OP37FP AD [Analog Devices], OP37FP Datasheet - Page 12

• Manufacturer Part Number: OP37FP
• Description: Low Noise, Precision, High Speed Operational Amplifier
• Manufacturer: AD [Analog Devices]

OP37
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
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
previous designs have made direct coupling difficult, if not
impossible, to use.
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
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.
140
120
100
100
80
60
40
50
10
5
1
50
10
OP27/37
OP08/108
100
5534
OP07
1k
100
REGISTER
NOISE ONLY
UNBALANCED
R
S
R
– SOURCE RESISTANCE –
500
S
= 100 ,
FREQUENCY – Hz
B
1k
and I
R
1k
S
= 0
OS
of only ± 40 nA and 35 nA
e.g. R
e.g. R
10k
1 R
2 R
S
S
S
S
5k
UNMATCHED
MATCHED
= R
= 10k , R
T
V
V
AC TRIM @ 10kHz
R
R
R
A
S
CM
S
S1
S1
S2
= 25 C
=
= 0
10k
= 10k , R
= 20V p-p
100k
BALANCED
R
15V
S1
S
= 1k
= R
B
. TCV
S2
S2
RS)2 +
= 0
= 5k
1
2
50k
1M
OS
of
At R
With R
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.
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
I
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.
OS
error also can be three times the V
S
< 1 kΩ key the OP37’s low voltage noise is maintained.
S
500
100
100
1k
50
10
50
10
< 1 kΩ, total noise increases, but is dominated by the
5
1
50
50
OP07
OP27/37
OP08/108
5534
OP27/37
OP08/108
100
100
OP07
5534
REGISTER
NOISE ONLY
REGISTER
NOISE ONLY
R
R
S
S
– SOURCE RESISTANCE –
– SOURCE RESISTANCE –
500
500
1k
1k
1
2
e.g. R
e.g. R
e.g. R
e.g. R
OS
1 R
2 R
1 R
2 R
S
S
S
S
S
S
S
S
spec.).
UNMATCHED
= R
MATCHED
= 10k , R
5k
5k
UNMATCHED
MATCHED
= R
= 10k , R
R
R
S1
R
R
S1
S2
S1
S1
S2
= 10k , R
10k
10k
= 10k , R
S1
S1
= R
= R
1
S2
S2
S2
S2
2
= 0
= 5k
= 0
= 5k
50k
50k
B
.