LOG114AIRGVR datasheet(13/30 Pages) BURR-BROWN

13 / 30 page

LOG114

SBOS301A − MAY 2004 − REVISED MARCH 2007

www.ti.com

DESIGN EXAMPLE FOR DUAL-SUPPLY

CONFIGURATION

Given these conditions:

D V+ = 5V and V− = −5V

D 100pA ≤ Input signal

D The stage following the LOG114 is an analog-to-

digital converter (ADC) with +5V supply and

+2.5V reference voltage, so VO4 swings from

+0.5V to +2.5V.

Due to LOG114 symmetry, you can 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 pins 3 (I1) and

pin 4 (I2).)

Select the magnitude of the reference current.

Since the signal (I2) spans eight decades, set I1 to

µA − four decades above the minimum I2 value.

(Note that it does not have to be placed in the

middle. If I2 spanned seven decades, I1 could be 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

(

∆VO4 per ∆I2) when the current signal decreases.

Using Equation (1) calculate the expected range of

log outputs at VLOGOUT:

For I

2 + 10mA :

LOGOUT + 0.375

log

+ 0.375

log

10mA

+* 1.5V

For I

2 + 100pA :

LOGOUT + 0.375

log

+ 0.375

log

100pA

+) 1.5V

Therefore, the expected voltage range at the output

of amplifier A3 is:

* 1.5V v V

LOGOUT v) 1.5V

The A4 amplifier scales and offsets the VLOGOUT

signal for use by the ADC using the equation:

O4 +*SFACTOR

LOGOUT ) VOFFSET

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:

≤ VO4 ≤ +2.5V

This output results in a 2.5V range for the 3V VLOGOUT

range, or 2.5V/3V scaling factor.

When I2 = 10mA, VLOGOUT = −1.5V. Using the

equation in step 5:

O4 +*SFACTOR

LOGOUT ) VOFFSET

+*2.5V 3V(*1.5V) ) V

OFFSET

Therefore, VOFFSET = 0V

The A4 amplifier configuration for VO4 = −2.5/3(VLOGOUT)

+ 0V is seen in Figure 3.

The overall transer function is:

O4 +*0.249

log

) 1.5V

Internal A4 Output Amplifier

100k

Ω

REF

+2.5V

LOGOUT

O4 = − 2/3 (VLOGOUT)

100pA

10mA

4 amplifier usedtoscale andoffset VLOGOUT for 0V to 2.5V output.

82.5k

Ω

100k

Ω

37.4k

Ω

+2.5V

+5V

−5V

Figure 3.

Operational

Amplifier

Configuration

for

Scaling the Output Going to ADC Stage.

(3)

(4)

(5)

(6)

(7)

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