10

TC811

© 2001 Microchip Technology Inc.

DS21472A

3-1/2 Digit Analog-To-Digital Converter with

Hold and Differential Reference Inputs

TC811-7 11/5/96

Analog Common (Pin 32)

The Analog Common pin is set at a voltage potential

is tied internally to an N channel FET capable of sinking

100

should an external load attempt to pull the common line

µA.

Analog common is therefore easily pulled to a more negative

+

and V

–

Analog Common during the Auto Zero cycle. During the

mon. If V

–

a common-mode voltage exists. This is rejected by the

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

powered applications, Analog Common and V

–

connected, removing common-mode voltage concerns. In

systems where V

–

or to a given voltage, Analog Common should be connected

to V

–

The Analog Common pin serves to set the analog

section reference or common point. The TC811 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 TC811 power source. The Analog Com-

voltage. The analog common potential has a voltage coef-

ficient of 0.001%/%.

7.0V), Analog Common is a very stable potential with

excellent temperature stability (typically 35ppm/

°C). This

potential can be used to generate the TC811 reference

voltage. An external voltage reference will be unnecessary

in most cases because of the 35ppm/

°C temperature coef-

ficient. See TC811 Internal Voltage Reference discussion.

TEST (Pin 37)

used as the negative power supply connection when inter-

facing the TC811 to external CMOS logic. The TEST pin is

tied to the internally generated negative logic supply through

a 500

Ω resistor. The TEST pin may be used to sink up to

1mA. See the applications section for additional information

on using TEST as a negative digital logic supply.

minus sign will be activated. Do not operate in this mode for

more than several minutes, because when TEST is pulled to

which may cause damage to the LCD.

APPLICATIONS INFORMATION

Decimal Point and Annunciator Drive

The TEST pin is connected to the internally generated

digital logic supply ground through a 500

Ω resistor. The

TEST pin may be used as the negative supply for external

CMOS gate segment drivers. LCD display annunciators for

decimal points, low battery indication, or function indication

may be added without adding an additional supply. No more

than 1mA should be supplied by the TEST pin. The TEST pin

Internal Voltage Reference

The TC811 Analog Common voltage temperature sta-

bility has been significantly improved. This improved device

can be used to upgrade old systems and design new

systems without external voltage references. External R and

C values do not need to be changed, however, noise

Zero Capacitor section). Figure 10 shows Analog Common

supplying the necessary voltage reference for the TC811.

TC811

BP

TEST

V+

V+

GND

TO LCD

DECIMAL

POINT

TO LCD

BACK-

PLANE

4049

TC811

DECIMAL

POINT

SELECT

V

+

V

+

TEST

GND

4030

TO LCD

DECIMAL

POINTS

BP

HDLR

TO "HOLD"

ANNUNCIATOR

Figure 9. Display Annunciator Drivers