2002-2012 Microchip Technology Inc. DS21457D-page 9
TC7116/A/TC7117/A
3.1.5 DIFFERENTIAL INPUT
This input can accept differential voltages anywhere
within the Common mode range of the input amplifier
or, specifically, from 1V below the positive supply to 1V
above the negative supply. In this range, the system
has a CMRR of 86dB, typical. However, since the inte-
grator also swings with the Common mode voltage,
care must be exercised to ensure that the integrator
output does not saturate. A worst-case condition would
be a large, positive Common mode voltage with a near
full scale negative differential input voltage. The nega-
tive input signal drives the integrator positive, when
most of its swing has been used up by the positive
Common mode voltage. For these critical applications,
the integrator swing can be reduced to less than the
recommended 2V full scale swing with little loss of
accuracy. The integrator output can swing within 0.3V
of either supply without loss of linearity.
3.1.6 ANALOG COMMON
This pin is included primarily to set the Common mode
voltage for battery operation (TC7116/TC7116A), or for
any system where the input signals are floating, with
respect to the power supply. The analog common pin
sets a voltage 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 attributes of a reference voltage.
When the total supply voltage is large enough to cause
the Zener to regulate (>7V), the analog common volt-
age will have a low voltage coefficient (0.001%), low
output impedance (15), and a temperature coeffi-
cient of less than 20ppm/°C, typically, and 50 ppm max-
imum. The TC7116/TC7117 temperature coefficients
are typically 80ppm/°C.
An external reference may be used, if necessary, as
shown in Figure 3-2.
FIGURE 3-2: Using an External
Reference
Analog common is also used as V
IN
- return during
auto-zero and de-integrate. If V
IN
- is different from ana-
log 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, V
IN
- 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 it can be conve-
niently referenced to analog common, it should be. 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 30mA or more of current to hold the
voltage 3V below the positive supply (when a load is
trying to pull the analog common line positive). How-
ever, there is only 10A of source current, so analog
common may easily be tied to a more negative voltage,
thus overriding the internal reference.
3.1.7 TEST
The TEST pin serves two functions. On the TC7117/
TC7117A, it is coupled to the internally generated digi-
tal supply through a 500 resistor. Thus, it can be used
as a negative supply for externally generated segment
drivers, such as decimal points, or any other presenta-
tion the user may want to include on the LCD.
(Figure 3-3 and Figure 3-4 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 10mA under these conditions.
FIGURE 3-3: Simple Inverter for Fixed
Decimal Point
V+
V+
1.2V REF
COMMON
TC7116
TC7116A
TC7117
TC7117A
6.8kΩ
V
REF
+
20kΩ
TC7116
TC7116A
BP
TEST
37
21
V+
V+
GND
To LCD
Decimal
Point
To LCD
Backplane
4049
