LOG100 Datasheet, PDF (9/9 Pages) Burr-Brown (TI) – Precision LOGARITHMIC AND LOG RATIO AMPLIFIER

scale the log amp to use as much of the ±10V output range
as possible. Thus, with the range of I₁ from I_{1 MIN} to
I_{1 MAX} ;
For I_{1 MAX}
+ 10V = K log I_{1 MAX}/I₂
(24)
For I_{1 MIN}
– 10V = K log I_{1 MIN}/I₂
(25)
Addition of these two equations and solving for I₂ shows that
its optimum value, I_{2 OPT}, is the geometric mean of I_{1 MAX} and
I_{1 MIN}.
I_{2 OPT} = I_{1 MAX} x I_{1 MIN}
(26)
K_OPT =
10
I
log ^{1 MAX}
I_{2 OPT}
(27)
Since K is selectable in discrete steps, use the largest value
of K available which does not exceed K_OPT.
NEGATIVE INPUT CURRENTS
The LOG100 will function only with positive input currents
(conventional current flow into pins 1 and 14). Some current
sources (such as photomultiplier tubes) provide negative
input currents. In such situations, the circuit in Figure 10
may be used.⁽¹⁾
VOLTAGE INPUTS
The LOG100 gives the best performance with current in-
puts. Voltage inputs may be handled directly with series
resistors, but the dynamic input range is limited to approxi-
mately three decades of input voltage by voltage noise and
offsets. The transfer function of equation (20) applies to this
configuration.
NOTE: (1) More detailed information may be found in “Properly Designed Log
Amplifiers Process Bipolar Input Signals” by Larry McDonald, EDN, 5 Oct. 80,
pp 99–102.
I_IN
Q_A
D₁
Q_B
National
LM394
D₂
I_OUT
FIGURE 10. Current Inverter.
ANTILOG CONFIGURATION (an implicit technique)
–V_CC
9
1
7
I_REF
LOG100
14
6
10
54 3
R
+
V_OUT
–
+V_CC
V_IN
V_IN
V_OUT = I_REF R Antilog – _K
C_C = 0.01µF
K = 1 when V_IN connected to pin 3.
K = 3 when V_IN connected to pin 4.
K = 5 when V_IN connected to pin 5.
FIGURE 11. Connections for Antilog Function.
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no responsibility for the use of this information, and all use of such information shall be entirely at the user’s own risk. Prices and specifications are subject to change
without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not authorize or warrant
any BURR-BROWN product for use in life support devices and/or systems.
®
9
LOG100