Datasheet

Data Sheet ADA4528-1/ADA4528-2
Rev. D | Page 21 of 28
Source Resistance
With 5.6 nV/√Hz of broadband noise at 1 kHz (V
SY
= 2.5 V
and A
V
= +100), the ADA4528-1/ADA4528-2 are among the
lowest noise zero-drift amplifiers currently available in the
industry. Therefore, it is important to carefully select the input
source resistance to maintain a total low noise.
The total input referred broadband noise (e
n
total) from any
amplifier is primarily a function of three types of noise: input
voltage noise, input current noise, and thermal (Johnson) noise
from the external resistors.
These uncorrelated noise sources can be summed up in a root
sum squared (rss) manner using the following equation:
e
n
total = [e
n
2
+ 4 kTR
S
+ (i
n
× R
S
)
2
]
1/2
where:
e
n
is the input voltage noise of the amplifier (V/√Hz).
k is the Boltzmann’s constant (1.38 × 10
−23
J/K).
T is the temperature in Kelvin (K).
R
S
is the total input source resistance (Ω).
i
n
is the input current noise of the amplifier (A/√Hz).
The total equivalent rms noise over a specific bandwidth is
expressed as
e
n,rms
= e
n
total × √BW
where BW is the bandwidth in hertz.
This analysis is valid for broadband noise calculation. If the
bandwidth of concern includes the chopping frequency, more
complicated calculations must be made to include the effect of
the noise energy spectrum at the chopping frequency (see the
Residual Voltage Ripple section).
With a low source resistance of R
S
< 1 kΩ, the voltage noise
of the amplifier dominates. As source resistance increases, the
thermal noise of R
S
dominates. As the source resistance increases
further, where R
S
> 100 kΩ, the current noise becomes the main
contributor to the total input noise. A good selection table for low
noise op amps can be found in the AN-940 Application Note, Low
Noise Amplifier Selection Guide for Optimal Noise Performance.
Voltage Noise Density with Different Gain Configurations
Figure 65 shows the voltage noise density vs. closed-loop gain of a
zero-drift amplifier from a leading competitor. The voltage noise
density of the amplifier increases from 11 nV/√Hz to 21 nV/√Hz
as the closed-loop gain decreases from 1000 to 1.
24
20
16
12
8
4
0
1 10 100 1000
09437-061
VOLTAGE NOISE DENSITY (nV/√Hz)
CLOSED-LOOP GAIN (V/V)
V
SY
= 5V
f = 100Hz
COMPETITOR A
Figure 65. Competitor A: Voltage Noise Density vs. Closed-Loop Gain
Figure 66 shows the voltage noise density vs. frequency of the
ADA4528-1/ADA4528-2 for three different gain configurations.
The ADA4528-1/ADA4528-2 offer a constant input voltage
noise density of 6 nV/√Hz to 7 nV/√Hz, regardless of the gain
configuration.
1
10
100
1 10 100 1k 10k
VOLTAGE NOISE DENSITY (nV/√Hz)
FREQUENCY (Hz)
A
V
= 10
A
V
= 100
A
V
= 1
V
SY
= 5V
V
CM
= V
SY
/2
09437-062
Figure 66. Voltage Noise Density vs. Frequency with Different Gain
Configurations