ADA4896-2ACP-EBZ AD [Analog Devices], ADA4896-2ACP-EBZ Datasheet - Page 19

• Manufacturer Part Number: ADA4896-2ACP-EBZ
• Description: 1 nV/?Hz, Low Power
• Manufacturer: AD [Analog Devices]

Data Sheet
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.
vn _ R
The output noise spectral density can be calculated by
where:
k is Boltzmann’s constant.
T is the absolute temperature (degrees Kelvin).
iep and ien represent the amplifier input current noise spectral
density (pA/√Hz).
ven is the amplifier input voltage noise spectral density (nV/√Hz).
R
R
Figure 48.
Source resistance noise, amplifier voltage noise ( ven ), and the
voltage noise from the amplifier current noise ( iep × R
subject to the noise gain term (1 + R
1 nV/√Hz input voltage noise and 2.8 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
used 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.
vn _ R
vout
S
F
4
is the source resistance, as shown in Figure 48.
and R
kTR
S
G
_
=
=
en
F
4kT × R
4kT × R
+
G
=
are the feedback network resistances, as shown in
⎛
⎜ ⎜
⎝
1
+
Figure 48. Noise Sources in Typical Connection
S
G
R
R
G
F
⎞
⎟ ⎟
⎠
2
[
R
R
4
G
S
kTRs
+
ien
iep
iep
2
R
ven
S
R
2
F
+
F
/R
ven
G
). Note that with a
2
]
vn _ R
+
⎛
⎜ ⎜
⎝
R
R
F
G
F
=
⎞
⎟ ⎟
⎠
2
+ vout_en –
4kT × R
4
kTR
S
) are all
G
F
+
ien
Rev. | Page 19 of 28
(6)
2
B
R
F
2
CAPACITANCE DRIVE
Capacitance at the output of an amplifier creates a delay within the
feedback path that, if within the bandwidth of the loop, can create
excessive ringing and oscillation. The ADA4896-2/ADA4897-1/
ADA4897-2
Placing a small snub resistor (R
output and the capacitive load mitigates the problem. Figure 50
shows the effect of using a snub resistor (R
peaking for the worst-case frequency response (gain of +2).
Using R
trade-off that the closed-loop gain is reduced by 0.8 dB due to
attenuation at the output. R
100 Ω to maintain an acceptable level of peaking and closed-
loop gain (see Figure 50).
500
–1
–2
–3
–4
–5
0.5
50
3
2
1
0
5
0.1
SNUB
ADA4896-2/ADA4897-1/ADA4897-2
50
V
V
G = +2
V
S
OUT
IN
Figure 50. Using a Snub Resistor to Reduce Peaking
R
= +5V
= 100 Ω eliminates the peaking entirely, with the
show the most peaking at a gain of +2 (see Figure 9).
SNUB
= 200mV p-p
249Ω
Figure 49. RTI Noise vs. Source Resistance
R
= 100Ω
1
Due to Output Capacitive Load
ADA4896-2
AMPLIFIER NOISE
RESISTOR NOISE
SOURCE RESISTANCE (Ω)
249Ω
AMPLIFIER AND
500
R
1
2
FREQUENCY (MHz)
TOTAL
R
SNUB
SNUB
SNUB
= 50Ω
can be adjusted from 0 Ω to
R
SNUB
) in series with the amplifier
R
RESISTANCE NOISE
SNUB
R
1kΩ
V
L
OUT
10
5k
SOURCE
= 0Ω
SNUB
C
39pF
L
) on reducing the
100
50k