2005 Microchip Technology Inc.
DS21461C-page 13
TC7136/TC7136A
7.0
DEVICE PIN FUNCTIONAL
DESCRIPTION
7.1
Differential Signal Inputs
VIN+ (Pin 31), VIN- (Pin 30)
The TC7136/A is designed with true differential inputs
and accepts input signals within the input stage Com-
mon mode voltage range (VCM). The typical range is
V+ – 1V to V- + 1V. Common mode voltages are
removed from the system when the TC7136A operates
from a battery or floating power source (isolated from
measured system), Common mode voltage removed
in battery operation with VIN = analog common and VIN-
is
connected
to
analog
common
(VCOM) (see
FIGURE 7-1:
COMMON MODE VOLTAGE REMOVED IN BATTERY OPERATION WITH
VIN = ANALOG COMMON
In systems where Common mode voltages exist, the
86dB Common mode rejection ratio minimizes error.
Common mode voltages do, however, affect the inte-
grator output level. A worst case condition exists if a
large positive VCM exists in conjunction with a full scale
negative differential signal. The negative signal drives
the integrator output positive along with VCM (see
put swing can be reduced below the recommended 2V
full scale swing. The integrator output will swing within
0.3V of V+ or V- without increased linearity error.
FIGURE 7-2:
COMMON MODE
VOLTAGE REDUCES
AVAILABLE INTEGRATOR
SWING
(VCOM ≠ VIN)
7.2
Differential Reference
VREF+ (Pin 36), VREF- (Pin 35)
The reference voltage can be generated anywhere
within the V+ to V- power supply range.
To prevent rollover type errors being induced by large
Common mode voltages, CREF should be large com-
pared to stray node capacitance. The TC7136/A offers
a significantly improved analog common temperature
coefficient. This potential provides a very stable volt-
age, suitable for use as a voltage reference. The
temperature coefficient of analog common is typically
35ppm/°C.
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 100
μA.
This FET will hold the common line at 3V below V+ if an
external load attempts to pull the common line toward
V+. Analog common source current is limited to 1
μA.
Analog common is, therefore, easily pulled to a more
negative voltage (i.e., below V+ – 3V).
VBUF
CAZ
VINT
BP
POL
Segment
Drive
OSC1
OSC3
OSC2
V-
V+
VREF+
VREF-
ANALOG
COMMON
V-
V+
GND
Measured
System
Power
Source
9V
LCD
TC7136
TC7136A
+
V-
V+
V-
RI
+
–
VIN
CI
Integrator
VI =
[
VCM = VIN
Input Buffer
CI = Integration capacitor
RI = Integration resistor
4000
FOSC
tI = Integration time =
Where:
VI
–
+
–
+
tI
CI
VCM