7106_15 Datasheet, PDF (9/18 Pages) Unisonic Technologies – CONVERTERS

7106
CMOS IC
„ DETAILED DESCRIPTION(Cont.)
ANALOG COMMON
This pin is included primarily to set the common mode voltage for battery operation (UTC 7106) 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 6V. 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 80ppm/℃.
The UTC 7106, with its negligible dissipation, suffers from none of these problems. In either case, an external
reference can easily be added, as shown in Fig.2
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 IC, 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.
V+
V+
V
REF HI
REF LO
UTC 7106
FIGURE 2 A.
6.8V
ZENER
Iz
V-
V
UTC 7106 _20k
REF HI
REF LO
COMMON
6.8k
ICL8069
1.2V
REFERENCE
FIGURE 2B.
Fig.2 Using an External Reference
UNISONIC TECHNOLOGIES CO., LTD
www.unisonic.com.tw
9 of 18
QW-R502-018.E