7 Application and Implementation

Note

Information in the following applications sections is not part of the TI component specification,

and TI does not warrant its accuracy or completeness. TI’s customers are responsible for

determining suitability of components for their purposes, as well as validating and testing their design

implementation to confirm system functionality.

7.1 Applications Information

7.1.1 Transfer Function

The ideal transfer function of the LOG114 is:

VLOGOUT=0.375×log I1I2

(1)

This transfer function can be seen graphically in the typical characteristic curve, Figure 5-6.

When a pedestal, or offset voltage (VCOM) is connected to the COM pin, an additional offset term is introduced

into the equation:

VLOGOUT=0.375×log I1I2 +VCOM

(2)

7.1.2 Input Current Range

To maintain specified accuracy, limit the input current range of the LOG114 from 100pA to 3.5mA. Input currents

outside of this range can compromise the LOG114 performance. Input currents larger than 3.5mA result in

increased nonlinearity. An absolute maximum input current rating of 10mA is included to prevent excessive

power dissipation that can damage the input transistor.

7.1.3 Setting the Reference Current

When the LOG114 is used to compute logarithms, either I1 or I2 can be held constant to become the reference

current (IREF) to which the other is compared. As shown in Figure 7-1, I2 is used as IREF and is generated using

the on-chip 2.5V VREF pin.

LOG114

SBOS301B – MAY 2004 – REVISED MARCH 2025

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