LOG114AIRGVR datasheet(18/30 Pages) BURR-BROWN

18 / 30 page

LOG114

SBOS301A − MAY 2004 − REVISED MARCH 2007

www.ti.com

VOLTAGE INPUTS

The LOG114 provides the best performance with cur-

rent inputs. Voltage inputs may be handled directly by

using a low-impedance voltage source with series resis-

tors, but the dynamic input range is limited to approxi-

mately three decades of input voltage. This limitation

exists because of the magnitude of the required input

voltage and size of the corresponding series resistor.

For 10nA of input current, a 10V voltage source and a

Ω resistor would be required. Voltage and current

noise from these sources must be considered and can

limit the usefulness of this technique.

APPLICATION CIRCUITS

LOG RATIO

One of the more common uses of log ratio amplifiers is

to measure absorbance. See Figure 10 for a typical ap-

plication. Absorbance of the sample is A = log

λ1′/λ1. If

D1 and D2 are matched, A ∝ (0.375V) log(I1/I2).

Sample

1′

Light

Source

Q 1

R 1

LO G114

R 3

V O4

(2)

− IN

+IN4

V LOGOUT

(1)

+IN

− IN

VO5

V CM IN

VREF

2.5VREF

−

V REF GND

Q 2

V+

Com

+5V

NO TE S: (1) VLOGOUT =0.375 × log(I1/I2).

(2) VO4 =0.375 × K × log(I1/I2)

K= 1 + R6/R5.

Figure 10. Using the LOG114 to Measure Absorbance.

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