VO4=−0.347×log I1I2 +1.25V

(21)

The external resistor values for A4 can be seen in Figure 7-8.

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

100k

37.4k

V–

2.5M

Input Signal

100pA to 10mA

100k

82.5k

IREF

1μF

0.1μF

5V

0.1μF

–5V

Figure 7-8. Dual Supply Configuration Example for Best Accuracy Over Eight Decades

7.2.2 Design Example for Single-Supply Configuration

Given these conditions:

Table 7-2. Example Design Parameters for Single-Supply Parameters

Parameter

Example Value

Positive supply voltage

5V

Negative supply voltage

0V

Input signal

100pA to 10mA

Reference voltage

2.5V

Output voltage

0.5V to 2.5V

1. Choose either I1 or I2 as the signal input pin. For this example, I2 is used. Choosing I1 as the reference

current makes the resistor network around A4 simpler. (Note: Current only flows into the I1 and I2 pins.)

2. Select the magnitude of the reference current. Since the signal (I2) spans eight decades, set I1 to 1μA − four

decades above the minimum I2 value, and four decades below the maximum 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 (22)

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

−1.5V≤VLOGOUT≤1.5V

(23)

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SBOS301B – MAY 2004 – REVISED MARCH 2025

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