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ICL8048BCJE

Log Amplifier

Intersil 

ICL8048

Total Error = x2 + y2 + z2at (A)

If required, this error can be referred to the system output

through the voltage gain of the antilog circuit, using the

voltage gain versus input voltage plot.

The numerical values of x, y, and z in the above equation are

obtained from the maximum error voltage plots. For

example, with the ICL8048BC, the maximum error at the

output is 30mV at 25oC. This means that the measured

output will be within 30mV of the theoretical transfer

function, provided the unit has been adjusted per the

procedures described previously. Figure 11 illustrates this

point.

8

6

4

IREF = 1mA

THEORETICAL

2

IREF = 1µA

30mV

0

TRANSFER FN

-2

-4

IREF = 1nA

30mV

-6

-8

10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3

INPUT CURRENT (A)

Actual output will lie

within shaded area for

ICL8048BC at 25oC

FIGURE 11. TRANSFER FUNCTION FOR CURRENT INPUTS

To determine the maximum error over the operating

temperature range, the 0oC to 70oC absolute error values

given in the table of electrical specifications should be used.

For intermediate temperatures, assume a linear increase in

the error between the 25oC value and the 70oC value.

It is important to note that the ICL8048 requires positive

values of IREF, and the input current must also be positive.

Application of negative IIN to the ICL8048 or negative IREF

will cause malfunction, and if maintained for long periods,

would lead to device degradation. Some protection can be

provided by placing a diode between pin 7 and ground.

Setting Up the Reference Current

The input current reference pin (IREF) is not a true virtual

ground. For the ICL8048, a fraction of the output voltage is

seen on Pin 16 (Figure 7). This does not constitute an

appreciable error provided VREF is much greater than this

voltage. A 10V or 15V reference satisfies this condition.

Alternatively, IREF can be provided from a true current

source. One method of implementing such a current source

is shown in Figure 12.

+15V

+15V

R1

VREF

-

741

+

IREF = VREF/R1

2N2609

10kΩ

2N2219

(TO PIN 16 ON ICL8048)

FIGURE 12.

IREF

Log of Ratio Circuit, Division

The ICL8048 may be used to generate the log of a ratio by

modulating the IREF input. The transfer function remains the

same, as defined by Equation 7:

VOUT

=

–K l o g10

----I--I--N------

IREF

(EQ. 7)

Clearly it is possible to perform division using just one

ICL8048, followed by an antilog amplifier. For multiplication,

it is generally necessary to use two log amps, summing their

outputs into an antilog amp.

To avoid the problems caused by the IREF input not being a

true virtual ground (discussed in the previous section), the

circuit of Figure 12 is again recommended if the IREF input is

to be modulated.

Definition of Terms

In the definitions which follow, it will be noted that the various

error terms are referred to the output of the log amp, and to

the input of the antilog amp. The reason for this is explained

on the previous page.

Dynamic Range. The dynamic range of the ICL8048 refers

to the range of input voltages or currents over which the

device is guaranteed to operate.

Error, Absolute Value. The absolute error is a measure of

the deviation from the theoretical transfer function, after

performing the offset and scale factor adjustments as

outlined, (ICL8048). It is expressed in mV and referred to the

linear axis of the transfer function plot. Thus, in the case of

the ICL8048, it is a measure of the deviation from the

theoretical output voltage for a given input current or voltage.

The absolute error specification is guaranteed over the

dynamic range.

6

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