2002-2012 Microchip Technology Inc. DS21458D-page 15
TC7126/A
7.3 Analog Common (Pin 32)
The analog common pin is set at a voltage potential
approximately 3V below V+. The potential is between
2.7V and 3.35V below V+. Analog common is tied inter-
nally to an N-channel FET capable of sinking 100A.
This FET will hold the common line at 3V should an
external load attempt to pull the common line toward
V+. Analog common source current is limited to 1A.
Therefore, analog common is easily pulled to a more
negative voltage (i.e., below V+ – 3V).
The TC7126A connects the internal V
IN
+ and V
IN
-
inputs to analog common during the auto-zero phase.
During the reference integrate phase, V
IN
- is con-
nected to analog common. If V
IN
- is not externally con-
nected to analog common, a Common mode voltage
exists, but is rejected by the converter’s 86dB Common
mode rejection ratio. In battery operation, analog com-
mon and V
IN
-
are usually connected, removing Com-
mon mode voltage concerns. In systems where V
IN
- is
connected to power supply ground or to a given
voltage, analog common should be connected to V
IN
-.
The analog common pin serves to set the analog sec-
tion reference, or common point. The TC7126A is spe-
cifically designed to operate from a battery, or in any
measurement system where input signals are not refer-
enced (float) with respect to the TC7126A’s power
source. The analog common potential of V+ – 3V gives
a 7V end of battery life voltage. The common potential
has a 0.001%/% voltage coefficient and a 15 output
impedance.
With sufficiently high total supply voltage (V+ – V- > 7V),
analog common is a very stable potential with excellent
temperature stability (typically 35ppm/°C). This poten-
tial can be used to generate the TC7126A’s reference
voltage. An external voltage reference will be unneces-
sary in most cases because of the 35ppm/°C tempera-
ture coefficient. See Section 7.5 “TC7126A Internal
Voltage Reference”, TC7126A Internal Voltage
Reference discussion.
7.4 TEST (Pin 37)
The TEST pin potential is 5V less than V+. TEST may
be used as the negative power supply connection for
external CMOS logic. The TEST pin is tied to the inter-
nally generated negative logic supply through a 500
resistor. The TEST pin load should be no more than
1mA. See Section 5.0 “DIGITAL SECTION”, Digital
Section for additional information on using TEST as a
negative digital logic supply.
If TEST is pulled HIGH (to V+),
all segments plus the
minus sign will be activated. DO NOT OPERATE IN
THIS MODE FOR MORE THAN SEVERAL MINUTES.
With TEST = V+, the LCD segments are impressed
with a DC voltage which will destroy the LCD.
7.5 TC7126A Internal Voltage
Reference
The TC7126A’s analog common voltage temperature
stability has been significantly improved (Figure 7-3).
The “A” version of the industry standard TC7126 device
allows users to upgrade old systems and design new
systems, without external voltage references. External
R and C values do not need to be changed. Figure 7-4
shows analog common supplying the necessary
voltage reference for the TC7126A.
FIGURE 7-3: Analog Common Temp.
Coefficient
FIGURE 7-4: TC7126A Internal Voltage
Reference Connection
Typical
Maximum
Typical
Typical
200
180
160
140
120
100
80
60
40
20
0
Analog Commom
Temperature Coefficient (ppm/°C)
TC7126A
ICL7136
No
Maximum
Specified
No
Maximum
Specified
ICL7126
V-
ANALOG
COMMON
TC7126A
V
REF
+
32
35
36
26
240kΩ
10kΩ
V
REF
-
V
REF
1
+
9V
SET V
REF
= 1/2 V
REF
V+
