Datasheet
Data Sheet OP27
Rev. H | Page 15 of 21
COMMENTS ON NOISE
The OP27 is a very low noise, monolithic op amp. The out-
standing input voltage noise characteristics of the
OP27
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 OP27A/OP27E has
I
B
and I
OS
of only ±40 nA and 35 nA at 25°C respectively. 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
, V
OS
, and TCV
OS
of 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 noise advantage of the
OP27 disappears when
high source resistors are used. Figure 38, Figure 39, Figure 40
compare the observed total noise of the
OP27 with the noise
performance of other devices in different circuit applications.
2/1
2
2
2
)(
)(
)
(
»
»
»
»
¼
º
«
«
«
«
¬
ª
u
NoiseResistor
RNoiseCurrent
NoiseVoltage
NoiseTotal
S
Figure 38 shows noise vs. source resistance at 1000 Hz. The
same plot applies to wideband noise. To use this plot, multiply
the vertical scale by the square root of the bandwidth.
R
S
—SOURCE RESISTANCE (
:
)
10
50 10k
5
500 1k 5k
1
100
50
100 50k
R
S1
R
S2
OP07
5534
OP27/37
REGISTER
NOISE ONLY
OP08/108
1
2
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
:
00317-038
TOTAL NOIS
E (nV/
Hz)
Figure 38. Noise vs. Source Resistance (Including Resistor Noise) at 1000 Hz
At R
S
< 1 kΩ, the low voltage noise of the OP27 is maintained.
With R
S
< 1 kΩ, total noise increases but is dominated by the
resistor noise rather than current or voltage noise. lt is only
beyond R
S
of 20 kΩ 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
OP27 and OP07 noise occurs in the 15 kΩ
to 40 kΩ region.
Figure 39 shows the 0.1 Hz to 10 Hz p-p noise. Here the picture
is less favorable; resistor noise is negligible and current noise
becomes important because it is inversely proportional to the
square root of frequency. The crossover with the
OP07 occurs
in the 3 kΩ to 5 kΩ range depending on whether balanced or
unbalanced source resistors are used (at 3 kΩ the I
B
and I
OS
error also can be 3× the V
OS
spec).
R
S
—SOURCE RESISTANCE (
:
)
100
50
10k
p-p NOISE (nV)
50
500 1k
5k
10
1k
500
100 50k
R
S1
R
S2
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
:
OP07
5534
OP27/37
REGISTER
NOISE ONLY
OP08/108
1
2
00317-039
Figure 39. Peak-to-Peak Noise (0.1 Hz to 10 Hz) as Source Resistance
(Includes Resistor Noise)
For low frequency applications, the OP07 is better than the
OP27/OP37 when RS > 3 kΩ. The only exception is when gain
error is important.
Figure 40 illustrates the 10 Hz noise. As expected, the results are
between the previous two figures.
10
50
10k
5
500 1k 5k
1
100
50
100 50k
OP07
5534
OP27/37
REGISTER
NOISE ONLY
OP08/108
R
S1
R
S2
1 R
S
UNMATCHED
e.g. R
S
= R
S1
= 10k
:
, R
S2
= 0
2 R
S
MA
TCHED
e.g. R
S
= 10k
:
, R
S1
= R
S2
= 5k
:
1
2
00317-040
R
S
—SOURCE RESISTANCE (
:
)
TOTAL NOISE (nV/
Hz)
Figure 40. 10 Hz Noise vs. Source Resistance (Includes Resistor Noise)
Audio Applications