ADA4896-2ACP-EBZ AD [Analog Devices], ADA4896-2ACP-EBZ Datasheet - Page 18

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

ADA4896-2/ADA4897-1/ADA4897-2
DC ERRORS
Figure 46 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 equation reduces to the familiar forms for noninverting
and inverting op amp gain expressions, as follows:
For noninverting gain (V
For 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.
V
V
V
CMRR is the common-mode rejection ratio.
PSRR is the power supply rejection ratio.
A is the dc open-loop gain.
V
V
⎛
⎜
⎝
V
CM
P
PNOM
OFFSET
OUT
OFFSET
is the power supply voltage.
is the common-mode voltage.
V
V
V
ERROR
is the specified power supply voltage.
Figure 46. Typical Connection Diagram and DC Error Sources
OUT
OUT
OUT
NOM
– V
– V
NOM
=
IN
IP
= 1
= 1
is the offset voltage at the specified supply voltage,
=
+
+
+
⎛
⎜
⎜
⎝
⎛
⎜
⎜
⎝
⎛ −
⎜
⎜
⎝
CMRR
R
R
V
G
S
+
R
+
R
CM
G
R
R
R
R
F
G
F
G
F
⎞
⎟
⎟
⎠
IP
⎞
⎟
⎟
⎠
⎞
⎟
⎟
⎠
×
+
I
I
B
= 0 V)
B
×
×
V
+
–
V
IN
+ V
V
V
IN
P
IP
= 0 V)
IP
PSRR
OS
−
R
−
V
F
–
⎛
⎜
⎜
⎝
PNOM
R
R
F
G
⎞
⎟
⎟
⎠
+
×
V
V
OUT
A
IN
+ V
⎞
⎟
⎠
×
OUT
⎛
⎜
⎜
⎝
1
–
+
R
R
G
F
⎞
⎟
⎟
⎠
Rev. | Page 18 of 28
(1)
(2)
(3)
(4)
B
The output error due to the input currents can be estimated as
BIAS CURRENT CANCELLATION
To cancel the output voltage error due to unmatched bias
currents at the inputs, R
To compensate for the unmatched bias currents at the two
inputs, set R
Table 11. Setting R
Value of R
Greater Than R
Less Than R
Table 12 shows sample values for R
and when R
Table 12. Examples of R
Gain
+2
+10
V
OUT
ERROR
=
Figure 47. Using R
F
(
R
249
249
||R
S
R
F
BP
F
||R
F
(Ω)
||
G
and R
S
G
R
R
R
< R
G
G
S
)
BN
×
BN
S
.
⎛
⎜
⎜
⎝
and R
Value of R
R
0
R
249
27.4
as shown in Table 11.
1
F
BP
G
BP
+
||R
BN
(Ω)
and R
and R
R
R
G
R
R
and R
F
G
− R
BN
BP
BP
⎞
⎟
⎟
⎠
BN
to Cancel Bias Current Errors
×
S
BN
to Cancel Bias Current Error
BP
I
R
50
50
BP
B
can be used (see Figure 47).
R
BP
S
(Ω)
−
F
Settings
(Ω)
and R
−
R
S
×
BN
⎛
⎜
⎜
⎝
R
74.5
Value of R
0
R
0
1
when R
S
BP
Data Sheet
+
− R
(Ω)
R
R
F
F
G
||R
⎞
⎟
⎟
⎠
F
G
||R
×
BN
R
0
25.3
I
G
BN
(Ω)
B
> R
+
(Ω)
(5)
S