
7
Differential Reference
The ref erence v oltage can be generated an ywhere within the
power supply voltage of the converter. The main source of
common mode error is a roll-over voltage caused by the
ref erence capacitor losing or gaining charge to stra y capacity
on its nodes. If there is a large common mode voltage, the
ref erence capacitor can gain charge (increase v oltage) when
called up to de-integrate a positive signal but lose charge
(decrease voltage) when called up to de-integ r ate a negative
input signal. This difference in reference for positive or
negative input voltage will give a roll-over error. However, by
selecting the reference capacitor such that it is large enough
in comparison to the stray capacitance, this error can be
held to less than 0.5 count worst case. (See Component
Value Selection.)
Analog COMMON
This pin is included primarily to set the common mode
voltage for battery operation (ICL7136) 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 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 z ener 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/oC.
The limitations of the on chip reference should also be
recognized, however. With the ICL7137, the internal heating
which results from the LED drivers can cause some
degradation in performance. Due to their higher thermal resis-
tance, plastic parts are poorer in this respect than ceramic.
The combination of reference Temperature Coefficient (TC),
internal chip dissipation, and package ther mal 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 ma y require sev er al 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 e xternal 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 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 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 ma y 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
e xternally generated segment drivers such as decimal points
or any other presentation 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.
ICL7136
V
REF LO
ICL7137
REF HI
V+
V-
6.8V
ZENER
IZ
FIGURE 4A.
ICL7136
V
REF HI
REF LO
COMMON
V+
ICL8069
1.2V
REFERENCE
6.8kΩ
20kΩ
ICL7137
FIGURE 4B.
FIGURE 4. USING AN EXTERNAL REFERENCE
ICL7136
V+
BP
TEST
21
37 TO LCD
BACKPLANE
TO LCD
DECIMAL
POINT
1MΩ
FIGURE 5. SIMPLE INVERTER FOR FIXED DECIMAL POINT
ICL7136, ICL7137