Datasheet
Data Sheet ADA4895-1/ADA4895-2
Rev. A | Page 17 of 24
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
–
10186-042
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 expressed as follows:
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 the errors due to the
amplifier offset voltage and input currents. The output error due
to the offset voltage can be estimated as follows:
ERROR
OUT
V
= (4)
+×
+
−
++
G
F
OUTPNOM
P
CM
OFFSET
R
R
A
V
PSRR
VV
CMRR
V
V
NOM
1
where:
NOM
OFFSET
V
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.
CMRR is the common-mode rejection ratio.
V
P
is the power supply voltage.
V
PNOM
is the specified power supply voltage.
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 follows:
+−
×
+×−×
+×=
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, Resistors R
BP
and R
BN
can be used (see Figure 47).
R
G
R
S
R
BP
R
BN
R
F
10186-043
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 Resistors R
BP
and R
BN
as shown in Table 10.
Table 10. Setting R
BP
and R
BN
to Cancel Bias Current Error
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