參數(shù)資料
型號(hào)： LOG114AIRGVT
廠(chǎng)商： TEXAS INSTRUMENTS INC
元件分類(lèi)： 模擬運(yùn)算功能
英文描述： LOG OR ANTILOG AMPLIFIER, PQCC16
封裝： 4 X 4 MM, GREEN, PLASTIC, QFN-16
文件頁(yè)數(shù)： 14/29頁(yè)
文件大小： 637K
代理商： LOG114AIRGVT
LOG114
SBOS301A MAY 2004 REVISED MARCH 2007
www.ti.com
21
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:
V
LOGOUT + 0.375
log
I
1
2
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:
V
LOGOUT + 0.375
log
I
1
2
) 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
a
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 fullscale 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:
V
LOGOUT IDEAL + 0.375
log
I
1
2
The actual transfer function with the major components
of error is:
0.375(1
" DK)
log
I
1
I
2
" 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
+
V
LOGOUT IDEAL*VLOGOUT TYP
V
LOGOUTIDEAL
100%
(17)
(18)
(19)
(20)
(21)
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