OP27_15 Datasheet, PDF (15/20 Pages) Analog Devices – Low Noise, Precision Operational Amplifier

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 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.
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.
⎡(Voltage Noise)² + ⎤^{1/ 2}
⎢
⎥
Total Noise = _⎢(Current Noise × R_S )² +⎥
⎢
⎢⎣^(Resistor
Noise)²
⎥
⎥⎦
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.
100
50
1
OP08/108
2
OP07
10
5
5534
OP27/37
1 R_S UNMATCHED
e.g. R_S = R_S1 = 10kΩ, R _S2 = 0
2 R_S MATCHED
e.g. R_S = 10kΩ, R _S1 = R_S2 = 5kΩ
RS1
REGISTER
RS2
NOISE ONLY
1
50 100
500 1k
5k 10k
50k
R_S—SOURCE RESISTANCE (Ω)
Figure 38. Noise vs. Source Resistance (Including Resistor Noise) at 1000 Hz
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.
OP27
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).
1k
OP08/108
500 ₅₅₃₄
OP07
1
2
100
OP27/37
50
1 R_S UNMATCHED
e.g. R_S = R_S1 = 10kΩ, R _S2 = 0
2 R_S MATCHED
e.g. R_S = 10kΩ, R _S1 = R_S2 = 5kΩ
RS1
REGISTER
RS2
NOISE ONLY
10
50 100
500 1k
5k 10k
50k
R_S—SOURCE RESISTANCE (Ω)
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.
100
50
OP08/108
1
2
OP07
10
5534
5
OP27/37
1 R_S UNMATCHED
e.g. R_S = R_S1 = 10kΩ, R _S2 = 0
2 R_S MATCHED
e.g. R_S = 10kΩ, R _S1 = R_S2 = 5kΩ
RS1
REGISTER
RS2
NOISE ONLY
1
50 100
500 1k
5k 10k
50k
R_S—SOURCE RESISTANCE (Ω)
Figure 40. 10 Hz Noise vs. Source Resistance (Includes Resistor Noise)
Audio Applications
Rev. F | Page 15 of 20