TC7126 View Datasheet(PDF) - TelCom Semiconductor Inc => Microchip

Part Name

Description

Manufacturer

TC7126

3-1/2 DIGIT ANALOG-TO-DIGITAL CONVERTERS

TelCom Semiconductor Inc => Microchip 

TC7126

TC7126A

3-1/2 DIGIT

ANALOG-TO-DIGITAL CONVERTERS

SEGMENT

DRIVE

LCD

MEASURED

SYSTEM

V+

V–

GND

V+ V– GND

POWER

SOURCE

VBUFF

VI+N

CAZ VINT

VI–N

TC7126A

ANALOG

COMMON

VR–EF

VR+ EF

V+

POL BP

OSC1

OSC3

OV –SC2

+

9V

Figure 5. Common-Mode Voltage Removed in Battery Operation With VIN = Analog Common

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 V+IN and VI–N in-

puts to analog common during the auto-zero phase. During

the reference-integrate phase, VI–N is connected to analog

common. If V+IN is not externally connected to analog com-

mon, a common-mode voltage exists, but is rejected by the

converter's 86 dB common-mode rejection ratio. In battery

operation, analog common and VI–N are usually connected,

removing common-mode voltage concerns. In systems where

VI–N is connected to power supply ground or to a given

voltage, analog common should be connected to VI–N.

The analog common pin serves to set the analog sec-

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

cally designed to operate from a battery or in any measure-

ment system where input signals are not referenced (float)

+

VIN

–

VCM

INPUT

BUFFER

+

RI

–

CI

–

+

VI

INTEGRATOR

[ VI

Where:

=

TI

RI CI

VCM – VIN

TI

=

Integration time

=

4000

fOSC

CI = Integration capacitor

RI = Integration resistor

Figure 6. Common-Mode Voltage Reduces Available Integrator

Swing (VCOM ≠ VIN)

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 15Ω output impedance.

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

analog common is a very stable potential with excellent

temperature stability (typically 35 ppm/°c). This potential

can be used to generate the TC7126A's reference voltage.

An external voltage reference will be unnecessary in most

cases because of the 35 ppm/°C temperature coefficient.

See "TC7126A Internal Voltage Reference" discussion.

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 internally-generated

negative logic supply through a 500Ω resistor. The TEST pin

load should not be more than 1 mA. See "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.

TC7126A Internal Voltage Reference

The TC7126A's analog common voltage temperature

stability has been significantly improved (Figure 7). The "A"

version of the industry-standard TC7126 device allows

users to upgrade old systems and design new systems

without external voltage references. External R and C val-

ues do not need to be changed. Figure 10 shows analog

common supplying the necessary voltage reference for the

TC7126A.

3-226

TELCOM SEMICONDUCTOR, INC.

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