OP470
5k
500
1/4
OP470
V
1
20V p-p
50k
50
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 R
S
< 1 kW
the total noise is dominated by the voltage noise of the OP470.
As R
S
rises above 1 kW, total noise increases and is dominated
by resistor noise rather than by voltage or current noise of the
OP470. When R
S
exceeds 20 kW, current noise of the OP470
becomes the major contributor to total noise.
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 R
S
> 5 kW.
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.
100
1/4
OP470
V
2
CHANNEL SEPARATION = 20 LOG
V
1
V
2
/1000
Figure 2. Channel Separation Test Circuit
+18V
2
3
A
11
–18V
4
1
+1V
5
6
B
7
+1V
TOTAL NOISE – nV/ Hz
OP11
10
OP400
OP471
9
10
C
8
–1V
13
12
D
14
–1V
OP470
Figure 3. Burn-In Circuit
APPLICATIONS INFORMATION
Voltage and Current Noise
1
100
RESISTOR
NOISE ONLY
1k
10k
RS – SOURCE RESISTANCE –
100k
TOTAL NOISE – nV/ Hz
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.
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
).
TOTAL NOISE AND SOURCE RESISTANCE
Figure 4. Total Noise vs. Source Resistance (Including
Resistor Noise) at 1 kHz
100
OP11
OP400
10
OP471
OP470
RESISTOR
NOISE ONLY
1
100
The total noise of an op amp can be calculated by:
E
n
=
where:
(
E
n
)
+
(
i
n
R
S
)
+
(
E
t
)
2
2
2
1k
10k
RS – SOURCE RESISTANCE –
100k
Figure 5. Total Noise vs. Source Resistance (Including
Resistor Noise) at 10 Hz
E
n
= total input referred noise
e
n
= up amp voltage noise
i
n
= op amp current noise
e
t
= source resistance thermal noise
R
S
= source resistance
REV. A
–9–
AD [ ANALOG DEVICES ]
