TC7135
DS21460D-page 8 © 2007 Microchip Technology Inc.
4.0 ANALOG SECTION
FUNCTIONAL DESCRIPTION
4.1 Differential Inputs
The TC7135 operates with differential voltages
(+INPUT, pin 10 and -INPUT, pin 9) within the input
amplifier Common mode range, which extends from 1V
below the positive supply to 1V above the negative
supply. Within this Common mode voltage range, an
86 dB Common mode rejection ratio is typical.
The integrator output also follows the Common mode
voltage and must not be allowed to saturate. A worst-
case condition exists, for example, when a large
positive Common mode voltage with a near full scale
negative differential input voltage is applied. The
negative 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 4V full scale swing, resulting in some
loss of accuracy. The integrator output can swing within
0.3V of either supply without loss of linearity.
4.2 Analog Common Input
The ANALOG COMMON pin is used as the -INPUT
return during auto-zero and de-integrate. If -INPUT is
different from ANALOG COMMON, a Common mode
voltage exists in the system. However, this signal is
rejected by the excellent CMRR of the converter. In
most applications, –INPUT 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 reference voltage is referenced
to ANALOG COMMON.
4.3 Reference Voltage Input
The reference voltage input (REF IN) must be a
positive voltage with respect to ANALOG COMMON. A
reference voltage circuit is shown in Figure 4-1.
FIGURE 4-1: Using An External
Reference.
MCP1525
2.5 V
REF
V+
10 kΩ
10 kΩ
V+
REF
IN
ANALOG
COMMON
Analog Ground
TC7135
1µF
