Datasheet

ADA4941-1
Rev. C | Page 17 of 24
Table 7, Table 8, and Table 9 show typical error budgets for the
circuits shown in Figure 48, Figure 49, and Figure 50.
R
F
= 1.0 kΩ, R
G
= 4.99 kΩ, R
S
_IN = 825 Ω, R
S
_REF = 0 Ω
Table 7. Output Voltage Error Budget for G = 2.4 Amplifier
Shown in Figure 48
Error
Source
Typical
Value VOP_error VON_error V
O
_dm_error
V
OS
_A1 0.1 mV +0.12 mV −0.12 mV +0.24 mV
I
BP
_A1 3 µA +2.48 mV −2.48 mV −4.96 mV
I
BN
_A1 3 µA −2.48 mV +2.48 mV +4.96 mV
V
OS
_A2 0.1 mV 0 mV +0.2 mV +0.2 mV
Total V
O
_error, dm = 0.44 mV
R
F
= 0 Ω, R
G
= ∞, R
S
_IN = 0 Ω, R
S
_REF = 0 Ω
Table 8. Output Voltage Error Budget for Amplifier Shown
in Figure 49
Error
Source
Typical
Value
VOP_error VON_error V
O
_dm_error
V
OS
_A1 0.1 mV +0.1 mV −0.1 mV +0.2 mV
I
BP
_A1 3 µA +2.48 mV −2.48 mV −4.96 mV
I
BN
_A1 3 µA −2.48 mV +2.48 mV +4.96 mV
V
OS
_A2 0.1 mV 0 mV +0.2 mV +0.2 mV
Total V
O
_error, dm = 0.4 mV
R
F
= 1.02 kΩ, R
G
= 665 Ω, R
S
_IN = 402 Ω, R
S
_REF = 0 Ω
Table 9. Output Voltage Error Budget for G = 5 Amplifier
Shown in Figure 50
Error
Source
Typical
Value VOP_error VON_error V
O
_dm_error
V
OS
_A1
0.1 mV
+0.25 mV
−0.25 mV
+0.5 mV
I
BP
_A1 3 µA +1.21 mV −1.21 mV −2.4 mV
I
BN
_A1 3 µA −1.21 mV +1.21 mV +2.4 mV
V
OS
_A2 0.1 mV 0 mV +0.2 mV +0.2 mV
Total V
O
_error, dm = 0.7 mV
OUTPUT VOLTAGE NOISE
1kΩ
1kΩ
R
G
R
F
R
S
ip
–
A2
in
–
A2
vn
–
A1
500Ω
A2
A1
REF
IN
2
8
4
5
FB
OUT+
+
–
VOP
1
OUT–
+
–
VON
vn
–
A2
R
S–
REF
ip
–
A1
in
–
A1
05704-057
√4kT (1kΩ)
√4kT (1kΩ)
√4kT (500Ω)
√4kT (R
S–
REF)√4kTR
S
√4kTR
G
√4kTR
F
Figure 52. Noise Sources
Figure 52 shows the major contributors to the ADA4941-1
differential output voltage noise. The differential output noise
mean-square voltage equals the sum of twice the noise mean-
square voltage contributions from the noninverting channel
(A1), plus the noise mean-square voltage terms associated with
the inverting channel (A2).
[ ]
[ ]
2
2
2
2
2
2
2
2
_41
24242
_2)_(1
2)_(12
_,
nVONkTR
R
R
R
R
kTRkTR
RA1inRA1ip
R
R
A1vn
R
R
ndmV
S
G
F
G
F
G
F
F
S
G
F
G
F
O
+








×








+
×+






×+
+×+








××








+
×+








×








+
=
(13)
where
VON_n
2
is calculated as
( )
[ ]
[ ]
)(16(500)16(1000)8
)_(1000)_500)(_(4
4
22
22
_REFRkTkTkT
A2inREFRA2ip
vn_A2VON_n
S
S
++
+++
+=
(14)
where:
vn_A1
and
vn_A2
are the input voltage noises of A1 and A2,
each equal to 2.1 nV/√Hz.
in_A1
,
in_A2
,
ip_A1
, and
ip_A2
are amplifier input current
noise terms, each equal to 1 pA/√Hz.
R
S
, R
F
, and R
G
are the external source, feedback, and gain
resistors, respectively.
kT is Boltzmann’s constant times absolute temperature, equal to
4.2 x 10
-21
W-s at room temperature.
R
S
_REF is any source resistance at the REF pin.
When A1 is used as a unity gain follower, the output voltage
noise spectral density is at its minimum, 10 nV/√Hz. Higher
voltage gains have higher output voltage noise.
Table 10, Table 11, and Table 12 show the noise contributions
and output voltage noise for the circuits in Figure 48, Figure 49,
and Figure 50.