Datasheet

ADA4896-2/ADA4897-1/ADA4897-2 Data Sheet
Rev. | Page 18 of 28
DC ERRORS
Figure 46 shows a typical connection diagram and the major
dc error sources.
R
G
– V
IN
+
R
S
– V
IP
+
I
B
+
I
B
–
+ V
OUT
–
R
F
+ V
OS
–
09447-031
Figure 46. Typical Connection Diagram and DC Error Sources
The ideal transfer function (all error sources set to 0 and
infinite dc gain) can be written as
IN
G
F
IP
G
F
OUT
V
R
R
V
R
R
V ×
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−×
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+= 1
(1)
This equation reduces to the familiar forms for noninverting
and inverting op amp gain expressions, as follows:
For noninverting gain (V
IN
= 0 V)
IP
G
F
OUT
V
R
R
V
×
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+=
1
(2)
For inverting gain (V
IP
= 0 V)
IN
G
F
OUT
V
R
R
V ×
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−
=
(3)
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
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+×
⎟
⎠
⎞
⎜
⎝
⎛
+
−
++
=
G
F
OUTPNOM
P
CM
OFFSET
OUT
R
R
A
V
PSRR
VV
CMRR
V
V
V
NOM
ERROR
1
(4)
where:
is the offset voltage at the specified supply voltage,
which is measured with the input and output at midsupply.
V
CM
is the common-mode voltage.
V
P
is the power supply voltage.
V
PNOM
is the specified power supply voltage.
CMRR is the common-mode rejection ratio.
PSRR is the power supply rejection ratio.
A is the dc open-loop gain.
NOM
OFFSET
V
The output error due to the input currents can be estimated as
+−
×
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+×−×
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+×=
B
G
F
S
B
G
F
G
F
OUT
I
R
R
RI
R
R
RRV
ERROR
11)||(
(5)
BIAS CURRENT CANCELLATION
To cancel the output voltage error due to unmatched bias
currents at the inputs, R
BP
and R
BN
can be used (see Figure 47).
R
G
R
S
R
BP
R
BN
R
F
09447-048
Figure 47. Using R
BP
and R
BN
to Cancel Bias Current Error
To compensate for the unmatched bias currents at the two
inputs, set R
BP
and R
BN
as shown in Table 11.
Table 11. Setting R
BN
and R
BP
to Cancel Bias Current Errors
Value of R
F
||R
G
Value of R
BP
(Ω) Value of R
BN
(Ω)
Greater Than R
S
R
F
||R
G
− R
S
0
Less Than R
S
0 R
S
− R
F
||R
G
Table 12 shows sample values for R
BP
and R
BN
when R
F
||R
G
> R
S
and when R
F
||R
G
< R
S
.
Table 12. Examples of R
BN
and R
BP
Settings
Gain R
F
(Ω) R
G
(Ω) R
S
(Ω) R
BP
(Ω) R
BN
(Ω)
[phone] 74.5 0
+10 249 27.4 50 0 25.3
B