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

• Manufacturer Part Number: ADA4896-2
• Description: 1 nV/?Hz, Low Power, Rail-to-Rail Output Amplifiers
• Manufacturer: AD [Analog Devices]

DC ERRORS
Figure 45 shows a typical connection diagram and the major dc
error sources.
The ideal transfer function (all error sources set to 0 and
infinite dc gain) can be written as
This reduces to the familiar forms for inverting and
noninverting op amp gain expressions, as follows:
(Noninverting gain, V
(Inverting gain, V
The total output voltage error is the sum of errors due to the
amplifier offset voltage and input currents. The output error
due to the offset voltage can be estimated as
where:
which is measured with the input and output at midsupply.
VCM is the common-mode voltage.
V
V
CMRR is the common-mode rejection ratio.
PSRR is the power supply rejection ratio.
A is the dc open-loop gain.
The output error due to the input currents can be estimated as
Note that setting R
error due to the input bias current.
V
⎛
⎜
⎝
V
V
V
P
PNOM
OFFSET
OUT
OUT
OFFSET
is the power supply voltage.
V
V
V
ERROR
ERROR
OUT
is the specified power supply voltage.
OUT
OUT
Figure 45. Typical Connection Diagram and DC Error Sources
NOM
– V
– V
NOM
=
=
IN
IP
=
= 1
= 1
is the offset voltage at the specified supply voltage,
(
+
+
⎛ −
⎜
⎜
⎝
+
⎛
⎜
⎜
⎝
⎛
⎜
⎜
⎝
R
R
CMRR
F
R
R
VCM
+
+
R
G
S
G
||
F
R
R
R
R
R
IP
⎞
× ⎟ ⎟
⎠
F
G
G
F
S
G
= 0 V)
equal to R
)
⎞
× ⎟ ⎟
⎠
⎞
× ⎟ ⎟
⎠
V
×
+
IN
IN
I
I
B
V
V
⎛
⎜
⎜
⎝
B
V
+
1
–
= 0 V)
IP
IP
P
+ V
+
PSRR
−
−
R
R
OS
⎛
⎜
⎜
⎝
V
R
G
F
F
F
–
R
R
PNOM
||R
⎞
⎟
⎟
⎠
G
F
I
B
⎞
× ⎟ ⎟
⎠
G
−
compensates for the voltage
−
+
V
R
V
IN
S
OUT
A
×
⎛
⎜
⎜
⎝
1
⎞
× ⎟
⎠
+
+ V
⎛
⎜
⎜
⎝
R
R
1
OUT
G
F
+
⎞
× ⎟ ⎟
⎠
–
R
R
G
F
I
B
⎞
⎟
⎟
⎠
+
Rev. 0 | Page 19 of 28
(1)
(2)
(3)
(4)
(5)
NOISE CONSIDERATIONS
Figure 46 illustrates the primary noise contributors for the
typical gain configurations. The total rms output noise is
the root-mean-square of all the contributions.
The output noise spectral density can be calculated by
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
R
Figure 46
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,
the noise contributions of the amplifier are relatively small for
source resistances between approximately 50 Ω and 700 Ω.
Figure 47
source resistance. In addition, the value of the feedback resistors
used impacts 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
vn _ R
vout
S
F
4
is the source resistance, as shown in
and R
kTR
S
G
_
500
=
=
en
0.5
50
F
5
4kT × R
4kT × R
50
+
G
=
.
shows the total RTI noise due to the amplifier vs. the
are the feedback network resistances, as shown in
⎛
⎜ ⎜
⎝
1
+
Figure 46. Noise Sources in Typical Connection
S
G
R
R
Figure 47. RTI Noise vs. Source Resistance
F
G
⎞
⎟ ⎟
⎠
2
[
R
R
4
G
S
AMPLIFIER NOISE
kTRs
SOURCE RESISTANCE (Ω)
RESISTOR NOISE
AMPLIFIER AND
500
TOTAL
+
ADA4896-2/ADA4897-1
ien
iep
iep
2
R
ven
S
R
2
F
RESISTANCE NOISE
+
F
/R
ven
Figure 46
G
SOURCE
5k
). Note that with a
2
]
vn _ R
+
⎛
⎜ ⎜
⎝
R
R
G
F
F
.
=
⎞
⎟ ⎟
⎠
2
+ vout_en –
4kT × R
4
kTR
S
) are all
50k
G
F
+
ien
(6)
2
R
F
2