OP27
COMMENTS ON NOISE
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 IB and IOS
error also can be 3× the VOS spec).
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 IB
and IOS 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 IB, VOS, and TCVOS of previous
designs have made direct coupling difficult, if not impossible,
to use.
1k
OP08/108
500
5534
OP07
1
2
100
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.
OP27/37
1 RS UNMATCHED
50
e.g. RS = RS1 = 10k
2 RS MATCHED
Ω, RS2 = 0
e.g. RS = 10kΩ, RS1 = RS2 = 5kΩ
RS1
RS2
REGISTER
NOISE ONLY
10
50
100
500
1k
5k
10k
50k
1/ 2
2
⎡
⎤
⎥
⎥
⎥
(VoltageNoise) +
R —SOURCE RESISTANCE (Ω)
S
⎢
TotalNoise = (Current Noise× R )2 +
Figure 39. Peak-to-Peak Noise (0.1 Hz to 10 Hz) as Source Resistance
(Includes Resistor Noise)
⎢
S
⎢
(Resistor Noise)2
⎢
⎥
⎣
⎦
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 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.
Figure 40 illustrates the 10 Hz noise. As expected, the results are
between the previous two figures.
100
50
100
1
50
OP08/108
1
2
2
OP07
10
OP08/108
1 RS UNMATCHED
OP07
10
5
5534
e.g. RS = RS1 = 10k
2 RS MATCHED
Ω, RS2 = 0
5534
e.g. RS = 10k
Ω
, RS1 = RS2 = 5k
Ω
1 RS UNMATCHED
OP27/37
RS1
5
e.g. RS = RS1 = 10k
2 RS MATCHED
Ω, RS2 = 0
RS2
e.g. RS = 10kΩ, RS1 = RS2 = 5kΩ
REGISTER
OP27/37
NOISE ONLY
RS1
1
50
100
500
1k
5k
10k
50k
RS2
REGISTER
NOISE ONLY
R —SOURCE RESISTANCE (Ω)
S
1
50
100
500
1k
5k
10k
50k
Figure 38. Noise vs. Source Resistance (Including Resistor Noise) at 1000 Hz
R —SOURCE RESISTANCE (Ω)
S
At RS < 1 kΩ, the low voltage noise of the OP27 is maintained.
With RS < 1 kΩ, total noise increases but is dominated by the
resistor noise rather than current or voltage noise. lt is only
beyond RS 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
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