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TC811

3-1/2 DIGIT A/D CONVERTER WITH HOLD AND DIFFERENTIAL REFERENCE INPUTS

TelCom Semiconductor Inc => Microchip 

3-1/2 DIGIT A/D CONVERTER WITH HOLD AND

DIFFERENTIAL REFERENCE INPUTS

TC811

Analog Common (Pin 32)

The Analog Common pin is set at a voltage potential

approximately 3.0V below V+. This potential is guaranteed

to be between 2.70V and 3.35V below V+. Analog common

is tied internally to an N channel FET capable of sinking

100µA. This FET will hold the common line at 3.0V below V+

should an external load attempt to pull the common line

toward V+. Analog common source current is limited to 1µA.

Analog common is therefore easily pulled to a more negative

voltage (i.e. below V+– 3.0V).

The TC811 connects the internal VI+N and VIN– inputs to

Analog Common during the Auto Zero cycle. During the

reference integrate phase VIN– is connected to Analog

Common. If VIN– is not externally connected to Analog

Common, a common-mode voltage exists. This is rejected

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

battery powered applications, Analog Common and VIN– are

usually connected, removing common-mode voltage con-

cerns. In systems where VIN– is connected to the power

supply ground or to a given voltage, Analog Common should

be connected to VIN– .

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-

mon potential of V+ – 3.0V gives a 7V end of battery life

voltage. The analog common potential has a voltage coef-

ficient of 0.001%/%.

With a sufficiently high total supply voltage (V+ – V– >

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)

The TEST pin potential is 5V less the V+. TEST may be

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 500Ω 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.

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

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

more than several minutes, because when TEST is pulled to

V+, the LCD Segments are impressed with a DC voltage

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 500Ω 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

potential is approximately 5V below V+.

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

performance will be improved by increasing CAZ (See Auto

Zero Capacitor section). Figure 10 shows Analog Common

supplying the necessary voltage reference for the TC811.

V+

TC811

BP

TEST

V+ BP

TC811

HDLR

DECIMAL

POINT

SELECT

TEST

V+

4049

GND

TO LCD

DECIMAL

POINT

TO LCD

BACK-

PLANE

V+

TO LCD

DECIMAL

POINTS

4030

GND

TO "HOLD"

ANNUNCIATOR

Figure 9. Display Annunciator Drivers

3-146

TELCOM SEMICONDUCTOR, INC.

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