7.2 Typical Applications

7.2.1 Design Example for Dual-Supply Configuration

Given these conditions:

Table 7-1. Example Design Parameters for Dual-Supply Parameters

Parameter

Example Value

Positive supply voltage

5V

Negative supply voltage

-5V

Input signal

100pA to 10mA

Reference voltage

2.5V

Output voltage

0V to 2.5V

1. Due to LOG114 symmetry, choose either I1 or I2 as the signal input pin. Choosing I1 as the reference makes

the resistor network around A4 simpler. (Note: Current must flow into I1 and I2 pins.)

2. Select the magnitude of the reference current. The signal (I2) spans eight decades, therefore set I1 to

1μA − four decades above the minimum I2 value. (Note that the value does not have to be placed in the

middle. If I2 spanned seven decades, I1 can set three decades above the minimum and four decades below

the maximum I2 value.) This configuration results in more swing amplitude in the negative direction, which

provides more sensitivity (ΔVO4 per ΔI2) when the current signal decreases.

3. Use Equation 1 to calculate the expected range of log outputs at VLOGOUT:

For I2=10mA:

VLOGOUT=0.375×log 1μA10mA =−1.5V For I2=100pA:

VLOGOUT=0.375×log 1μA100pA =1.5V

(15)

Therefore, the expected voltage range at the output of amplifier A3 is:

−1.5V≤VLOGOUT≤1.5V

(16)

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

VO4= −GA4×VLOGOUT +VOFFSET

(17)

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:

0V≤VO4≤2.5V

(18)

This output results in a 2.5V range for the 3V VLOGOUT range, therefore a gain of 5/6 is needed for A4.

5. When I2 = 10mA, VLOGOUT = −1.5V. Using Equation 19 in step 4:

0V=−2.5V3V× −1.5V +VOFFSET

(19)

Therefore, VOFFSET = 1.25V

This makes the A4 formula:

VO4=−56VLOGOUT +1.25

(20)

Replacing VLOGOUT with Equation 1 gives the system's overall function to be:

LOG114

SBOS301B – MAY 2004 – REVISED MARCH 2025

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