TC820CPL datasheet(8/28 Pages) MICROCHIP

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TC820

DS21476B-page 8

3.0

DETAILED DESCRIPTION

The TC820 is a 3-3/4 digit measurement system com-

bining an integrating analog-to-digital converter, fre-

quency counter, and logic level tester in a single

package. The TC820 supersedes the TC7106 in new

designs by improving performance and reducing sys-

tem cost. The TC820 adds features that are difficult,

expensive, or impossible to provide with older A/D con-

verters (see Table 3-1). The high level of integration

permits TC820 based instruments to deliver higher per-

formance and more features, while actually reducing

parts count. Fabricated in low power CMOS, the TC820

directly drives a 3-3/4 digit (3999 maximum) LCD.

With a maximum range of 3999 counts, the TC820 pro-

vides 10 times greater resolution in the 200mV to

400mV range than traditional 3-1/2 digit meters. An

auto-zero cycle ensures a zero reading with a 0V input.

CMOS processing reduces analog input bias current to

only 1pA. Rollover error (the difference in readings for

equal magnitude but opposite polarity input signals) is

less than ±1 count. Differential reference inputs permit

ratiometric measurements for ohms or bridge trans-

ducer applications.

The TC820's frequency counter option simplifies

design of an instrument well-suited to both analog and

digital troubleshooting: voltage, current, and resistance

measurements, plus precise frequency measurements

to 4MHz (higher frequencies can be measured with an

external prescaler), and a simple logic probe. The fre-

quency counter will automatically adjust its range to

match the input frequency, over a four-decade range.

Two logic level measurement inputs permit a TC820

based meter to function as a logic probe. When com-

bined with external level shifters, the TC820 will display

logic levels on the LCD and also turn on a piezoelectric

buzzer when the measured logic level is low.

Other TC820 features simplify instrument design and

reduce parts count. On-chip decimal point drivers are

included, as is a low battery detection annunciator. A

piezoelectric buzzer can be controlled with an external

switch or by the logic probe inputs. Two oscillator

options are provided: a crystal can be used if high accu-

racy frequency measurements are desired, or a simple

RC option can be used for low-end instruments.

A "peak reading hold" input allows the TC820 to retain

the highest A/D or frequency reading. This feature is

useful in measuring motor starting current, maximum

temperature, and similar applications.

A family of instruments can be created with the TC820.

No additional design effort is required to create instru-

ments with 3-3/4 digit resolution.

The TC820 operates from a single 9V battery, with typ-

ical power of 10mW. Packages include a 40-pin plastic

DIP, 44-pin plastic flat package (PQFP), and 44-pin

PLCC.

TABLE 3-1:

COMPETITIVE EVALUATION

3.1

General Theory of Operation

3.1.1

DUAL SLOPE CONVERSION

PRINCIPLES

The TC820 analog-to-digital converter operates on the

principle of dual slope integration. An understanding of

the dual slope conversion technique will aid the user in

following the detailed TC820 theory of operation follow-

ing this section. A conventional dual slope converter

measurement cycle has two distinct phases:

Input Signal Integration

Reference Voltage Integration (De-integration)

Referring to Figure 3-1, the unknown input signal to be

converted is integrated from zero for a fixed time period

(tINT), measured by counting clock pulses. A constant

reference voltage of the opposite polarity is then inte-

grated until the integrator output voltage returns to

zero. The reference integration (de-integration) time

(tDEINT) is then directly proportional to the unknown

input voltage (VIN).

Features Comparison

TC820

7106

3-3/4 Digit Resolution

Yes

Auto-Ranging Frequency

Counter

Yes

Logic Probe

Yes

Decimal Point Drive

Yes

Peak Reading Hold

(Frequency or Voltage)

Yes

Display Hold

Yes

Simple 10:1 Range Change

Yes

Buzzer Drive

Yes

Low Battery Detection

with Annunciator

Yes

Over Range Detection

with Annunciator

Yes

Low Drift Reference

Yes

Under Range/Over Range

Logic Output

Yes

Input Overload Display

"OL"

"1"

LCD Annunciator Driver

Yes

LCD Drive Type

Triplexed

Direct

LCD Pin Connections

LCD Elements

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