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LOG102

8

SBOS211A

FIGURE 10. Absorbance Measurement.

I

2

I

1

14

1

9

10

5

V

OUT

6

V+

V–

C

C

LOG102

D

2

D

1

Sample

λ

1

λ

λ

1

Light

Source

APPLICATION CIRCUITS

LOG RATIO

One of the more common uses of log ratio amplifiers is

to measure absorbance. A typical application is shown in

Figure 10.

Absorbance of the sample is A = log

λ

(3)

(4)

FIGURE 9. Reverse Biased Photodiode Using Pin 10 on

LOG102.

FIGURE 8. Precision Current Inverter/Current Source.

1.5k

Ω

+5V

+5V

1.5k

Ω

1.5k

Ω

100k

Ω

10nA to 1mA

LOG102

10nA to 1mA

100k

Ω

100k

Ω

+3.3V

+3.3V

1/2 OPA2335

1/2 OPA2335

NOTE: OPA2335 Available Q2 2002

Photodiode

Back Bias

Pin 1 or Pin 14

100k

Ω

I

1

Signal

C

C

(–V

I

2

REF

(–V

–V

RB

LOG102

6

9

10

14

15

V+

V

OUT

V–

DATA COMPRESSION

In many applications the compressive effects of the logarith-

mic transfer function are useful. For example, a LOG102

preceding a 12-bit Analog-to-Digital (A/D) converter can

produce the dynamic range equivalent to a 20-bit converter.

NOTES: (1) –V

V