tc7107a Microchip Technology Inc., tc7107a Datasheet - Page 15

• Manufacturer Part Number: tc7107a
• Description: 3-1/2 Digit Analog-to-digital Converters
• Manufacturer: Microchip Technology Inc.

8.0
8.1
The TC7106A/7017A is designed with true differential
inputs and accepts input signals within the input stage
common mode voltage range (V
is V+ – 1.0 to V+ + 1V. Common mode voltages are
removed from the system when the TC7106A/
TC7107A operates from a battery or floating power
source (isolated from measured system) and V
connected to analog common (V
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. Integrator output saturation must be
prevented. A worst-case condition exists if a large
positive V
negative differential signal. The negative signal drives
the integrator output positive along with V
Figure ). For such applications the integrator output
swing can be reduced below the recommended 2.0V
full scale swing. The integrator output will swing within
0.3V of V+ or V- without increasing linearity errors.
FIGURE 8-1:
Reduces Available Integrator Swing (V
© 2006 Microchip Technology Inc.
+
–
V
V
CM
IN
DEVICE PIN FUNCTIONAL
DESCRIPTION
Differential Signal Inputs
V
CM
IN
+
–
Input Buffer
+ (Pin 31), V
exists in conjunction with a full scale
Where:
Common Mode Voltage
T
C
R
V
R
I
I
I
I
= Integration Time =
=
= Integration Capacitor
= Integration Resistor
I
IN
R
T
- (Pin 30)
I
CM
COM
C
I
I
). The typical range
) (see Figure 8-2).
[
–
+
C
V
I
Integrator
CM
COM
– V
CM
IN
4000
F
≠
OSC
[
IN
V
(see
V
- is
IN
I
)
8.2
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
compared to stray node capacitance.
The TC7106A/TC7107A circuits have a significantly
lower analog common temperature coefficient. This
gives a very stable voltage suitable for use as a
reference. The temperature coefficient of analog
common is 20ppm/°C typically.
8.3
The analog common pin is set at a voltage potential
approximately 3.0V below V+. The potential is between
2.7V and 3.35V below V+. Analog common is tied inter-
nally to the N channel FET capable of sinking 20mA.
This FET will hold the common line at 3.0V should an
external load attempt to pull the common line toward
V+. Analog common source current is limited to 10μA.
Analog common is, therefore, easily pulled to a more
negative voltage (i.e., below V+ – 3.0V).
The TC7106A connects the internal V
inputs to analog common during the auto-zero cycle.
During the reference integrate phase, V
nected to analog common. If V
nected to analog common, a Common mode voltage
exists. This is rejected by the converter’s 86dB Com-
mon mode rejection ratio. In battery operation, analog
common and V
Common mode voltage concerns. In systems where V-
is connected to the power supply ground, or to a given
voltage, analog common should be connected to V
TC7106/A/TC7107/A
Differential Reference
V
Analog Common (Pin 32)
REF
+ (Pin 36), V
IN
- are usually connected, removing
IN
REF
REF
- is not externally con-
- (Pin 35)
should be large
DS21455C-page 15
IN
+ and V
IN
- is con-
IN
IN
-.
-