This result is acceptable in a dual−supply system (V+ = 5V, V− = −5V) where the output can swing below

ground, but this result does not work in a single supply 5V system. Therefore, an offset voltage must be

added to the system.

4. Select an offset voltage, VCOM to use for centering the output between (V−) + 0.6V and (V+) − 0.6V, which

is the full-scale output capability of the A3 amplifier. Choosing VCOM = 2.5V, and recalculating the expected

voltage output range for VLOGOUT using Equation 2, results in:

1V≤VLOGOUT≤4V

(24)

5. The A4 amplifier scales and offsets the VLOGOUT signal for use by the ADC using the equation:

VO4=−GA4×VLOGOUT+VOFFSET

(25)

The A4 amplifier is specified with a rated output swing capability from (V−) +0.5V to (V+) − 0.5V.

Therefore, choose the final A4 output:

0.5V≤VO4≤2.5V

(26)

This output results in a 2V range for the 3V VLOGOUT range, therefore a gain of 2/3 is needed for A4.

6. When I2 = 10mA, VLOGOUT = 1V, and VO4 = 2.5V. Using Equation 25 in step 5:

2.5V=−2V3V× 1V +VOFFSET

(27)

Therefore, VOFFSET = 3.17V

The A4 amplifier configuration for VO4 = −2/3(VLOGOUT) + 3.17 is seen in Figure 7-10.

The overall transfer function is:

VO4=−0.25×log I1I2 +1.5V

(28)

A

1

+

–

R1

R3

A

3

+

–

R2

R4

A

2

+

–

I2

I1

VCM IN

COM

A

4

+

–

VO4

–IN4

+IN4

V+

VREF GND

I2

I1

LOG114

VLOGOUT

REF

VREF

A

5

–

+

VO5

+IN5

–IN5

316k

V–

100k

66.5k

I1

1 F

4.096V

Reference

RREF

1.62M

2.5V

Input current from

photodiode or

current source

I2

100k

0.1 F

5V

A.

In single−supply configuration, VCM IN must be connected to ≥ 1V.

B.

The cathode of the photodiode is returned to VREF resulting in zero bias across the photodiode. The cathode can returned to a voltage

more positive than VCM IN to create a reverse bias for reducing photodiode capacitance, which increases speed.

Figure 7-9. Single-Supply Configuration Example for Measurement Over Eight Decades

LOG114

SBOS301B – MAY 2004 – REVISED MARCH 2025

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