icl8049 Intersil Corporation, icl8049 Datasheet - Page 3

• Manufacturer Part Number: icl8049
• Description: Antilog Amplifier
• Manufacturer: Intersil Corporation

ICL8049 Detailed Description
The ICL8049 relies on the same logarithmic properties of
the transistor as the ICL8048. The input voltage forces a
specific 'V
difference is converted into a difference of collector currents
by the transistor pair. The equation governing the behavior
of the transistor pair is derived from (2) on the previous page
and is as follows:
When numerical values for q/kT are put into this equation, it
is found that a 'V
change the collector current ratio by a factor of ten. But for
ease of application, it is desirable that a 1V change at the
input generate a tenfold change at the output. The required
input attenuation is achieved by the network comprising R
and R
per volt may be selected, R
have a value of 1k:, adjustable r20%, for one decade per
volt. R
chip; its temperature characteristics are chosen to
compensate the temperature dependence of Equation 1, as
explained on the previous page.
The overall transfer function is as follows:
Substituting V
2k:
R
4
2
V OUT
1
. In order that scale factors other than one decade
V+
V-
is a thin film resistor deposited on the monolithic
15k:
R
15k:
I
--------- -
I
OUT
R
REF
BE
3
5
=
OUT
between Q
100k:
R OUT I REF exp
R
(LOW
680:
1k:
=
T.C.)
6
= I
I C1
--------
I
C2
BE
ex p
OUT
V
IN
=
of 59mV (at +25
R
2
x R
exp
---------------------
1
R
2
1
is external to the chip. It should
OUT
and Q
15.9k:
-R 2
q'V BE
----------------- -
FIGURE 1. ICL8049 OFFSET AND SCALE FACTOR ADJUSTMENT
+
2
kT
R
------------------------ -
R
gives:
R
1
2
1
–
1
2
R 2
+
16
u
(Figure 1). This V
R
qV IN
-------- -
2
o
k T
C) is required to
u
qV IN
------------
kT
+
-
A
1
ICL8049
(1)
(2)
(3)
BE
7
1
3
200pF
For voltage references Equation 3 becomes
ICL8049 Offset and Scale Factor
Adjustment
As with the log amplifier, the antilog amplifier requires three
adjustments. The first step is to null out the offset voltage of
A
of Q
grounded input. The second step forces V
adjusted for V
point on the transfer function. The third step applies a
specific input and adjusts the output to the correct voltage.
This sets the scale factor. Referring to Figure 1 the exact
procedure for 1 decade/volt is as follows:
The procedure outlined above optimizes the performance
over a 3 decade range at the output (i.e., V
to 10V). For a more limited range of output voltages, for
example 1V to 10V, it would be better to use a precise 1V
supply and adjust for V
or starting points, different values for R
needed, but the same basic procedure applies.
C
1. Connect the input (pin #16) to +15V. This reverse biases
2. Connect the input to Ground. Adjust R
3. Connect the input to a precise 2V supply and adjust R
2
1
V
. This is accomplished by reverse biasing the base-emitter
(+15V)
V
OUT
the base-emitter of Q
nect the input from +15V.
Disconnect the input from Ground.
V
REF
2
OUT
Q
. A
3
1
R
15k:
REF
Q
2
=
= 100mV.
I
2
REF
then operates as a unity gain buffer with a
V
OUT
REF
= 10V. This step essentially “anchors” one
u
15
R
----------- -
R
OUT
OUT
REF
2
. Adjust R
= 1V. For other scale factors and/
12
exp
+
-
A
2k:
R
2
---------------------
7
R 1
V+
7
–
for V
13
+
R
2
R 2
2
IN
4
OUT
and R
for V
= 0; the output is
OUT
10
14
u
= 0V. Discon-
V
qV IN
-------- -
OUT
OUT
from 10mV)
kT
REF
R
10k:
= 10V.
OUT
I
OUT
will be
2
for
(4)