OP270
5k⍀
TOTAL NOISE AND SOURCE RESISTANCE
The total noise of an op amp can be calculated by:
500⍀
1/2
OP270
2
2
2
V
20V
p-p
1
En =
e
n
+ i R
+ e
(
)
(
)
( )
n
S
t
where:
5k⍀
En = total input referred noise
en = op amp voltage noise
in = op amp current noise
50⍀
1/2
OP270
V
2
et = source resistance thermal noise
RS = source resistance
V
1
CHANNEL SEPARATION = 20 log
V /1000
2
The total noise is referred to the input and at the output would
be amplified by the circuit gain.
Figure 1. Channel Separation Test Circuit
+18V
Figure 3 shows the relationship between total noise at 1 kHz
and source resistance. For RS < 1 kW the total noise is dominated
by the voltage noise of the OP270. As RS rises above 1 kW, total
noise increases and is dominated by resistor noise rather than by
the voltage or current noise of the OP270. When RS exceeds
20 kW, current noise of the OP270 becomes the major contributor
to total noise.
8
100k⍀
2
1
1/2
OP270
3
200k⍀
100
6
7
1/2
OP270
5
100k⍀
OP200
10
4
–18V
OP270
Figure 2. Burn-In Circuit
RESISTOR
NOISE ONLY
1
100
APPLICATIONS INFORMATION
VOLTAGE AND CURRENT NOISE
1k
10k
100k
R
– SOURCE RESISTANCE (⍀)
S
The OP270 is a very low noise dual op amp, exhibiting atypical
voltage noise of only 3.2 nV/÷Hz @ 1 kHz. The exceptionally
low noise characteristic of the OP270 is achieved in part by
operating the input transistors at high collector currents since
the voltage noise is inversely proportional to the square root of
the collector current. Current noise, however, is directly propor-
tional to the square root of the collector current. As a result, the
outstanding voltage noise performance of the OP270 is gained
at the expense of current noise performance, which is normal for
low noise amplifiers.
Figure 3. Total Noise vs. Source Resistance
(Including Resistor Noise) at 1 kHz
Figure 4 also shows the relationship between total noise and
source resistance, but at 10 Hz. Total noise increases more
quickly than shown in Figure 3 because current noise is inversely
proportional to the square root of frequency. In Figure 4, current
noise of the OP270 dominates the total noise when RS > 5 kW.
Figures 3 and 4 show that to reduce total noise, source resistance
must be kept to a minimum. In applications with a high source
resistance, the OP200, with lower current noise than the OP270,
will provide lower total noise.
To obtain the best noise performance in a circuit, it is vital to
understand the relationship between voltage noise (en), current
noise (in), and resistor noise (et).
–8–
REV. C
ADI [ ADI ]
