Datasheet

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LOG104
10
SBOS243C
(8)
(9)
(10)
(11)
(12)
(13)
INDIVIDUAL ERROR COMPONENTS
The ideal transfer function with current input is:
VV
I
I
OUT
=
(
)
•05
1
2
.log
The actual transfer function with the major components of
error is:
VVK
II
II
Nm V
OUT
B
B
OS O
=
(
)
±
(
)
±±05 1
11
22
. log
–
–
∆
The individual component of error is:
∆K = gain accuracy (0.15%, typ), as specified in
specification table.
I
B1
= bias current of A
1
(5pA, typ)
I
B2
= bias current of A
2
(5pA, typ)
N = log conformity error (0.01%, 0.06%, typ)
0.01% for n = 5, 0.06% for n = 7.5
V
OSO
= output offset voltage (3mV, typ)
m = number of decades over which N is specified:
Example: what is the error when
I
1
= 1µA and I
2
= 100nA
VmV
OUT
=±
(
)
−
−
±
(
)
(
)
±
−−
−−
0 5 0 0015
10 5 10
10 5 10
2 0 00015 3 0
612
712
. . log
•
•
..
= 0.5055V
Since the ideal output is 0.5V, the error as a percent of
reading is
%
.
.
%.%error =•=
0 5055
05
100 1 1
For the case of voltage inputs, the actual transfer function is
VVK
V
R
I
E
R
V
R
I
E
R
Nm V
OUT
B
OS
B
OS
OSO
=
(
)
±
(
)
±
±
±±05 1
1
1
1
1
1
2
2
2
2
2
.log
–
–
∆
Where
E
R
and
E
R
OS OS1
1
2
2
are considered to be zero for large
values of resistance from external input current sources.