OP270 Data Sheet
Rev. F | Page 12 of 20
APPLICATIONS INFORMATION
VOLTAGE AND CURRENT NOISE
The OP270 is a very low noise dual op amp, exhibiting a typical
voltage noise density of only 3.2 nV/√Hz at 1 kHz. Because the
voltage noise is inversely proportional to the square root of the
collector current, the exceptionally low noise characteristic of
the OP270 is achieved in part by operating the input transistors
at high collector currents. Current noise, however, is directly
proportional to the square root of the collector current. As a
result, the outstanding voltage noise density 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 relationships among voltage noise (e
n
), current
noise (i
n
), and resistor noise (e
t
).
TOTAL NOISE AND SOURCE RESISTANCE
The total noise of an op amp can be calculated by
222
)()()(
tsnnn
eRieE
where:
E
n
is the total input-referred noise.
e
n
is the op amp voltage noise.
i
n
is the op amp current noise.
e
t
is the source resistance thermal noise.
R
S
is the source resistance.
The total noise is referred to the input and at the output is
amplified by the circuit gain.
Figure 32 shows the relationship between total noise at 1 kHz
and source resistance. When R
S
is less than 1 kΩ, the total noise
is dominated by the voltage noise of the OP270. As R
S
rises
above 1 kΩ, 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 kΩ, the current noise of the OP270
becomes the major contributor to total noise.
00352-033
100
10
1
100 1k 10k 100k
TOTAL NOISE (nV/
Hz)
SOURCE RESISTANCE (Ω)
RESISTOR
NOISE ONLY
OP200
OP270
Figure 32. Total Noise vs. Source Resistance
(Including Resistor Noise) at 1 kHz
Figure 33 also shows the relationship between total noise and
source resistance, but at 10 Hz. Total noise increases more
quickly than shown in Figure 32 because current noise is
inversely proportional to the square root of frequency. In
Figure 33, the current noise of the OP270 dominates the total
noise when R
S
is greater than 5 kΩ.
Figure 32 and Figure 33 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, can provide lower total noise.
00352-034
100
10
1
100 1k 10k 100k
TOTAL NOISE (nV/
Hz)
SOURCE RESISTANCE (Ω)
RESISTOR
NOISE ONLY
OP200
OP270
Figure 33. Total Noise vs. Source Resistance
(Including Resistor Noise) at 10 Hz
Figure 34 shows peak-to-peak noise vs. source resistance over
the 0.1 Hz to 10 Hz range. At low values of R
S
, the voltage noise
of the OP270 is the major contributor to peak-to-peak noise,
with current noise becoming the major contributor as R
S
increases. The crossover point between the OP270 and the
OP200 for peak-to-peak noise is at a source resistance of 17 kΩ.
00352-035
1k
100
10
100
1k
10k 100k
PEAK-TO-PEAK NOISE (nV)
SOURCE RESISTANCE (Ω)
RESISTOR
NOISE ONLY
OP200
OP270
Figure 34. Peak-to-Peak Noise (0.1 Hz to 10 Hz) vs. Source Resistance
(Including Resistor Noise)
