TC7136 datasheet(13/22 Pages) MICROCHIP

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DS21461B-page 13

TC7136/TC7136A

7.0

DEVICE PIN FUNCTIONAL

DESCRIPTION

7.1

Differential Signal Inputs

VIN+(Pin31), VIN-(Pin 30)

The TC7136/A is designed with true differential inputs

and accepts input signals within the input stage Com-

mon mode voltage range (VCM). The typical range is

V+ – 1V to V- + 1V. Common mode voltages are

removed from the system when the TC7136A operates

from a battery or floating power source (isolated from

measured system), Common mode voltage removed

in battery operation with VIN = analog common and VIN-

connected

analog

common

(VCOM)(see

Figure 7-1).

FIGURE 7-1:

COMMON MODE VOLTAGE REMOVED IN BATTERY OPERATION WITH

VIN = ANALOG COMMON

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. A worst case condition exists if a

large positive VCM exists in conjunction with a full scale

negative differential signal. The negative signal drives

the integrator output positive along with VCM (see

Figure 7-2.) For such applications, the integrator out-

put swing can be reduced below the recommended 2V

full scale swing. The integrator output will swing within

0.3V of V+ or V- without increased linearity error.

FIGURE 7-2:

COMMON MODE

VOLTAGE REDUCES

AVAILABLE INTEGRATOR

SWING

(VCOM ≠ VIN)

7.2

Differential Reference

VREF+(Pin36), VREF-(Pin 35)

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, CREF should be large com-

pared to stray node capacitance. The TC7136/A offers

a significantly improved analog common temperature

coefficient. This potential provides a very stable volt-

age, suitable for use as a voltage reference. The

temperature coefficient of analog common is typically

35ppm/°C.

7.3

Analog Common(Pin32)

The analog common pin is set at a voltage potential

approximately 3V below V+. The potential is between

2.7V and 3.35V below V+. Analog common is tied inter-

nally to an N-channel FET, capable of sinking 100

µA.

This FET will hold the common line at 3V below V+ if an

external load attempts to pull the common line toward

V+. Analog common source current is limited to 1

µA.

Analog common is, therefore, easily pulled to a more

negative voltage (i.e., below V+ – 3V).

VBUF

CAZ

VINT

POL

Segment

Drive

OSC1

OSC3

OSC2

V-

V+

VREF+

VREF-

ANALOG

COMMON

V-

V+

GND

Measured

System

Power

Source

LCD

TC7136

TC7136A

V-

V+

V-

–

VIN

Integrator

VI =

[

VCM = VIN

Input Buffer

CI = Integration capacitor

RI = Integration resistor

4000

FOSC

tI = Integration time =

Where:

–

VCM

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