ICL8048BCJE View Datasheet(PDF) - Intersil
Part Name
Description
Manufacturer
ICL8048BCJE Datasheet PDF : 8 Pages
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ICL8048
V+
VREF (+15V)
IREF
V+
IIN
VIN
RIN
4
R4
2kΩ
5
2
-
Q1
A1
1
+
RREF
16
R3
Q2
7
R5
2kΩ
+
A2
-
10 VOUT
GND
C1
A1 OUTPUT
GAIN 15
R1
15.9kΩ
150pF
R0
680Ω (LOW T.C.)
10kΩ
R2
1kΩ
FIGURE 7. ICL8048 OFFSET AND SCALE FACTOR ADJUSTMENT
carefully designed to provide the necessary compensation.
Resistor R2 is external and should be a low T.C. type; it
should have a nominal value of 1kΩ to provide 1V/decade,
and must have an adjustment range of ±20% to allow for
production variations in the absolute value of R1.
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 Q1 of collector current and opens the feedback loop
around A1. Instead, it is necessary to zero the offset voltage
of A1 and A2 separately, and then to adjust the scale factor.
Referring to Figure 7, this is done as follows:
1. Temporarily connect a 10kΩ resistor (R0) between pins 2
and 7. With no input voltage, adjust R4 until the output of
A1 (pin 7) is zero. Remove R0.
Note that for a current input, this adjustment is not neces-
sary since the offset voltage of A1 does not cause any er-
ror for current source inputs.
2. Set IIN = IREF = 1mA. Adjust R5 such that the output of A2
(pin 10) is zero.
3. Set IIN = 1µA, IREF = 1mA. Adjust R2 for VOUT = 3V (for
a 1V/decade scale factor) or 6V (for a 2V/decade scale
factor).
Step #3 determines the scale factor. Setting IIN = 1µA
optimizes the scale factor adjustment over a fairly wide
dynamic range, from 1mA to 1nA. Clearly, if the ICL8048 is to
be used for inputs which only span the range 100µA to 1mA, it
would be better to set IIN = 100µA in Step #3. Similarly,
adjustment for other scale factors would require different IIN
and VOUT values.
Applications Information
ICL8048 Scale Factor Adjustment
The scale factor adjustment procedures outlined previously
for the ICL8048, are primarily directed towards setting up 1V
(∆VOUT) per decade (∆IIN or ∆VIN) for the log amp, or one
decade (∆VOUT) per volt (∆VIN) for the antilog amp.
This corresponds to K = 1 in the respective transfer functions:
VOUT
=
-K log10
----I--I--N------
IREF
(EQ. 5)
By adjusting R2 (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 R2 required
to give a specific value of K can be determined from
Equation 6. It should be remembered that R1 has a ±20%
tolerance in absolute value, so that allowance shall be made
for adjusting the nominal value of R2 by ±20%.
R2 = (---K-----–--9---0-4--.-1-0---5----9----) Ω
(EQ. 6)
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 IIN = (VIN - VOFFSET)/RIN, this conversion
is accurate only when VIN is much greater than the offset
voltage. A substantial reduction of VOFFSET would allow
voltage operation over a 120dB range.
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