OP270
5k
TOTAL NOISE AND SOURCE RESISTANCE
The total noise of an op amp can be calculated by:
500
1/2
OP270
V
1
20V
p-p
E
n
=
where:
(
e
n
)
+
(
i
n
R
S
)
+
(
e
t
)
2
2
2
5k
E
n
= total input referred noise
e
n
= op amp voltage noise
i
n
= op amp current noise
V
2
V
CHANNEL SEPARATION = 20 log
2
1
50
1/2
OP270
e
t
= source resistance thermal noise
R
S
= source resistance
The total noise is referred to the input and at the output would
be amplified by the circuit gain.
Figure 3 shows the relationship between total noise at 1 kHz
and source resistance. For R
S
< 1 kW the total noise is dominated
by the voltage noise of the OP270. As R
S
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 R
S
exceeds
20 kW, current noise of the OP270 becomes the major contributor
to total noise.
100
V /1000
Figure 1. Channel Separation Test Circuit
+18V
8
100k
2
1/2
OP270
1
3
200k
6
1/2
OP270
7
5
100k
TOTAL NOISE (nV/
Hz)
OP200
10
4
–18V
OP270
RESISTOR
NOISE ONLY
1
100
1k
10k
100k
Figure 2. Burn-In Circuit
APPLICATIONS INFORMATION
VOLTAGE AND CURRENT NOISE
R
S
– SOURCE RESISTANCE ( )
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.
To obtain the best noise performance in a circuit, it is vital to
understand the relationship between voltage noise (e
n
), current
noise (i
n
), and resistor noise (e
t
).
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 R
S
> 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.
–8–
REV. C
AD [ ANALOG DEVICES ]
