OP37
140
R
S
= 0
120
T
A
= 25 C
V
S
= 15V
V
CM
= 20V p-p
AC TRIM @ 10kHz
R
S
= 0
1k
OP08/108
500
5534
OP07
p-p NOISE – nV
CMRR – dB
100
R
S
= 1k
BALANCED
R
S
= 100 ,
UNBALANCED
1
100
OP27/37
50
2
80
1k
60
1 R
S
e.g. R
S
2 R
S
e.g. R
S
UNMATCHED
= R
S1
= 10k , R
S2
= 0
MATCHED
= 10k , R
S1
= R
S2
= 5k
R
S1
40
10
100
1k
10k
FREQUENCY – Hz
100k
1M
REGISTER
NOISE ONLY
10
50
100
R
S2
10k
500
1k
5k
R
S
– SOURCE RESISTANCE –
50k
Figure 4b. TBD
Comments on Noise
Figure 6. Peak-to-Peak Noise (0.1 Hz to 10 Hz) vs. Source
Resistance (Includes Resistor Noise)
The OP37 is a very low-noise monolithic op amp. The outstanding
input voltage noise characteristics of the OP37 are achieved
mainly by operating the input stage at a high quiescent current.
The input bias and offset currents, which would normally increase,
are held to reasonable values by the input bias current cancellation
circuit. The OP37A/E has I
B
and I
OS
of only
±
40 nA and 35 nA
respectively at 25°C. This is particularly important when the input
has a high source resistance. In addition, many audio amplifier
designers prefer to use direct coupling. The high I
B
. TCV
OS
of
previous designs have made direct coupling difficult, if not
impossible, to use.
100
At R
S
< 1 kΩ key the OP37’s low voltage noise is maintained.
With R
S
< 1 kΩ, total noise increases, but is dominated by the
resistor noise rather than current or voltage noise. It is only
beyond Rs of 20kil that current noise starts to dominate. The
argument can be made that current noise is not important for
applications with low to-moderate source resistances. The
crossover between the OP37 and OP07 and OP08 noise occurs
in the 15 kΩ to 40 kΩ region.
100
50
1
2
OP08/108
50
1
TOTAL NOISE – nV/ Hz
TOTAL NOISE – nV/ Hz
10
OP07
5534
OP08/108
2
OP07
10
1 R
S
e.g. R
S
2 R
S
e.g. R
S
UNMATCHED
= R
S1
= 10k , R
S2
= 0
MATCHED
= 10k , R
S1
= R
S2
= 5k
R
S1
5
OP27/37
1 R
S
e.g. R
S
2 R
S
e.g. R
S
UNMATCHED
= R
S1
= 10k , R
S2
= 0
MATCHED
= 10k , R
S1
= R
S2
= 5k
R
S1
5
5534
OP27/37
REGISTER
NOISE ONLY
REGISTER
NOISE ONLY
1
50
100
R
S2
R
S2
10k
500
1k
5k
R
S
– SOURCE RESISTANCE –
50k
1
50
100
500
1k
5k
10k
R
S
– SOURCE RESISTANCE –
50k
Figure 7. !0 Hz Noise vs. Source resistance (Inlcludes
Resistor Noise)
Figure 5. Noise vs. Resistance (Including Resistor Noise
@ 1000 Hz)
Voltage noise is inversely proportional to the square-root of bias
current, but current noise is proportional to the square-root of
bias current. The OP37’s noise advantage disappears when high
source-resistors are used. Figures 5, 6, and 7 compare OP-37
observed total noise with the noise performance of other devices
in different circuit applications.
Total noise = [( Voltage noise)2 + (current noise
(resistor noise_]1/2
RS)2 +
Figure 6 shows the 0.1 Hz to 10 Hz peak-to-peak noise. Here
the picture is less favorable; resistor noise is negligible, current
noise becomes important because it is inversely proportional to
the square-root of frequency. The crossover with the OP-07
occurs in the 3 kΩ to 5 kΩ range depending on whether bal-
anced or unbalanced source resistors are used (at 3 kΩ the I
B
.
I
OS
error also can be three times the V
OS
spec.).
Therefore, for low-frequency applications, the OP07 is better
than the OP27/37 when Rs > 3 kΩ. The only exception is when
gain error is important. Figure 3 illustrates the 10 Hz noise. As
expected, the results are between the previous two figures.
For reference, typical source resistances of some signal sources
are listed in Table I.
Figure 5 shows noise versus source resistance at 1000 Hz. The
same plot applies to wideband noise. To use this plot, just multiply
the vertical scale by the square-root of the bandwidth.
–12–
REV. A
AD [ ANALOG DEVICES ]
