of input voltage. This limitation exists because of the magnitude of the required input voltage and size of the

corresponding series resistor. For 10nA of input current, a 10V voltage source and a 1GΩ resistor are required.

Voltage and current noise from these sources must be considered and can limit the usefulness of this technique.

7.1.6 High-Current Linearity Correction

The LOG114 is capable of handling a wide dynamic range of currents, from less than 100pA in a carefully

designed PCB to 10mA in high-current applications. The LOG114 was designed for high speeds, therefore the

transistors that provide feedback around amplifiers A1 and A2 within the LOG114 have a small series resistance,

RS. This small series resistance causes a deviation from the LOG114 transfer function at input currents that

exceed approximately 1mA. The modified equation for VLOGOUT that shows this deviation and is given in

Equation 5.

VLOGOUT=0.375×log I1I2 +I1×RS+2

(5)

The high-current linearity correction circuit (refer to Figure 7-7) creates an error signal that is proportional to input

current I1 by using R2, R3 and R4, and amplifier A5. Resistor R1 is used to properly level-shift the resulting output

signal. The signal at the output from amplifier A5 is then coupled to the input of amplifier A4 in a manner that

subtracts the error signal from the output, VLOGOUT.

A

1

+

–

A

3

+

–

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

V–

3

IPhotodiode

R1

1k

R3

5k

R4

21.5k

VBIAS

(from the Level Shifter)

R2

VERR

Figure 7-7. High-Current Linearity Correction Circuit

7.1.7 Error Sources

7.1.7.1 Accuracy

Accuracy considerations for a log ratio amplifier are somewhat more complicated than for other amplifiers. This

complexity exists because the transfer function is nonlinear and has two inputs, each of which can vary over a

wide dynamic range. The accuracy for any combination of inputs is determined from the total error specification.

7.1.7.2 Total Error

The total error is the deviation of the actual output from the ideal output. Thus:

VLOGOUT actual =VLOGOUT ideal ±Total_Error

(6)

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LOG114

SBOS301B – MAY 2004 – REVISED MARCH 2025

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