ICL8048BCJE 데이터 시트보기 (PDF) - Intersil
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ICL8048BCJE Datasheet PDF : 8 Pages
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ICL8048
0.1µF
1K
0.1µF
0.1µF
0V
-
ICL7650
+
RIN
VIN
IIN
VOFFSET 2
1
33kΩ
4
-
A1
+
C1
150pF
33kΩ
VREF
(+15V)
5
Q1
RREF
IREF
Q2
R5
2kΩ
16 12 13
R3
ICL8048
+
A2
-
R1
15.9kΩ
A1 OUTPUT
7
GAIN
15 R2
680Ω
(LOW T.C.) 1kΩ
V+
10
VOUT
FIGURE 9. ICL8048 OFFSET NULLED BY ICL7650
12
IREF = 1mA
10
8
K=2
6
K=1
4
K = 0.5
2
0
-2
10-10 10-9
10-8 10-7 10-6 10-5
INPUT CURRENT (A)
10-4
10-3
FIGURE 8. EFFECT OF VARYING “K” ON THE LOG AMPLIFIER
Figure 9 shows the ICL8048 in an automatic offset nulling
configuration using the ICL7650S. The extremely low offset
voltage of the ICL7650S forces its non-inverting input (and thus
pin 2 of the ICL8048) to the same potential as its inverting input
by nulling the first stage of the log amp. Since VOFFSET is now
within a few µV of ground potential, RIN can perform its voltage
to current conversion much more accurately, and without an
offset trimmer pot. Step 1 of the offset and scale factor
adjustment is eliminated, simplifying calibration.
NOTE: The ICL7650S op amp has a maximum supply voltage of
18V. The ICL8048 will operate at this voltage, but IREF must be
limited to 200µA or less for proper calibration and operation. Best
performance will be achieved when the ICL7650S has a ±3V to ±8V
supply and the ICL8048 is at its recommended ±15V supply. See
A053 for a method of powering the ICL7650S from a ±15V source.
Frequency Compensation
Although the op amps in the ICL8048 are compensated for
unity gain, some additional frequency compensation is
required. This is because the log transistors in the feedback
loop add to the loop gain. In the ICL8048, 150pF should be
connected between Pins 2 and 7 (Figure 7).
Error Analysis
Performing a meaningful error analysis of a circuit containing a
log and antilog amplifiers is more complex than dealing with a
similar circuit involving only op amps. In this data sheet every
effort has been made to simplify the analysis task, without in
any way compromising the validity of the resultant numbers.
The key difference in making error calculations in log/antilog
amps, compared with op amps, is that the gain of the former
is a function of the input signal level. Thus, it is necessary,
when referring errors from output to input, or vice versa, to
check the input voltage level, then determine the gain of the
circuit by referring to the graphs given in the Typical
Performance Curves section.
The various error terms in the log amplifier, the ICL8048, are
Referred To the Output (RTO) of the device. The errors are
expressed in this way because in the majority of systems a
number of log amps interface with an antilog amp, as shown
in Figure 10.
INPUT LOG AMP
A
LOG AMP
B
INPUT
ERROR DUE TO A (RTO)
= xmV
ERROR DUE TO C (RTI)
= zmV
ANTI LOG OUTPUT
AMP
A
C
ERROR DUE TO B (RTO)
= ymV
FIGURE 10.
It is very straightforward to estimate the system error at node
(A) by taking the square root of the sum-of-the-squares of
the errors of each contributing block.
5
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