TC7135CPI datasheet(8/22 Pages) MICROCHIP

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TC7135

DS21460B-page 8

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

86dB 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 posi-

tive Common mode voltage with a near full scale neg-

ative 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, with some loss of

accuracy. The integrator output can swing within 0.3V

of either supply without loss of linearity.

4.2

Analog Common Input

ANALOG COMMON is used as the -INPUT return dur-

ing 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 applica-

tions, –INPUT will be set at a fixed, known voltage

(power supply common, for instance). In this applica-

tion, 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 voltagewithrespect toANALOG COMMON. A

reference voltage circuit is shown in Figure 4-1.

FIGURE 4-1:

USING AN EXTERNAL

REFERENCE

MCP1525

2.5 VREF

V+

10k

V+

REF

ANALOG

COMMON

Analog Ground

TC7135

µF

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