OP27
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/E 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.
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
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).
1k
OP08/108
500
5534
OP07
p-p NOISE (nV)
1
100
OP27/37
50
2
1 R
S
e.g. R
S
2 R
S
e.g. R
S
UNMATCHED
= R
S1
= 10k
Ω
, R
S2
= 0
MATCHED
= 10k
Ω
, R
S1
= R
S2
= 5k
Ω
R
S1
REGISTER
NOISE ONLY
10
50
100
R
S2
⎡
(
Voltage Noise
)
2
+
⎤
⎢
⎥
Total Noise
= ⎢
(
Current Noise
×
R
S
)
2
+ ⎥
⎢
⎥
2
⎢
(
Resistor Noise
)
⎥
⎣
⎦
1/ 2
10k
500
1k
5k
R
S
—SOURCE RESISTANCE (
Ω
)
50k
Figure 39. Peak-to-Peak Noise (0.1 Hz to 10 Hz) as Source Resistance
(Includes Resistor Noise)
same plot applies to wideband noise. To use this plot, multiply
the vertical scale by the square root of the bandwidth.
100
For low frequency applications, the
is better than the
OP27/OP37 when RS > 3 kΩ. The only exception is when gain
error is important.
between the previous two figures.
100
50
1
TOTAL NOISE (nV/
√
Hz)
OP08/108
2
10
1 R
S
e.g. R
S
2 R
S
e.g. R
S
UNMATCHED
= R
S1
= 10k
Ω
, R
S2
= 0
MATCHED
= 10k
Ω
, R
S1
= R
S2
= 5k
Ω
R
S1
50
1
2
OP08/108
TOTAL NOISE (nV/
√
Hz)
OP07
5
5534
OP27/37
REGISTER
NOISE ONLY
100
10
OP07
5534
5
OP27/37
R
S2
1 R
S
e.g. R
S
2 R
S
e.g. R
S
UNMATCHED
= R
S1
= 10k
Ω
, R
S2
= 0
MATCHED
= 10k
Ω
, R
S1
= R
S2
= 5k
Ω
R
S1
500
1k
5k
10k
R
S
—SOURCE RESISTANCE (
Ω
)
50k
00317-038
1
50
Figure 38. Noise vs. Source Resistance (Including Resistor Noise) at 1000 Hz
100
500
1k
5k
10k
R
S
—SOURCE RESISTANCE (
Ω
)
50k
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 40. 10 Hz Noise vs. Source Resistance (Includes Resistor Noise)
Audio Applications
Rev. F | Page 15 of 20
00317-040
1
50
REGISTER
NOISE ONLY
R
S2
00317-039
AD [ ANALOG DEVICES ]
