Datasheet

Data Sheet ADA4896-2/ADA4897-1/ADA4897-2
Rev. | Page 19 of 28
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
=
09447-034
Figure 48. Noise Sources in Typical Connection
The output noise spectral density can be calculated by
[]
2
2
2
2
2
2
2
4414
_
FG
G
F
S
G
F
F
RienkTR
R
R
venRiepkTRs
R
R
kTR
envout
+
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+++
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
++
=
(6)
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
S
is the source resistance, as shown in Figure 48.
R
F
and R
G
are the feedback network resistances, as shown in
Figure 48.
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 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.
50 500
NOISE (nV/
√
Hz)
SOURCE RESISTANCE (Ω)
5
0.5
50
500
5k 50k
TOTAL
AMPLIFIER NOISE
AMPLIFIER AND
RESISTOR NOISE
SOURCE
RESISTANCE NOISE
09447-057
Figure 49. RTI Noise vs. Source Resistance
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 show the most peaking at a gain of +2 (see Figure 9).
Placing a small snub resistor (R
SNUB
) in series with the amplifier
output and the capacitive load mitigates the problem. Figure 50
shows the effect of using a snub resistor (R
SNUB
) on reducing the
peaking for the worst-case frequency response (gain of +2).
Using R
SNUB
= 100  eliminates the peaking entirely, with the
trade-off that the closed-loop gain is reduced by 0.8 dB due to
attenuation at the output. R
SNUB
can be adjusted from 0  to
100  to maintain an acceptable level of peaking and closed-
loop gain (see Figure 50).
–5
–4
–3
–2
–1
0
1
2
3
NORMALIZED CLOSED-LOOP GAIN (dB)
FREQUENCY (MHz)
0.1 1 10 100
R
SNUB
= 50Ω
R
SNUB
= 0Ω
R
SNUB
= 100Ω
ADA4896-2
R
L
1kΩ
R
1
249Ω
R
2
249Ω
C
L
39pF
R
SNUB
V
IN
V
OUT
V
S
= +5V
V
OUT
= 200mV p-p
G = +2
09447-058
Figure 50. Using a Snub Resistor to Reduce Peaking
Due to Output Capacitive Load
B