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參數(shù)資料
| 型號： | ICL8048BCJE |
| 廠商： | INTERSIL CORP |
| 元件分類： | 運(yùn)動(dòng)控制電子 |
| 英文描述： | Log Amplifier |
| 中文描述： | LOG OR ANTILOG AMPLIFIER, CDIP16 |
| 封裝： | FRIT SEALED, CERDIP-16 |
| 文件頁數(shù)： | 4/7頁 |
| 文件大?。?/td> | 85K |
| 代理商： | ICL8048BCJE |
4
carefully designed to provide the necessary compensation.
Resistor R
2
is external and should be a low T.C. type; it
should have a nominal value of 1k
to provide 1V/decade,
and must have an adjustment range of
±
20% to allow for
production variations in the absolute value of R
1
.
ICL8048 Offset and Scale Factor
Adjustment
A log amp, unlike an op amp, cannot be offset adjusted by
simply grounding the input. This is because the log of zero
approaches minus infinity; reducing the input current to zero
starves Q
1
of collector current and opens the feedback loop
around A
1
. Instead, it is necessary to zero the offset voltage
of A
1
and A
2
separately, and then to adjust the scale factor.
Referring to Figure 7, this is done as follows:
1. Temporarily connect a 10k
resistor (R
0
) between pins 2
and 7. With no input voltage, adjust R
4
until the output of
A
1
(pin 7) is zero. Remove R
0
.
Note that for a current input, this adjustment is not neces-
sary since the offset voltage of A
1
does not cause any er-
ror for current source inputs.
2. SetI
IN
=I
REF
=1mA.AdjustR
5
suchthattheoutputofA
2
(pin 10) is zero.
3. Set I
IN
= 1
μ
A, I
REF
= 1mA. Adjust R
2
for V
OUT
= 3V (for
a 1V/decade scale factor) or 6V (for a 2V/decade scale
factor).
Step #3 determines the scale factor. Setting I
IN
= 1
μ
A
optimizesthescalefactoradjustmentoverafairlywidedynamic
range, from 1mA to 1nA. Clearly, if the ICL8048 is to be used
for inputs which only span the range 100
μ
A to 1mA, it would be
better to set I
IN
= 100
μ
A in Step #3. Similarly, adjustment for
other scale factors would require different I
IN
and V
OUT
values.
Applications Information
ICL8048 Scale Factor Adjustment
The scale factor adjustment procedures outlined previously
for the ICL8048, are primarily directed towards setting up 1V
(
V
OUT
) per decade (
I
IN
or
V
IN
) for the log amp, or one
decade (
V
OUT
) per volt (
V
IN
) for the antilog amp.
This corresponds to K = 1 in the respective transfer functions:
By adjusting R
2
(Figure 7) the scale factor “K” in Equation 5
can be varied. The effect of changing K is shown graphically
in Figure 8 for the log amp. The nominal value of R
2
required
to give a specific value of K can be determined from
Equation 6. It should be remembered that R
1
has a
±
20%
tolerance in absolute value, so that allowance shall be made
for adjusting the nominal value of R
2
by
±
20%.
)
ICL8048 Automatic Offset Nulling Circuit
The ICL8048 is fundamentally a logarithmic current
amplifier. It can be made to act as a voltage amplifier by
placing a resistor between the current input and the voltage
source but, since I
IN
= (V
IN
- V
OFFSET
)/R
IN
, this conversion
is accurate only when V
IN
is much greater than the offset
voltage. A substantial reduction of V
OFFSET
would allow
voltage operation over a 120dB range.
I
IN
R
IN
A
1
+
-
R
4
2k
V+
5
4
A
2
+
-
R
5
2k
V+
GND
1
2
C
1
Q
1
Q
2
R
3
7
R
2
R
1
1k
15.9k
GAIN
A
1
OUTPUT
R
0
10k
680
(LOW T.C.)
150pF
R
REF
I
REF
V
REF
(+15V)
16
10V
OUT
15
V
IN
FIGURE 7. ICL8048 OFFSET AND SCALE FACTOR ADJUSTMENT
VOUT
-K log10
--------------
=
(EQ. 5)
R2
--------–
=
(EQ. 6)
INPUT CURRENT (A)
0
-2
10
-10
10
I
REF
= 1mA
O
12
K = 2
K = 1
K = 0.5
8
6
4
2
10
-9
10
-8
10
-7
10
-6
10
-5
10
-4
10
-3
FIGURE 8. EFFECT OF VARYING “K” ON THE LOG AMPLIFIER
ICL8048
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