OP37
140
1k
T
= 25ꢃC
A
S
OP08/108
5534
V
= ꢂ15V
R
= 0
S
500
V
= 20V p-p
CM
120
100
80
AC TRIM @ 10kHz
R
= 0
S
OP07
1
R
= 1kꢄ
S
BALANCED
2
100
50
OP27/37
R
= 100ꢄ,
1kꢄ UNBALANCED
S
1 R UNMATCHED
S
e.g.R = R = 10kꢄ, R = 0
S
S1
S2
2 R MATCHED
S
e.g.R = 10kꢄ, R = R = 5kꢄ
S
S1
S2
60
R
S1
R
S2
REGISTER
NOISE ONLY
40
10
10
100
1k
10k
100k
1M
50
100
500
1k
5k
10k
50k
FREQUENCY – Hz
R
– SOURCE RESISTANCE –ꢄ
S
Figure 4b. CMRR vs. Frequency
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 IB and IOS 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 IB. TCVOS of
previous designs have made direct coupling difficult, if not
impossible, to use.
At RS < 1 kW key the OP37’s low voltage noise is maintained.
With RS < 1 kW, total noise increases, but is dominated by the
resistor noise rather than current or voltage noise. It is only
beyond Rs of 20 kW 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 kW to 40 kW region.
100
50
1
2
100
OP08/108
50
1
OP07
10
OP08/108
5534
1 R UNMATCHED
S
2
5
e.g.R = R = 10kꢄ, R = 0
S
S
S1
S2
OP07
10
2 R MATCHED
OP27/37
e.g.R = 10kꢄ, R = R = 5kꢄ
S
S1 S2
R
S1
1 R UNMATCHED
S
5
5534
R
S2
e.g.R = R = 10kꢄ, R = 0
REGISTER
S
S1
S2
2 R MATCHED
NOISE ONLY
S
1
e.g.R = 10kꢄ, R = R = 5kꢄ
S
S1 S2
OP27/37
50
100
500
1k
5k
10k
50k
R
S1
R
– SOURCE RESISTANCE –ꢄ
S
R
S2
REGISTER
NOISE ONLY
Figure 7. Noise vs. Source resistance (Includes Resistor
Noise @ 10 Hz)
1
50
100
500
1k
5k
10k
50k
R
– SOURCE RESISTANCE –ꢄ
S
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 OP07
occurs in the 3 kW to 5 kW range depending on whether bal-
anced or unbalanced source resistors are used (at 3 kW the IB.
IOS error also can be three times the VOS spec.).
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.
Therefore, for low-frequency applications, the OP07 is better
than the OP27/37 when Rs > 3 kW. The only exception is when
gain error is important. Figure 7 illustrates the 10 Hz noise. As
expected, the results are between the previous two figures.
Total noise = [( Voltage noise)2 + (current noise ϫ RS)2 +
(resistor noise_]1/2
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. B
ADI [ ADI ]
