7
FN3084.5
Differential Reference
The reference voltage can be generated anywhere within the
power supply voltage of the converter. The main source of
common mode error is a roll-over voltage caused by the
reference capacitor losing or gaining charge to stray capacity
on its nodes. If there is a large common mode voltage, the
reference capacitor can gain charge (increase voltage) when
called up to de-integrate a positive signal but lose charge
(decrease voltage) when called up to de-integrate 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 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 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 (<6.8V), the
COMMON voltage will have a low voltage coefficient
(0.001%/V), low output impedance (≅15Ω), and a
temperature coefficient typically less than 80ppm/
o
C.
The limitations of the on-chip reference should also be
recognized, however. The reference Temperature Coefficient
(TC), can cause some degradation in performance.
Temperature changes of 2
o
C to 8
o
C, typical for instruments,
can give a scale factor error of a count or more. Also the
common voltage will have a poor voltage coefficient when the
total supply voltage is less than that which will cause the zener
to regulate (<7V). These problems are eliminated if an
external reference is used, as shown in Figure 5.
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 volt-
age 2.8V below the positive supply (when a load is trying to
pull the common line positive). However, there is only 1µA of
source current, so COMMON may easily be tied to a more
negative voltage thus overriding the internal reference.
FIGURE 5A.
FIGURE 5B.
FIGURE 5.
ICL7126
V+
REF LO
REF HI
V+
V-
6.8V
ZENER
I
Z
ICL7126
V+
REF HI
REF LO
COMMON
V+
ICL8069
1.2V
REFERENCE
27kΩ
200kΩ
ICL7126
