OP37 Datasheet, PDF (12/16 Pages) Analog Devices – Low Noise, Precision, High Speed Operational Amplifier

OP37
140
R_S = 0
120
100
80
R_S = 100⍀,
1k⍀ UNBALANCED
60
T_A = 25؇C
V_S = ؎15V
V_CM = 20V p-p
AC TRIM @ 10kHz
R_S = 0
R_S = 1k⍀
BALANCED
40
10
100
1k
10k
100k
1M
FREQUENCY – Hz
Figure 4b. TBD
Comments on 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
50
1
OP08/108
2
OP07
10
5
5534
OP27/37
1 R_S UNMATCHED
e.g. R_S = R_S1 = 10k⍀, R _S2 = 0
2 R_S MATCHED
e.g. R_S = 10k⍀, R _S1 = R_S2 = 5k⍀
RS1
REGISTER
RS2
NOISE ONLY
1
50 100
500 1k
5k 10k
50k
R_S – SOURCE RESISTANCE – ⍀
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 Ï« RS)2 +
(resistor noise_]1/2
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.
1k
OP08/108
500 ₅₅₃₄
OP07
1
2
100
OP27/37
50
1 R_S UNMATCHED
e.g. R_S = R_S1 = 10k⍀, R _S2 = 0
2 R_S MATCHED
e.g. R_S = 10k⍀, R _S1 = R_S2 = 5k⍀
RS1
REGISTER
RS2
NOISE ONLY
10
50 100
500 1k
5k 10k
50k
R_S – SOURCE RESISTANCE – ⍀
Figure 6. Peak-to-Peak Noise (0.1 Hz to 10 Hz) vs. Source
Resistance (Includes Resistor Noise)
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
OP08/108
1
2
OP07
10
5534
5
OP27/37
1 R_S UNMATCHED
e.g. R_S = R_S1 = 10k⍀, R _S2 = 0
2 R_S MATCHED
e.g. R_S = 10k⍀, R _S1 = R_S2 = 5k⍀
RS1
REGISTER
RS2
NOISE ONLY
1
50 100
500 1k
5k 10k
50k
R_S – SOURCE RESISTANCE – ⍀
Figure 7. !0 Hz Noise vs. Source resistance (Inlcludes
Resistor Noise)
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.
–12–
REV. A