OP470
5kꢅ
The total noise is referred to the input and at the output would
be amplified by the circuit gain. Figure 4 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 OP470.
As RS rises above 1 kW, total noise increases and is dominated
by resistor noise rather than by voltage or current noise of the
OP470. When RS exceeds 20 kW, current noise of the OP470
becomes the major contributor to total noise.
500ꢅ
1/4
V
V
20V p-p
1
OP470
50kꢅ
50ꢅ
Figure 5 also shows the relationship between total noise and
source resistance, but at 10 Hz. Total noise increases more
quickly than shown in Figure 4 because current noise is inversely
proportional to the square root of frequency. In Figure 5, current
noise of the OP470 dominates the total noise when RS > 5 kW.
1/4
2
OP470
V
1
CHANNEL SEPARATION = 20 LOG
V /1000
2
From Figures 4 and 5 it can be seen that to reduce total noise,
source resistance must be kept to a minimum. In applications
with a high source resistance, the OP400, with lower current
noise than the OP470, will provide lower total noise.
Figure 2. Channel Separation Test Circuit
+18V
2
3
6
5
100
4
1
7
A
B
D
+1V
–1V
+1V
–1V
11
–18V
OP11
OP400
OP471
10
9
13
12
8
14
C
10
OP470
RESISTOR
NOISE ONLY
Figure 3. Burn-In Circuit
1
100
1k
10k
100k
APPLICATIONS INFORMATION
Voltage and Current Noise
R
– SOURCE RESISTANCE –ꢅ
S
Figure 4. Total Noise vs. Source Resistance (Including
Resistor Noise) at 1 kHz
The OP470 is a very low-noise quad op amp, exhibiting a typi-
cal voltage noise of only 3.2 nV÷Hz @ 1 kHz. The exceptionally
low-noise characteristics of the OP470 are in part achieved 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 OP470 is gained
at the expense of current noise performance, which is typical for
low noise amplifiers.
100
OP11
OP400
10
OP471
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).
OP470
RESISTOR
NOISE ONLY
TOTAL NOISE AND SOURCE RESISTANCE
The total noise of an op amp can be calculated by:
1
100
1k
10k
100k
R
– SOURCE RESISTANCE –ꢅ
S
2
2
2
En =
e
+ i R
+ e
) ( )
( )
(
n
n
S
t
Figure 5. Total Noise vs. Source Resistance (Including
Resistor Noise) at 10 Hz
where:
En = total input referred noise
en = up amp voltage noise
in = op amp current noise
et = source resistance thermal noise
RS = source resistance
REV. B
–9–
ADI [ ADI ]
