ad797bn Analog Devices, Inc., ad797bn Datasheet - Page 12

• Manufacturer Part Number: ad797bn
• Description: Ultralow Distortion, Ultralow Noise Op Amp
• Manufacturer: Analog Devices, Inc.

AD797
NOISE AND SOURCE IMPEDANCE
CONSIDERATIONS
The AD797’s ultralow voltage noise of 0.9 nV/√Hz is achieved
with special input transistors running at nearly 1 mA of
collector current. It is important then to consider the total
input referred noise (e
from voltage noise (e
(√4 kTr
where r
This equation is plotted for the AD797 in Figure 33. Because
optimum dc performance is obtained with matched source
resistances, this case is considered even though it is clear from
Equation 1 that eliminating the balancing source resistance
lowers the total noise by reducing the total r
At very low source resistance (r
noise dominates. As source resistance increases, the Johnson
noise of r
current noise component is larger than the resistor noise.
+IN
e
100
N
0.1
10
S
total
S
Q1
1
).
= total input source resistance.
10
S
I1
dominates until at higher resistances (r
I3
=
Q2
[
I2
Figure 33. Noise vs. Source Resistance
e
N
–IN
Figure 32. AD797 Block Diagram
2
N
+
), current noise (i
N
total), which includes contributions
C
4
SOURCE RESISTANCE (Ω)
100
CURRENT
MIRROR
kTr
TOTAL NOISE
C
N
A
S
I4
+
S
<50 Ω), the amplifiers’ voltage
(
i
B
N
/
C
r
N
C
S
1000
), and resistor noise
)
1
2
]
1
S
2 /
by a factor of two.
RESISTOR
NOISE
ONLY
S
A
> 2 kΩ); the
10000
OUT
Rev. E | Page 12 of 20
(1)
The AD797 is the optimum choice for low noise performance
provided the source resistance is kept <1 kΩ. At higher values of
source resistance, optimum performance with respect to noise
alone is obtained with other amplifiers from Analog Devices
(Table 3).
Table 3. Recommended Amplifiers for Different Source
Impedances
r
0 to <1 kΩ
1 kΩ to <10 kΩ
10 kΩ to <100 kΩ
>100 kΩ
LOW FREQUENCY NOISE
Analog Devices specifies low frequency noise as a peak-to-peak
(p-p) quantity in a 0.1 Hz to 10 Hz bandwidth. Several
techniques can be used to make this measurement. The usual
technique involves amplifying, filtering, and measuring the
amplifier’s noise for a predetermined test time. The noise
bandwidth of the filter is corrected for, and the test time is
carefully controlled because the measurement time acts as an
additional low frequency roll-off.
The plot in Figure 7 uses a slightly different technique. Here an
FFT based instrument (Figure 34) is used to generate a 10 Hz
“brickwall” filter. A low frequency pole at 0.1 Hz is generated
with an external ac coupling capacitor, the instrument being
dc coupled.
Several precautions are necessary to get optimum low frequency
noise performance.
•
•
•
•
S
, ohms
Care must be used to account for the effects of rS. Even
a 10 Ω resistor has 0.4 nV/√Hz of noise (an error of 9%
when root sum squared with 0.9 nV/√Hz).
The test setup must be fully warmed up to prevent eOS
drift from erroneously contributing to input noise.
Circuitry must be shielded from air currents. Heat flow out
of the package through its leads creates the opportunity for
a thermoelectric potential at every junction of different
metals. Selective heating and cooling of these by random
air currents appears as 1/f noise and obscure the true
device noise.
The results must be interpreted using valid statistical
techniques.
Recommended Amplifier
AD797
AD743/AD745, OP27/OP37, OP07
AD743/AD745, OP07
AD548, AD549, AD711, AD743/AD745