TC7136ACPL datasheet(10/12 Pages) TELCOM

10 / 12 page

3-256

TELCOM SEMICONDUCTOR, INC.

Figure 7. Common-Mode Voltage Removed in Battery Operation With VIN = Analog Common

In systems where common-mode voltages exist, the

86 dB common-mode rejection ratio minimizes error. Com-

mon-mode voltages do, however, affect the integrator out-

put level. A worst-case condition exists if a large positive

VCM exists in conjunction with a full-scale negative differ-

ential signal. The negative signal drives the integrator

output positive along with VCM (see Figure 8.) For such

applications, the integrator output swing can be reduced

below the recommended 2V full-scale swing. The integra-

tor output will swing within 0.3V of V+ or V– without in-

creased linearity error.

Differential Reference

REF (Pin 36), V

–

REF (Pin 35)

The reference voltage can be generated anywhere

within the V+ to V– power supply range.

To prevent roll-over type errors being induced by large

common-mode voltages, CREF should be large compared to

stray node capacitance.

The TC7136/A offers a significantly improved analog

common temperature coefficient. This potential provides a

very stable voltage, suitable for use as a voltage reference.

The temperature coefficient of analog common is typically

35 ppm/

°C.

ANALOG COMMON (Pin 32)

The analog common pin is set at a voltage potential

approximately 3V below V+. The potential is guaranteed to

be between 2.7V and 3.35V below V+. Analog common is

tied internally 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).

The TC7136/A connects the internal VIN

+ and VIN– in-

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

the reference-integrate phase, VIN

–

is connected to analog

common. If VIN

– 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 VIN

– are usually connected,

removing common-mode voltage concerns. In systems where

VIN

– is connected to the power supply ground or to a given

voltage, analog common should be connected to V

–

The analog common pin serves to set the analog sec-

tion reference, or common point. The TC7136A is specifi-

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

ment system where input signals are not referenced (float)

with respect to the TC7136A 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.

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

Figure 8. Common-Mode Voltage Reduces Available Integrator

Swing (VCOM

≠ V

IN)

VBUF

CAZ VINT

POL

SEGMENT

DRIVE

OSC1

OSC3

OSC2

–

VREF

–

ANALOG

COMMON

–

GND

MEASURED

SYSTEM

POWER

SOURCE

LCD

TC7136

TC7136A

–

VIN

C I

INTEGRATOR

VI =

[

VCM

VIN

–

INPUT

BUFFER

CI =

Integration capacitor

Integration resistor

4000

Integration time

T I ==

Where:

OSC

–

RI CI

TC7136

TC7136A

LOW POWER, 3-1/2 DIGIT

ANALOG-TO-DIGITAL CONVERTERS

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