8
FN3086.6
July 21, 2005
Analog COMMON
This pin is included primarily to set the common mode
voltage for battery operation or for any system where the
input signals are floating with respect to the power supply.
The COMMON pin sets a voltage that is approximately 2.8V
more negative than the positive supply. This is selected to
give a minimum end-of-life battery voltage of about 6.8V.
However, analog COMMON has some of the attributes of a
reference voltage. When the total supply voltage is large
enough to cause the zener to regulate (>7V), the COMMON
voltage will have a low voltage coefficient (0.001%/V), low
output impedance (≅15Ω), and a temperature coefficient
typically less than 150ppm/
o
C.
The limitations of the on chip reference should also be
recognized, however. Due to their higher thermal resistance,
plastic parts are poorer in this respect than ceramic. The
combination of reference Temperature Coefficient (TC), internal
chip dissipation, and package thermal resistance can increase
noise near full scale from 25µV to 80µV
P-P
. Also the linearity in
going from a high dissipation count such as 1000 (20 segments
on) to a low dissipation count such as 1111 (8 segments on) can
suffer by a count or more. Devices with a positive TC reference
may require several counts to pull out of an over range
condition. This is because over-range is a low dissipation
mode, with the three least significant digits blanked. Similarly,
units with a negative TC may cycle between over range and a
non-over range count as the die alternately heats and cools. All
these problems are of course eliminated if an external
reference is used.
The ICL7136, with its negligible dissipation, suffers from
none of these problems. In either case, an external
reference can easily be added, as shown in Figure 3.
Analog COMMON is also used as the input low return during
auto-zero and de-integrate. If IN LO is different from analog
COMMON, a common mode voltage exists in the system
and is taken care of by the excellent CMRR of the converter.
However, in some applications IN LO will be set at a fixed
known voltage (power supply common for instance). In this
application, analog COMMON should be tied to the same
point, thus removing the common mode voltage from the
converter. The same holds true for the reference voltage. If
reference can be conveniently tied to analog COMMON, it
should be since this removes the common mode voltage
from the reference system.
Within the lC, analog COMMON is tied to an N-Channel FET
that can sink approximately 3mA of current to hold the
voltage 2.8V below the positive supply (when a load is trying
to pull the common line positive). However, there is only
10µA of source current, so COMMON may easily be tied to a
more negative voltage thus overriding the internal reference.
TEST
The TEST pin serves two functions. On the ICL7136 it is
coupled to the internally generated digital supply through a
500Ω resistor. Thus it can be used as the negative supply for
externally generated segment drivers such as decimal points
or any other presentation the user may want to include on
the LCD display. Figures 4 and 5 show such an application.
No more than a 1mA load should be applied.
The second function is a “lamp test”. When TEST is pulled
high (to V+) all segments will be turned on and the display
should read “-1888”. The TEST pin will sink about 5mA
under these conditions.
CAUTION: On the ICL7136, in the lamp test mode, the segments have a
constant DC voltage (no square-wave) and may burn the LCD
display if left in this mode for several minutes.
ICL7136
V
REF LO
REF HI
V+
V-
6.8V
ZENER
I
Z
FIGURE 3A.
ICL7136
V
REF HI
REF LO
COMMON
V+
ICL8069
1.2V
REFERENCE
6.8kΩ
20kΩ
FIGURE 3B.
FIGURE 3. USING AN EXTERNAL REFERENCE
ICL7136
V+
BP
TEST
21
37
TO LCD
BACKPLANE
TO LCD
DECIMAL
POINT
1MΩ
FIGURE 4. SIMPLE INVERTER FOR FIXED DECIMAL POINT
ICL7136
