TC7126 datasheet(14/26 Pages) MICROCHIP

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TC7126/A

DS21458C-page 14

7.0

DEVICE PIN FUNCTIONAL

DESCRIPTION

(Pin Numbers Refer to the 40-Pin PDIP.)

7.1

Differential Signal Inputs

VIN+ (Pin 31), VIN- (Pin 30)

The TC7126A is designed with true differential inputs

and accepts input signals within the input stage

Common mode voltage range (VCM). Typical range is

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

removed from the system when the TC7126A operates

from a battery or floating power source (isolated from

measured system), and VIN- is connected to analog

common (VCOM) (see Figure 7-2).

In systems where Common mode voltages exist, the

TC7126A’s 86 dB Common mode rejection ratio

minimizes error. Common mode voltages do, however,

affect the integrator output level. A worst-case condi-

tion 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 7-1). For such applications,

the integrator output 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 7-1:

Common Mode Voltage

Reduces Available Integrator Swing (V

COM ≠ VIN)

7.2

Differential Reference

VREF+ (Pin 36), 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

compared to stray node capacitance.

The TC7126A offers a significantly improved analog

common

temperature

coefficient.

This

potential

provides a very stable voltage, suitable for use as a ref-

erence. The temperature coefficient of analog common

is typically 35ppm/°C for the TC7126A and 80 ppm/°C

for the TC7126.

FIGURE 7-2:

Common Mode Voltage Removed in Battery Operation with V

IN = Analog Common

–

VIN

Integrator

VI =

[

VCM – VIN

Input

Buffer

CI = Integration capacitor

RI = Integration resistor

4000

FOSC

tI = Integration time =

Where:

–

RI CI

VCM

VBUFF

CAZ

VINT

POL

Segment

Drive

OSC1

OSC3

OSC2

V-

V+

VREF+

VREF-

ANALOG

COMMON

V-

V+

V-

V+

GND

Measured

System

Power

Source

LCD

TC7126A

VIN-

VIN+

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