TC7126 datasheet(13/22 Pages) MICROCHIP

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DS21458B-page 13

TC7126/A

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 com-

pared to stray node capacitance.

The TC7126A offers a significantly improved analog

common temperature coefficient. This potential pro-

vides a very stable voltage, suitable for use as a refer-

ence. The temperature coefficient of analog common is

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

the TC7126.

FIGURE 7-2:

COMMON MODE VOLTAGE REMOVED IN BATTERY OPERATION WITH

VIN = ANALOG COMMON

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 should an

external load attempt to pull the common line toward

V+. Analog common source current is limited to 1

µA.

Therefore, analog common is easily pulled to a more

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

The TC7126A connects the internal VIN+and VIN-

inputs to analog common during the auto-zero phase.

During the reference integrate phase, VIN- is con-

nected to analog common. If VIN- is not externally con-

nected to analog common, a Common mode voltage

exists, but is rejected by the converter's 86dB Com-

mon mode rejection ratio. In battery operation, analog

common and VIN- are usually connected, removing

Common mode voltage concerns. In systems where

VIN- is connected to power supply ground or to a given

voltage, analog common should be connected to VIN-.

The analog common pin serves to set the analog sec-

tion reference, or common point. The TC7126A is spe-

cifically designed to operate from a battery, or in any

measurement system where input signals are not refer-

enced (float) with respect to the TC7126A's power

source. The analog common potential of V+ – 3V gives

a 7V end of battery life voltage. The common potential

has a 0.001%/% voltage coefficient and a 15

Ω output

impedance.

With sufficiently high total supply voltage (V+ – V- > 7V),

analog common is a very stable potential with excellent

temperature stability (typically 35ppm/°C). This poten-

tial can be used to generate the TC7126A's reference

voltage. An external voltage reference will be unneces-

sary in most cases because of the 35ppm/°C tempera-

ture coefficient. See Section 7.5, TC7126A Internal

Voltage Reference discussion.

7.4

TEST (Pin 37)

The TEST pin potential is 5V less than V+. TEST may

be used as the negative power supply connection for

external CMOS logic. The TEST pin is tied to the inter-

nally generated negative logic supply through a 500

Ω

resistor. The TEST pin load should be no more than

1mA. See Section 5.0, Digital Section for additional

information on using TEST as a negative digital logic

supply.

If TEST is pulled HIGH (to V+), all segments plus the

minus sign will be activated. DO NOT OPERATE IN

THIS MODE FOR MORE THAN SEVERAL MINUTES.

With TEST = V+, the LCD segments are impressed

with a DC voltage which will destroy the LCD.

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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