LOG104 Datasheet, PDF (9/13 Pages) Texas Instruments – LOGARITHMIC AND LOG RATIO AMPLIFIER

TOTAL ERROR
The total error is the deviation (expressed in mV) of the
actual output from the ideal output of V_OUT = 0.5V • log(I₁/I₂).
Thus,
V_OUT(ACTUAL) = V_OUT(IDEAL) ± Total Error.
(6)
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 I₁/I₂ is the largest of the two errors when I₁ and I₂ 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.
MEASURING AVALANCHE PHOTODIODE CURRENT
The wide dynamic range of the LOG104 is useful for measuring
avalanche photodiode current (APD), as shown in Figure 13.
LOG CONFORMITY
For the LOG104, log conformity is calculated the same as
linearity and is plotted I₁/I₂ on a semi-log scale. In many
applications, log conformity is the most important specifica-
tion. This is true 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 (I₁/I₂) 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)} = 0.5V/dec • 2Nm V
(7)
where N is the log conformity error, in percent.
+15V to +60V
500Ω
I_SHUNT
5kΩ
5kΩ
+5V
Irx = 1µA to 1mA
Receiver
APD
I to V
Converter
10Gbits/sec
INA168
SOT23-5
2
1
I_OUT
I_OUT = 0.1 • I_SHUNT
+5V
4
1
Q₁
Q₂
100µA
25kΩ
REF3025
2.5V
A₁
8
LOG104
SO-8
5
–5V
C_C
1.2kΩ
1kΩ
3
A₂
OPA703
6
V_OUT = 2.5V to 0V
FIGURE 14. High Side Shunt for Avalanche Photodiode (APD) Measures 3-Decades of APD Current.
LOG104
SBOS243B
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