This pin is included primarily to set the common mode
voltage for battery operation (ICL7116) or for any system
where the input signals are ﬂoating with respect to the power
supply. The COMMON pin sets a voltage that is approxi-
mately 2.8V less 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 (>6.8V), the COM-
MON voltage will have a low voltage coefﬁcient (0.001%/V),
low output impedance (≅15Ω), and a temperature coefﬁcient
typically less than 80ppm/oC.
The limitations of the on chip reference should also be
recognized, however. With the ICL7117, the internal heat-
ing which results from the LED drivers can cause some
degradation in performance. 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 80µVP-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 con-
dition. This is because over-range is a low dissipation
mode, with the three least signiﬁcant digits blanked. Simi-
larly, 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 ICL7116, with its negligible dissipation, suffers from
none of these problems. In either case, an external
reference can easily be added, as shown in Figure 4.
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 30mA 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.
FIGURE 4. USING AN EXTERNAL REFERENCE
The TEST pin serves two functions. On the ICL7116 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 annunciator the user may want to include on the
LCD display. Figures 5 and 6 show such an application. No
more than a 1mA load should be applied.
FIGURE 5. SIMPLE INVERTER FOR FIXED DECIMAL POINT
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
CAUTION: On the ICL7116, 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.