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LOG104

7

SBOS243B

FIGURE 7. Precision Current Inverter/Current Source.

1.5k

Ω

Photodiode

10nA to 1mA

10nA to 1mA

+5V

+5V

1.5k

Ω

+3.3V

+3.3V

1/2 OPA2335

1/2 OPA2335

OPA2335

1

2

BSH203

TLV271 or

Back Bias

LOG104

Pin 1 or Pin 8

OPERATION ON SINGLE SUPPLY

Many applications do not have the dual supplies required to

operate the LOG104. Figure 10 shows the LOG104 config-

ured for operation with a single +5V supply.

VOLTAGE INPUTS

The LOG104 gives the best performance 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.

FIGURE 8. Precision Current Inverter/Current Source.

1.5k

Ω

+5V

+5V

1.5k

Ω

1.5k

Ω

100k

Ω

10nA to 1mA

LOG104

10nA to 1mA

100k

Ω

100k

Ω

+3.3V

+3.3V

1/2 OPA2335

1/2 OPA2335

Photodiode

Back Bias

Pin 1 or Pin 8

100k

Ω

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 9.

Absorbance of the sample is A = log

λ

(3)

(4)

DATA COMPRESSION

In many applications the compressive effects of the logarith-

mic transfer function are useful. For example, a LOG104

preceding a 12-bit A/D converter can produce the dynamic

range equivalent to a 20-bit converter.

FIGURE 9. Absorbance Measurement.

I

2

I

1

8

1

5

6

3

V

OUT

4

V+

V–

C

C

LOG104

D

2

D

1

Sample

λ

1

λ

λ

1

Light

Source

8

1

3

6

4

5

1

2

35

4

–5V

1

µF

LOG104

1

µF

I

1

I

2

V

OUT

C

C

1

µF

Single Supply +5V

(1) TPS60402DBV negative charge pump.

FIGURE 10. Single +5V Power-Supply Operation.