MAT12

Data Sheet

Rev. A | Page 8 of 12

APPLICATIONS INFORMATION

FAST LOGARITHMIC AMPLIFIER

The circuit of Figure 16 is a modification of a standard logarith-

mic amplifier configuration. Running the MAT12 at 2.5 mA per

side (full scale) allows for a fast response with a wide dynamic

range. The circuit has a seven decade current range and a five

decade voltage range, and it is capable of 2.5 µs settling time to

1% with a 1 V to 10 V step. The output follows the equation:

IN

REF

2

2

3

O

V

V

q

kT

R

R

R

V

ln

+

=

To compensate for the temperature dependence of the kT/q term, a

resistor with a positive 0.35%/°C temperature coefficient is selected

for R2. The output is inverted with respect to the input and is

nominally −1 V/decade using the component values indicated.

1

4

3

2

8

7

5

6

–15V

330pF

R3

7.5kΩ

R2

500Ω

R2 = 0.35%/°C

330pF

VO

+15V

RS

4kΩ

R1

4kΩ

4kΩ

VIN

(0V TO 10V)

VREF

10V

AD8512

MAT12

1/2

AD8512

Figure 16. Fast Logarithmic Amplifier