AN-940 View Datasheet(PDF) - Analog Devices
Part Name
Description
Manufacturer
AN-940 Datasheet PDF : 12 Pages
| |||
Application Note
SELECTING LOW NOISE OP AMP
If an op amp is driven with a source resistance, the equivalent
noise input becomes the square root of the sum of the squares of
the amplifier’s voltage noise, the voltage generated by the source
resistance, and the voltage caused by the amplifiers current
noise flowing through the source impedance.
For very low source resistances, the noise generated by the
source resistance and amplifier current noise contribute
insignificantly to the total. In this case, the noise at the
input is effectively only the voltage noise of the op amp.
If the source resistance is high, the Johnson noise of the source
resistance may dominate both the op amp voltage noise and the
voltage due to the current noise. However, note that, because the
Johnson noise only increases with the square root of the resis-
tance, while the noise voltage due to the current noise is directly
proportional to the input impedance, the amplifier’s current noise
always dominates for a high enough value of input impedance.
When an amplifier’s voltage and current noise are high enough,
there may be no value of input resistance for which Johnson
noise dominates.
AN-940
An amplifier can be selected where its noise contribution is
negligible compared to the source resistance by using a figure
of merit, RS, OP, of an op amp. It can be calculated by using an
amplifier’s noise specification.
RS, OP
en
in
(7)
where:
en is input-referred voltage noise.
in is input-referred current noise.
Figure 6 shows a comparison of the voltage noise density of
a number of high voltage (up to 44 V) op amps from Analog
Devices, Inc., vs. RS, OP at 1 kHz. The diagonal line plots the
Johnson noise associated with resistance.
100
f = 1kHz
JOHNSON NOISE LINE
OF SOURCE RESISTANCE
AD8622/AD8624
OP285
OP467
AD8610/
OP271
AD8620
10
OPx177
OP275
OP213
OP270
ADA4004
ADA4075-2
OPx84
OP27/OP37
AD743/AD745
1
AD8597/AD8599
AD797
0.1
10
100
1k
10k
100k
1M
SOURCE RESISTANCE (Ω)
Figure 6. Analog Devices Op Amp Noise Plot
Rev. D | Page 7 of 12
Share Link: 