LOG114AIRGVR datasheet(11/30 Pages) BURR-BROWN

11 / 30 page

LOG114

SBOS301A − MAY 2004 − REVISED MARCH 2007

www.ti.com

APPLICATIONS INFORMATION

OVERVIEW

The LOG114 is a precision logarithmic amplifier that is

capable of measuring currents over a dynamic range of

eight decades. It computes the logarithm, or log ratio,

of an input current relative to a reference current ac-

cording to equation (1).

LOGOUT + 0.375

log

The output at VLOGOUT can be digitized directly, or scaled

for an ADC input using an uncommitted or external op

amp.

An offsetting voltage (VCom) can be connected to the

Com pin to raise the voltage at VLOGOUT. When an

offsetting voltage is used, the transfer function

becomes:

LOGOUT + 0.375

log

) V

Com

Either I1 or I2 can be held constant to serve as the refer-

ence current, with the other input being used for the in-

put signal. The value of the reference current is selected

such that the output at VLOGOUT (pin 9) is zero when the

reference current and input current are equal. An on-

chip 2.5V reference is provided for use in generating the

reference current.

Two additional amplifiers, A4 and A5, are included in the

LOG114 for use in scaling, offsetting, filtering, threshold

detection, or other functions.

BASIC CONNECTIONS

Figure 1 and Figure 2 show the LOG114 in typical dual

and single-supply configurations, respectively. To re-

duce the influence of lead inductance of power-supply

lines, it is recommended that each supply be bypassed

with a 10

µF tantalum capacitor in parallel with a 1000pF

ceramic capacitor as shown in Figure 1 and Figure 2.

Connecting these capacitors as close to the LOG114

V+ supply pin to ground as possible improves supply−

related noise rejection.

100k

Ω

66.5k

Ω

100k

Ω

56.2k

Ω

REF

2.5M

Ω

(2)

−IN4

+IN4

LOGOUT

(1)

+IN5

−IN

VO5

CM IN

16 VREF

2.5V

REF

−

Com

VREF GND

REF

µF

Input Signal

100pA to 10mA

V+

+5V

−5V

1000pF

µF10µF

LOG114

NOTE: (1) V

LOGOUT = 0.375 × log(I1/I2)

(2) VO4 = −0.249 × log(I1/I2)+ 1.5V

Figure 1. Dual Supply Configuration Example for Best Accuracy Over Eight Decades.

(1)

(2)

LOG114AIRGVR Datasheet(PDF) 11 Page - Burr-Brown (TI)

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