TC835 datasheet(9/24 Pages) MICROCHIP

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DS21478B-page 9

TC835

4.0

ANALOG SECTION

FUNCTIONAL DESCRIPTION

(In Reference to the 28-Pin Plastic Package)

4.1

Differential Inputs

(+INPUT (Pin 10) and

–INPUT (Pin 9))

The TC835 operates with differential voltages within

the input amplifier Common mode range. The input

amplifier Common mode range extends from 0.5V

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. The integrator output must not be allowed to

saturate. An example of a worst case condition would

be 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, with

the effect of reduced accuracy. The integrator output

can swing within 0.3V of either supply without loss of

linearity.

4.2

Analog Common Input (Pin 3)

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. 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

(REF IN (Pin 2))

The REF IN input 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 VREF

V+

10k

V+

REF

ANALOG

COMMON

Analog Ground

TC835

µF

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