Texas Instruments LOG114 Evaluation Module LOG114EVM LOG114EVM Datenbogen
Produktcode
LOG114EVM
+
OPA365
C2
100 nF
VCC
+5 V
R9
4.02 kW
R10
1 kW
2.5 V
from LOG114 VREF
VBIAS
I
=
REF
(2.5 V
2 V)
1 M
-
W
= 500 nA
I
I
I
2
1
V
=
LOGOUT
0.375 LOG
10
·
+ 2 V
( (
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.
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
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
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
the OPA365. The level shifter circuit is shown in
.
Figure 2. LOG114EVM Level Shifter
2.2
Signal Path
The LOG114 has two inputs, I
1
and I
2
. Input I
1
is used for the input signal (usually from a photodiode) and
I
2
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 I
2
is
given by
(1)
The input current must flow into I
1
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
). For proper operation, each input to the LOG114 (I
1
and I
2
) requires the current to flow into
the LOG114.
The output from the LOG114 amplifier A3 is called V
LOGOUT
. The relationship between V
LOGOUT
, I
1
and I
2
,
and the bias voltage generated by the level shifter is described by
.
(2)
3
SBOU110
–
September 2011
LOG114EVM User Guide
Copyright
©
2011, Texas Instruments Incorporated