TC7135CLI datasheet(8/24 Pages) MICROCHIP

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TC7135

DS21460C-page 8

 2004 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 Com-

mon 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 posi-

tive 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 VREF

V+

10 k

Ω

10 k

Ω

V+

REF

ANALOG

COMMON

Analog Ground

TC7135

1µF

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