LOG112, 2112

9

SBOS246C

www.ti.com

FIGURE 8. Precision Current Inverter/Current Source.

1k

Ω

+5V

+5V

1.5k

Ω

1.5k

Ω

100k

Ω

10nA to 1mA

LOG112

10nA to 1mA

100k

Ω

100k

Ω

+3.3V

+3.3V

1/2

OPA2335

1/2

OPA2335

Photodiode

Back Bias

Pin 1 or Pin 14

100k

Ω

VOLTAGE INPUTS

The LOG112 and LOG2112 give the best performances with

current inputs. Voltage inputs may be handled directly with

series resistors, but the dynamic input range is limited to

approximately three decades of input voltage by voltage

noise and offsets. The transfer function of Equation 13

applies to this configuration.

ACHIEVING HIGHER ACCURACY WITH HIGHER

INPUT CURRENTS

As input current to the LOG112 increases, output accuracy

degrades. For a 4.5mA input current on

±5V supplies and a

10mA input current on

±12V supplies, total output error can

be between 15% and 25%. Applying a common-mode volt-

transistors out of saturation and reduces output error to

approximately 10%. To avoid forward biasing a photodiode,

reverse bias the photodiode, apply a more positive voltage to

the cathode than the anode.

APPLICATION CIRCUITS

LOG RATIO

One of the more common uses of log ratio amplifiers is

to measure absorbance. See Figure 10 for a typical application.

Absorbance of the sample is A = log

λ

(3)

(4)

FIGURE 9. Extending Input Current Level and Improving Accuracy by Applying a Common-Mode Voltage.

A

2

A

1

Q

1

Q

2

C

C

I

1

V

REF – GND

V

O3

V+ = +5V

GND

LOG112

A

3

R

2

R

1

V

REF

100k

Ω

+2.5V

150k

Ω

R

REF

+IN3

V

LOGOUT

–IN3

V– = –5V

V

CM

I

2

V

REF

65

3

4

7

9

10

13

11

8

14

1

+1.5V