TC820CPL datasheet(13/22 Pages) TELCOM

13 / 22 page

3-161

TELCOM SEMICONDUCTOR, INC.

TC820

3-3/4 A/D CONVERTER WITH

FREQUENCY COUNTER

AND LOGIC PROBE

To prevent roll-over-type errors from being induced by

large common-mode voltages, CREF should be large com-

pared to stray node capacitance. A 0.1

µF capacitor is

typical.

The TC820 offers a significantly improved analog

common temperature coefficient, providing a very stable

voltage suitable for use as a voltage reference. The

temperature coefficient of analog common is typically

35ppm/

°C.

Figure 7. LCD During Logic Probe Operation

HIGH

LOW

"HIGH" ANNUNCIATOR WILL BE ON WHEN DP1/HI =

LOGIC HIGH

"LOW" ANNUNCIATOR AND BUZZER WILL BE ON

WHEN DP0/LO = LOGIC HIGH

Analog Common

The analog common pin is set at a voltage potential

approximately 3.3V below VDD. This potential is guaranteed

to be between 3.15V and 3.45V below VDD. Analog common

is tied internally to an N-channel FET capable of sinking

3mA. This FET will hold the common line at 3.3V below VDD

should an external load attempt to pull the common line

toward VDD. Analog common source current is limited to

µA, and is therefore easily pulled to a more negative

voltage (i.e., below VDD – 3.3V).

The TC820 connects the internal V

IN and V

–

IN inputs to

analog common during the auto-zero cycle. During the

reference integrate phase, V

–

IN is connected to analog

common. If V

–

IN 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 V

–

IN are usually

connected, removing common-mode voltage concerns. In

systems where V

–

IN is connected to the power supply ground

or to a given voltage, analog common should be connected

to V

–

IN.

The analog common pin serves to set the analog section

reference or common point. The TC820 is specifically

designed to operate from a battery or in any measurement

TC820

GND

–

MEASURED

SYSTEM

POWER

SOURCE

VIN

VDD

VSS

REF

–

REF

VIN

–

VBUF

CAZ

BP1

INT

BP3

BP2

OSC1

OSC2

OSC3 NC

SEGMENT

DRIVE

LCD

ANALOG

COMMON

Figure 8. Common-Mode Voltage Removed in Battery Operation With V

–

IN = Analog Common

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