LOG114 datasheet(14/29 Pages) BURR-BROWN

14 / 29 page

LOG114

SBOS301A − MAY 2004 − REVISED MARCH 2007

www.ti.com

DESIGN EXAMPLE FOR SINGLE-SUPPLY

CONFIGURATION

Given these conditions:

D V+ = 5V

D V− = GND

D 100pA ≤ Input signal ≤ 10mA

D The stage following the LOG114 is an analog to

digital converter (ADC) with +5V supply and

+2.5V reference voltage

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.)

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,

and four decades below the maximum 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

This result would be fine in a dual−supply system

(V+ = +5V, V− = −5V) where the output can swing

below ground, but does not work in a single supply

+5V system. Therefore, an offset voltage must be

added to the system.

Select an offset voltage, VCom to use for centering

the output between (V−) + 0.6V and (V+) − 0.6V,

which is the full-scale output capability of the A3

amplifier. Choosing VCom = 2.5V, and recalculating

the expected voltage output range for VLOGOUT using

Equation (2), results in:

) 1V v V

LOGOUT v) 4V

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:

+0.5V

≤ VO4 ≤ +2.5V

This output results in a 2V range for the 3V VLOGOUT

range, or 2V/3V scaling factor.

When I2 = 10mA, VLOGOUT = +1V, and VO4 = 2.5V.

Using the equation in step 5:

O4 +*SFACTOR

LOGOUT ) VOFFSET

2.5V

+*2V 3V(1V) ) V

OFFSET

Therefore, VOFFSET = 3.16V

The A4 amplifier configuration for VO4 = −2/3(VLOGOUT) +

3.16 is seen in Figure 4a.

The overall transer function is:

O4 +*0.249

log

) 1.5V

A similar process can be used for configuring an

external rail-to-rail output op amp, such as the OPA335.

Because the OPA335 op amp can swing down to 0V

using a pulldown resistor, RP, connected to −5V (for

details, refer to the OPA335 data sheet, available for

download at www.ti.com), the scaling factor is 2.5V/3V

and the corresponding VOFFSET is 3.3V. This circuit

configuration is shown in Figure 4b.

(8)

(9)

(10)

(11)

(12)

(13)

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