®

LOG100

9

FIGURE 10. Current Inverter.

Q

B

National

LM394

I

IN

I

OUT

D

1

D

2

Q

A

FIGURE 11. Connections for Antilog Function.

ANTILOG CONFIGURATION (an implicit technique)

scale the log amp to use as much of the

±10V output range

as possible. Thus, with the range of I

1

from I

1 MIN

to

I

For I

1 MAX

+ 10V = K log I

(24)

For I

1 MIN

– 10V = K log I

(25)

Addition of these two equations and solving for I

its optimum value, I

I

I

I

1 MAX

x I

1 MIN

(26)

K

(27)

Since K is selectable in discrete steps, use the largest value

of K available which does not exceed K

NEGATIVE INPUT CURRENTS

The LOG100 will function only with positive input currents

(conventional current flow into pins 1 and 14). Some current

sources (such as photomultiplier tubes) provide negative

input currents. In such situations, the circuit in Figure 10

VOLTAGE INPUTS

The LOG100 gives the best performance with current in-

puts. Voltage inputs may be handled directly with series

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

mately three decades of input voltage by voltage noise and

offsets. The transfer function of equation (20) applies to this

configuration.

I

REF

14

1

5

43

6

10

7

V

OUT

9

–V

CC

+V

CC

+

–

LOG100

R

C

V

IN

V

–

V

IN

K

K = 1 when V

K = 3 when V

K = 5 when V

The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN assumes

no responsibility for the use of this information, and all use of such information shall be entirely at the user’s own risk. Prices and specifications are subject to change

without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not authorize or warrant

any BURR-BROWN product for use in life support devices and/or systems.

NOTE: (1) More detailed information may be found in “Properly Designed Log

Amplifiers Process Bipolar Input Signals” by Larry McDonald, EDN, 5 Oct. 80,

pp 99–102.

10

I

2 OPT

I

1 MAX

log