+

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

2

1

V

=

LOGOUT

0.375 LOG

10

·

+ 2 V

( (

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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.

Figure 2. LOG114EVM Level Shifter

2.2

Signal Path

The LOG114 has two inputs, I

I

reference internal to the LOG114 and the 2-V output from the level shifter. The reference current into I

given by Equation 1.

(1)

The input current must flow into I

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 (I

the LOG114.

The output from the LOG114 amplifier A3 is called V

and the bias voltage generated by the level shifter is described by Equation 2.

(2)

3

SBOU110

– September 2011

LOG114EVM User Guide

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