A similar process can be used to configure an external rail-to-rail output op amp, such as the OPA383. The

OPA383 op amp can swing down to almost 0V (for details, refer to the OPA383 data sheet), therefore the scaling

factor can be approximated to be 2.5/3 and the corresponding VOFFSET is 1.24V. Figure 7-10 shows this circuit

configuration.

37.4k

100k

OPA383

–

+

V

REF

2.5V

100k

37.4k

V

LOGOUT

V

OUT = -5/6*VLOGOUT + 1.24

Figure 7-10. Operational Amplifier Configuration for Scaling and Offsetting the Output Going to ADC

Stage

7.2.3 Advantages of Dual−Supply Operation

The LOG114 performs well on a single 5V supply by level-shifting COM pin to half-supply and raising the

common-mode voltage (VCM IN pin) of the input amplifiers. This level-shift places the input amplifiers in the linear

operating range. However, there are also some advantages to operating the LOG114 on dual ±5V supplies.

These advantages include:

1. Eliminating the need for the 4.096V precision reference

2. Eliminating a small additional source of error arising from the noise and temperature drift of the level-shifting

voltage

3. Allowing increased magnitude of a reverse bias voltage on the photodiode

7.2.4 Log Ratio

One of the more common uses of log ratio amplifiers is to measure absorbance. See Figure 7-11 for a typical

application. Absorbance of the sample is A = log λ1′/λ1. If D1 and D2 are matched, A ∝ (0.375V) log(I1/I2).

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

R5

R6

V–

200

200

1.25k

1.25k

I2

I1

D1

D2

1

Sample

Light

Source

1

1'

5V

A.

VLOGOUT = 0.375 × log(I1/I2).

B.

VO4 = 0.375 × K × log(I1/I2), K = 1 + R6/R5.

Figure 7-11. Using the LOG114 to Measure Absorbance

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LOG114

SBOS301B – MAY 2004 – REVISED MARCH 2025

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