Datasheet

ADA4841-1/ADA4841-2
Rev. E | Page 14 of 20
The output noise spectral density can be calculated by
[]
2
2
2
2
2
2
2
4414
F
G
F
S
G
F
RienkTRg
R
R
venRienkTRs
R
R
kTRf
envout
+
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+++
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
++
=_
(6)
where:
k is Boltzmann’s Constant.
T is the absolute temperature, degrees Kelvin.
ien
is the amplifier input current noise spectral density, pA/√Hz.
ven
is the amplifier input voltage spectral density, nV/√Hz.
R
S
is the source resistance as shown in Figure 40.
R
F
and R
G
are the feedback network resistances, as shown in
Figure 40.
Source resistance noise, amplifier voltage noise (
ven
), and the
voltage noise from the amplifier current noise (
ien
× R
S
) are
all subject to the noise gain term (1 + R
F
/R
G
). Note that with a
2.1 nV/√Hz input voltage noise and 1.4 pA/√Hz input current,
the noise contributions of the amplifier are relatively small for
source resistances between approximately 200 Ω and 30 kΩ.
shows the total RTI noise due to the amplifier vs. the
source resistance. In addition, the value of the feedback resistors
used impacts the noise. It is recommended to keep the value of
feedback resistors between 250 Ω and 1 kΩ to keep the total
noise low.
Figure 41
1000
0.1
10 100k
05614-007
SOURCE RESISTANCE (Ω)
NOISE (nV/ Hz)
100
10
1
100 1k 10k
TOTAL AMPLIFIER NOISE
SOURCE RESISTANCE NOISE
AMPLIFIER + RESISTOR NOISE
Figure 41. RTI Noise vs. Source Resistance
HEADROOM CONSIDERATIONS
The ADA4841-1/ADA4841-2 are designed to provide maximum
input and output signal ranges with 16-bit to 18-bit dc linearity.
As the input or output headroom limits are reached, the signal
linearity degrades.
The input stage positive limit is almost exactly a volt below the
positive supply at room temperature. Input voltages above that
start to show clipping behavior. The positive input voltage limit
increases with temperature with a coefficient of about 2 mV/°C.
The lower supply limit is nominally below the minus supply;
therefore, in a standard gain configuration, the output stage
limits the signal headroom on the negative supply side. Figure 42
and Figure 43 show the nominal CMRR behavior at the limits of
the input headroom for three temperatures—this is generated
using the subtractor topology shown in Figure 44, which avoids
the output stage limitation.
300
–300
3.00 5.00
05614-055
COMMON-MODE VOLTAGE (V)
COMMON-MODE ERROR (μV)
260
220
180
140
100
60
20
–20
–60
–100
–140
–180
–220
–260
3.20 3.40 3.60 3.80 4.00 4.20 4.40 4.60 4.80
–40°C
+25°C
+125°C
Figure 42. +CMV vs. Common-Mode Error vs. V
OS
0
–800
–6.00
–4.00
05614-054
COMMON-MODE VOLTAGE (V)
COMMON-MODE ERROR (μV)
–50
–100
–150
–200
–250
–300
–350
–400
–450
–500
–550
–600
–650
–700
–750
–5.80 –5.60 –5.40 –5.20 –5.00 –4.80 –4.60 –4.40 –4.20
–40°C
+25°C
+125°C
Figure 43. −CMV vs. Common-Mode Error vs. V
OS
+ V
OUT
–
– V
CM
+
05614-051
Figure 44. Common-Range Subtractor