TC820
DS21476C-page 8 © 2007 Microchip Technology Inc.
3.0 DETAILED DESCRIPTION
The is a 3-3/4 digit measurement system combining an
integrating analog-to-digital converter, frequency
counter, and logic level tester in a single package. The
TC820 supersedes the TC7106 in new designs by
improving performance and reducing system cost. The
TC820 adds features that are difficult, expensive, or
impossible to provide with older A/D converters (see
Table 3-1). The high level of integration permits TC820
based instruments to deliver higher performance 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
provides 10 times greater resolution in the 200 mV to
400 mV 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 1 pA. 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
transducer 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
frequency 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
combined 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
accuracy 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
instruments with 3-3/4 digit resolution.
The TC820 operates from a single 9V battery, with
typical power of 10mW. Packages include a 40-lead
plastic DIP, 44-lead plastic flat package (PQFP), and
44-lead 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
following this section. A conventional dual slope con-
verter measurement cycle has two distinct phases:
1. Input Signal Integration.
2. 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
(t
INT
), measured by counting clock pulses. A constant
reference voltage of the opposite polarity is then
integrated until the integrator output voltage returns to
zero. The reference integration (de-integration) time
(t
DEINT
) is then directly proportional to the unknown
input voltage (V
IN
).
Features Comparison TC820 7106
3-3/4 Digit Resolution Yes No
Auto-Ranging Frequency
Counter
Yes No
Logic Probe Yes No
Decimal Point Drive Yes No
Peak Reading Hold
(Frequency or Voltage)
Yes No
Display Hold Yes No
Simple 10:1 Range Change Yes No
Buzzer Drive Yes No
Low Battery Detection
with Annunciator
Yes No
Over Range Detection
with Annunciator
Yes No
Low Drift Reference Yes No
Under Range/Over Range
Logic Output
Yes No
Input Overload Display "OL" "1"
LCD Annunciator Driver Yes No
LCD Drive Type Triplexed Direct
LCD Pin Connections 15 24
LCD Elements 36 23
