TC820CPL datasheet(14/22 Pages) TELCOM

14 / 22 page

3-162

TELCOM SEMICONDUCTOR, INC.

3-3/4 A/D CONVERTER WITH

FREQUENCY COUNTER

AND LOGIC PROBE

TC820

Table I. TC820 Control Input Truth Table

Logic Input

FREQ/

RANGE/

TC820

VOLTS

FREQ

LOGIC

Function

Logic Probe

A/D Converter,

VFULL SCALE = 2

VREF

A/D Converter,

VFULL SCALE = 20

VREF

Frequency

Frequency Counter

Counter Input

NOTES: 1. Logic "0" = DGND

2. Logic "1" = VDD

Figure 9.

Common-Mode Voltage Reduces Available

Integrator Swing (VCOM

VIN)

–

VIN

INTEGRATOR

INPUT

BUFFER

–

TI= Integration Time =

CI= Integration Capacitor

RI= Integration Resistor

4000

VCM

OSC

Where:

VI =

[

VCM

VIN

–

RI CI

FREQ/VOLTS

This input determines whether the TC820 is in the

analog-to-digital conversion mode or in the frequency counter

mode. When FREQ/VOLTS is connected to VDD, the TC820

will measure frequency at the RANGE/FREQ input. When

unconnected, or connected to DGND, the TC820 will oper-

ate as an analog-to-digital converter. This input has an

internal 5

µA pull-down to DGND.

LOGIC

The LOGIC input is used to activate the logic probe

function. When connected to VDD, the TC820 will enter the

logic probe mode. The LCD will show "OL" and all decimal

points will be off. The decimal point inputs directly control

"high" and "low" display annunciators. When LOGIC is

unconnected, or connected to DGND, the TC820 will per-

form analog-to-digital or frequency measurements as se-

lected by the FREQ/VOLTS input. The LOGIC input has an

internal 5

µA pull-down to DGND.

system where input signals are not referenced (float)

with respect to the TC820 power source. The analog common

potential of VDD – 3.3V gives a 7V end-of-battery-life voltage.

The analog common potential has a voltage coefficient of

0.001%/%.

With a sufficiently high total supply voltage (VDD – VSS

> 7V), analog common is a very stable potential with excellent

temperature stability (typically 35ppm/

°C). This potential

can be used to generate the TC820 reference voltage. An

external voltage reference will be unnecessary in most

cases, because of the 35ppm/

°C temperature coefficient.

See the applications section for details.

Function Control Input Pin

Functional Description

The TC820 operating modes are selected with the

function control inputs. The control input truth table is shown

in Table I. The high logic threshold is

≥ VDD - 1.5V and the

low logic level is

≤ DGND +1.5V.

RANGE/FREQ

The function of this dual-purpose pin is determined by

the FREQ/VOLTS input. When FREQ/VOLTS is connected

to VDD, RANGE/FREQ is the input for the frequency counter

function. Pulses at this input are counted with a time base

equal to fOSC/40,000. Since this input has CMOS input levels

(VDD - 1.5V and DGND +1.5V), an external buffer is recom-

mended.

When the TC820 analog-to-digital converter function is

selected, connecting RANGE/FREQ to VDD will divide the

integration time by 10. Therefore, the RANGE/FREQ input

can be used to perform a 10:1 range change without

changing external components.

DP0/LO, DP1/HI

The function of these dual-purpose pins is determined

by the LOGIC input. When the TC820 is in the analog-to-

digital converter mode, these inputs control the LCD decimal

points. The decimal point truth table is shown in Table II.

These inputs have internal 5

µA pull-downs to DGND when

the voltage/frequency measurement mode is active.

Table II. TC820 Decimal Point Truth Table

Decimal Point Inputs

DP1

DP0

LCD

3999

399.9

39.99

3.999

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