FEATURES

− Tiny for High Density Systems

− Precision on One Supply

− Fast Over Eight Decades

− Fully-Tested Function

Eight Decades, 100pA to 10mA

−5

°C to +75°C

APPLICATIONS

AMPLIFIER (EDFA)

OF ANALOG-TO-DIGITAL (ADC) CONVERTER

DESCRIPTION

The LOG114 is specifically designed for measuring

low-level

and

wide

dynamic

range

currents

in

communications,

lasers,

medical,

and

industrial

systems. The device computes the logarithm or log-ratio

of an input current or voltage relative to a reference

current

or

voltage

(logarithmic

transimpedance

amplifier).

High precision is ensured over a wide dynamic range of

input signals on either bipolar (

±5V) or single (+5V)

supply. Special temperature drift compensation circuitry

is included on-chip. In log-ratio applications, the signal

current may be from a high impedance source such as

a photodiode or resistor in series with a low impedance

voltage source. The reference current is provided by a

resistor in series with a precision internal voltage

reference, photo diode, or active current source.

out per decade of input current, which limits the output

so that it fits within a 5V or 10V range. The output can be

scaled and offset with one of the available additional

amplifiers, so it matches a wide variety of ADC input

ranges.

Stable

dc

performance

allows

accurate

measurement

of

low-level

signals

over

a

wide

temperature range. The LOG114 is specified over a

−5

°C to +75°C temperature range and can operate from

−40

°C to +85°C.

All trademarks are the property of their respective owners.

LOG114

SBOS301A − MAY 2004 − REVISED MARCH 2007

Single-Supply, High-Speed, Precision

LOGARITHMIC AMPLIFIER

www.ti.com

Copyright

 2004−2007, Texas Instruments Incorporated

Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of Texas Instruments

semiconductor products and disclaimers thereto appears at the end of this data sheet.

PRODUCTION DATA information is current as of publication date. Products

conform to specifications per the terms of Texas Instruments standard warranty.

Production processing does not necessarily include testing of all parameters.

Q

1

200

Ω

R

1

(1)

1250

Ω

R

2

R

5

R

6

200

Ω

1250

Ω

−IN

4

V

LOGOUT

−IN

5

V

CM IN

4

18

6

7

I

from a photodiode

or other current source

11

10

9(2)

14

5

3

15

13

12

16

2.5V

V

−

Com

Q

2

A

3

(4)

A

1

A

4

LOG114

A

5

I

REF

I

2

V+

REF

provide temperature compensation.

(3) V

(4) Differential Amplifier (A