LOG114EVM Datasheet, PDF (3/21 Pages) Texas Instruments – LOG114EVM User Guide

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LOG114EVM
2 LOG114EVM
The LOG114EVM comes completely assembled and constructed to operate on a single 5-V power supply.
The circuit used in the LOG114EVM is designed to operate over a wide range of input currents (10 nA to
10 mA) and scales the output voltage from 1.15 V to 2.75 V. There are three distinct sections to the
LOG114EVM circuit: a precision level shifter, the signal path, and the high-current linearity correction
section. Each section is described below to familiarize you with the circuit functionality and operation.
2.1 Level Shifter
A precision level shifter is required to operate the LOG114 on a single power supply. On the
LOG114EVM, the level shifter circuit is constructed by deriving a bias voltage of 2 V from the 2.5-V
voltage reference internal to the LOG114. This level shift of 2 V is required to keep the nodes internal to
the LOG114 properly biased when operating from a single power supply. Resistors R9 and R10 form a
voltage divider that is then buffered by the OPA365 amplifier. The output of the OPA365 is a fixed voltage
of 2 V, independent of the power-supply voltage or input signal. C2 is used to bypass the power supply for
the OPA365. The level shifter circuit is shown in Figure 2.
2.5 V
from LOG114 VREF
R10
1 kW
+
C2
100 nF
VCC
+5 V
R9
4.02 kW
OPA365
VBIAS
Figure 2. LOG114EVM Level Shifter
2.2 Signal Path
The LOG114 has two inputs, I₁ and I₂. Input I₁ is used for the input signal (usually from a photodiode) and
I₂ is a fixed-current reference signal. This fixed-current reference signal is derived from the 2.5-V voltage
reference internal to the LOG114 and the 2-V output from the level shifter. The reference current into I₂ is
given by Equation 1.
(2.5 V - 2 V)
I_REF = _{1 MW} = 500 nA
(1)
The input current must flow into I₁ and comes directly from the anode of a photodiode. In non-optical
applications, the input current could also be derived from a resistor, or a variety of other sources (see
Section 3.4). For proper operation, each input to the LOG114 (I₁ and I₂) requires the current to flow into
the LOG114.
The output from the LOG114 amplifier A3 is called V_LOGOUT. The relationship between V_LOGOUT, I₁ and I₂,
and the bias voltage generated by the level shifter is described by Equation 2.
( ^VLOGOUT ^{= 0.375·LOG}10
I₂
I₁
+2V
(2)
SBOU110 – September 2011
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LOG114EVM User Guide
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