Datasheet

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LOG101
9
SBOS242B
TOTAL ERROR
The total error is the deviation (expressed in mV) of the
actual output from the ideal output of V
OUT
= 1V • log (I
1
/I
2
).
Thus,
V
OUT(ACTUAL)
= V
OUT(IDEAL)
± Total Error.
It represents the sum of all the individual components of error
normally associated with the log amp when operated in the
current input mode. The worst-case error for any given ratio
of I
1
/I
2
is the largest of the two errors when I
1
and I
2
are
considered separately. Temperature can affect total error.
ERRORS RTO AND RTI
As with any transfer function, errors generated by the func-
tion itself may be Referred-to-Output (RTO) or Referred-to-
Input (RTI). In this respect, log amps have a unique property:
Given some error voltage at the log amp’s output, that error
corresponds to a constant percent of the input regardless of
the actual input level.
USING A LARGER REFERENCE VOLTAGE
REDUCES OFFSET ERRORS
Using a larger reference voltage to create the reference
current minimizes errors due to the LOG101’s input offset
voltage. Maintaining an increasing output voltage as a func-
tion of increasing photodiode current is also important in
many optical sensing applications. All zeros from the
A/D converter output represent zero or low-scale photodiode
current. Inputting the reference current into I
1
, and designing
I
REF
such that it is as large or larger than the expected
maximum photodiode current is accomplished using this
requirement. The LOG101 configured with the reference
current connecting I
1
and the photodiode current connecting
A
1
A
2
LOG101
Q
1
Q
2
OPA703
R
2
R
3
R
2
6
R
3
C
C
V
OUT
V
REF
I
REF
I
REF
=
I
MIN
to I
MAX
V
REF
R
1
V
OUT
= V
REF
– • (1V)LOG
( )
R
2
R
3
I
REF
I
PHOTO
I
1
I
2
8
1
I
PHOTO
A/D
Converter
R
1
V
MIN
to V
MAX
3
FIGURE 13. Technique for Using Full-Scale Reference Current Such that V
OUT
Increases with Increasing Photodiode Current.
to I
2
is shown in Figure 13. The OPA703 is configured as a
level shifter with inverting gain and is used to scale the
photodiode current directly into the A/D converter input
voltage range.
The wide dynamic range of the LOG101 is also useful for
measuring avalanche photodiode current (APD) (see Figure 14).
LOG CONFORMITY
For the LOG101, log conformity is calculated the same as
linearity and is plotted I
1
/I
2
on a semi-log scale. In many
applications, log conformity is the most important specifica-
tion. This is because bias current errors are negligible
(5pA compared to input currents of 100pA and above) and
the scale factor and offset errors may be trimmed to zero or
removed by system calibration. This leaves log conformity as
the major source of error.
Log conformity is defined as the peak deviation from the best
fit straight line of the V
OUT
versus log (I
1
/I
2
) curve. This is
expressed as a percent of ideal full-scale output. Thus, the
nonlinearity error expressed in volts over m decades is:
V
OUT(NONLIN)
= 1V/dec • 2NmV
where N is the log conformity error, in percent.
INDIVIDUAL ERROR COMPONENTS
The ideal transfer function with current input is:
VV
I
I
OUT
=
(
)
•1
1
2
log
The actual transfer function with the major components of
error is:
VVK
II
II
Nm V
OUT
B
B
OS O
=
(
)
±
(
)
±±11 2
11
22
∆ log
–
–
(6)
(7)
(8)
(9)