LOG114AIRGVR datasheet(21/30 Pages) BURR-BROWN

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LOG114

SBOS301A − MAY 2004 − REVISED MARCH 2007

www.ti.com

INSIDE THE LOG114

The LOG114 uses two matched logarithmic amplifiers

(A1 and A2 with logging diodes in the feedback loops) to

generate the outputs log (I1) and log (I2), respectively.

The gain of 6.25 differential amplifier (A3) subtracts the

output of A2 from the output of A1, resulting in [log (I1)

− log (I2)], or log (I1/I2). The symmetrical design of the

A1 and A2 logarithmic amps allows I1 and I2 to be used

interchangeably, and provides good bandwidth and

phase characteristics with frequency.

DEFINITION OF TERMS

Transfer Function

The ideal transfer function of the LOG114 is:

LOGOUT + 0.375

log

This transfer function can be seen graphically in the typ-

ical characteristic curve, VLOGOUT vs IREF.

When a pedestal, or offset, voltage (VCom) is connected

to the Com pin, an additional offset term is introduced

into the equation:

LOGOUT + 0.375

log

) V

Com

Accuracy

Accuracy considerations for a log ratio amplifier are

somewhat more complicated than for other amplifiers.

This complexity exists because the transfer function is

nonlinear and has two inputs, each of which can vary

over

wide

dynamic range. The accuracy for any combination of

inputs is determined from the total error specification.

Total Error

The total error is the deviation of the actual output from

the ideal output. Thus,

VLOGOUT(ACTUAL) = VLOGOUT(IDEAL) ± Total Error

It represents the sum of all the individual components

of error normally associated with the log amp when op-

erating in the current input mode. The worst-case error

for any given ratio of I1/I2 is the largest of the two errors

when I1 and I2 are considered separately. Temperature

can also affect total error.

Errors RTO and RTI

As with any transfer function, errors generated by the

function may be Referred-to-Output (RTO) or Referred-

to-Input (RTI). In this respect, log amps have a unique

property: given some error voltage at the log amp out-

put, that error corresponds to a constant percent of the

input, regardless of the actual input level.

Log Conformity

For the LOG114, log conformity is calculated in the

same way as linearity and is plotted as I1/I2 on a semi-

log scale. In many applications, log conformity is the

most important specification. This condition is true be-

cause bias current errors are negligible (5pA for the

LOG114), and the scale factor and offset errors may be

trimmed to zero or removed by system calibration.

These factors leave log conformity as the major source

of error.

Log conformity is defined as the peak deviation from the

best fit straight line of the VLOGOUT versus log (I1/I2)

curve. Log conformity is then expressed as a percent of

ideal full−scale output. Thus, the nonlinearity error ex-

pressed in volts over m decades is:

VLOGOUT (NONLIN) = 0.375V/decade • 2Nm

where N is the log conformity error, in percent.

INDIVIDUAL ERROR COMPONENTS

The ideal transfer function with current input is:

LOGOUT IDEAL + 0.375

log

The actual transfer function with the major components

of error is:

0.375(1

" DK)

log

" 2Nm " V

OSO

where:

∆K = gain error (0.4%, typ, as specified in the Electri-

cal Characteristics table)

IB1 = bias current of A1 (5pA, typ)

IB2 = bias current of A2 (5pA, typ)

m = number of decades over which the log

conformity error is specified

N = log conformity error (0.1%, typ for m = 5 decades;

0.9% typ for m = 7.5 decades)

VOSO = output offset voltage (11mV, typ for ±5V sup-

plies; 14mV, typ for +5V supplies)

To determine the typical error resulting from these error

components, first compute the ideal output. Then calcu-

late the output again, this time including the individual

error components. Then determine the error in percent

using Equation (21):

%error

LOGOUT IDEAL*VLOGOUT TYP

LOGOUTIDEAL

100%

(17)

(18)

(19)

(20)

(21)

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