resistor Noise)
The circuit of Figure 7 is a test setup for measuring peak-to-peak
voltage noise. To measure the 200 nV peak-to-peak noise speci-
fication of the OP470 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 typi-
cally changes 5
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.
For reference, typical source resistances of some signal sources
are listed in Table I.
R3
1.24k
R1
5
R2
5
OP470
DUT
OP27E
R5
909
R4
200
C1
2 F
C4
0.22 F
R10
65.4k
R11
65.4k
C3
0.22 F
R14
4.99k
R6
600k
D1
1N4148
D2
OP15E
1N4148
R9
306k
R8
10k
OP15E
R13
5.9k
e
OUT
C5
1 F
C2
0.032 F
R12
10k
GAIN = 50,000
V
S
= 5V
Figure 7. Peak-To-Peak Voltage Noise Test Circuit (0.1 Hz to 10 Hz)
–10–
REV. A
AD [ ANALOG DEVICES ]
