Datasheet

ADA4895-1/ADA4895-2 Data Sheet
Rev. A | Page 18 of 24
NOISE CONSIDERATIONS
Figure 48 illustrates the primary noise contributors for the
typical gain configurations. The total rms output noise is
the root mean square of all the contributions.
R
G
R
S
iep
ien
+ vout_en –
R
F
ven
4kT × R
S
vn _ R
S
=
4kT × R
G
vn _ R
G
=
4kT × R
F
vn _ R
F
=
10186-044
Figure 48. Noise Sources in Typical Gain Configurations
The output noise spectral density can be calculated as follows:
vout_en
= (6)
[ ]
2
2
2
2
2
2
2
4414
F
G
G
F
SS
G
F
F
RienkTR
R
R
venRiepkTR
R
R
kTR +








+++








++
where:
k is Boltzmann’s constant.
T is the absolute temperature (degrees Kelvin).
R
F
and R
G
are the feedback network resistances, as shown in
Figure 48.
R
S
is the source resistance, as shown in Figure 48.
iep
and
ien
represent the amplifier input current noise spectral
density (pA/√Hz).
ven
is the amplifier input voltage noise spectral density (nV/√Hz).
Source resistance noise, amplifier voltage noise (
ven
), and the
voltage noise from the amplifier current noise (
iep
× R
S
) are all
subject to the noise gain term (1 + R
F
/R
G
). Note that with a
1 nV/√Hz input voltage noise and a 1.7 pA/√Hz input current
noise, the noise contributions of the amplifier are relatively
small for source resistances from approximately 50 Ω to 700 Ω.
Figure 49 shows the total RTI noise due to the amplifier vs. the
source resistance. In addition, the value of the feedback resistors
affects the noise. It is recommended that the value of the feedback
resistors be maintained between 250 Ω and 1 kΩ to keep the
total noise low.
50 500
NOISE (nV/√Hz)
SOURCE RESISTANCE (Ω)
5
0.
5
50
500
5k 50k
AMPLIFIER NOISE
AMPLIFIER AND
RESISTOR NOISE
SOURCE
RESISTANCE NOISE
10186-045
Figure 49. RTI Noise vs. Source Resistance