LOG114EVM Datasheet, PDF (4/21 Pages) Texas Instruments – LOG114EVM User Guide

LOG114EVM
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The V_LOGOUT output is then scaled by resistors R7 and R8. These resistors divide the signal before
combining with the high-current linearity correction section. Combining the input signal and linearity
correction signal is accomplished with amplifier A4 and resistors R5 and R6. Capacitor C1 is used to
bypass the power supply for the LOG114. The signal path circuit is shown in Figure 3.
VLOGOUT
High-Current Linearity
Correction Signal
+
VCC
5V
C1
100 nF
R11
I_REF
1 MW
R7
10 kW
R8
4.3 kW
VREF
R6
10 kW
R5
10 kW
I_PHOTO
VBIAS
(from the
A1
Level Shifter)
V+
VREF
A4
VOUT
A3
A2
A5
V-
Figure 3. LOG114EVM Signal Path
2.3 High-Current Linearity Correction
The LOG114 is capable of handling a wide dynamic range of currents, from less than 100 pA in a carefully
designed PCB to 10 mA in high-current applications. Because the LOG114 was designed for high speeds,
the transistors that provide feedback around amplifiers A1 and A2 within the LOG114 have a small series
resistance, R_S. This small series resistance causes a deviation from the ideal LOG114 transfer function at
input currents that exceed approximately 1 mA. The modified equation for V_LOGOUT that shows this
deviation from the ideal is given in Equation 3.
( ^VLOGOUT ^{= 0.375·LOG}10
I₂
I₁
+ 2 V + I₁·R_S
(3)
4
LOG114EVM User Guide
Copyright © 2011, Texas Instruments Incorporated
SBOU110 – September 2011
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