TC7126_06 Datasheet, PDF (14/26 Pages) Microchip Technology – 3-1/2 Digit Analog-to-Digital Converters

TC7126/A
7.0 DEVICE PIN FUNCTIONAL
DESCRIPTION
(Pin Numbers Refer to the 40-Pin PDIP.)
7.1 Differential Signal Inputs
V_IN+ (Pin 31), V_IN- (Pin 30)
The TC7126A is designed with true differential inputs
and accepts input signals within the input stage
Common mode voltage range (V_CM). Typical range is
V+ – 1V to V- + 1V. Common mode voltages are
removed from the system when the TC7126A operates
from a battery or floating power source (isolated from
measured system), and V_IN- is connected to analog
common (V_COM) (see Figure 7-2).
In systems where Common mode voltages exist, the
TC7126A’s 86 dB Common mode rejection ratio
minimizes error. Common mode voltages do, however,
affect the integrator output level. A worst-case condi-
tion exists if a large positive V_CM exists in conjunction
with a full scale negative differential signal. The
negative signal drives the integrator output positive
along with V_CM (see Figure 7-1). For such applications,
the integrator output 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.
+
V_IN
–
V_CM
Input
Buffer
+
R_I
–
C_I
–
+
V_I
Integrator
[ ^VI
Where:
=
tI
RI CI
V_CM – V_IN
t_I
=
Integration
time
=
4000
F_OSC
C_I = Integration capacitor
R_I = Integration resistor
FIGURE 7-1:
Common Mode Voltage
Reduces Available Integrator Swing (V_COM ≠ V_IN)
7.2 Differential Reference
V_REF+ (Pin 36), V_REF- (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, C_REF should be large
compared to stray node capacitance.
The TC7126A offers a significantly improved analog
common temperature coefficient. This potential
provides a very stable voltage, suitable for use as a ref-
erence. The temperature coefficient of analog common
is typically 35ppm/°C for the TC7126A and 80 ppm/°C
for the TC7126.
Segment
Drive
LCD
Measured
System
V+
V-
GND
V+ V- _GND
Power
Source
V_BUFF
C_AZ V_INT
V_IN+
V_IN-
TC7126A
ANALOG
COMMON
V_REF-
V_REF+
V+
POL BP
OSC1
OSC3
OSC2
V-
+
9V
FIGURE 7-2:
Common Mode Voltage Removed in Battery Operation with V_IN = Analog Common
DS21458C-page 14
© 2006 Microchip Technology Inc.