TC820
DS21476C-page 10 © 2007 Microchip Technology Inc.
3.2.3 SIGNAL INTEGRATION PHASE
Upon completion of the auto-zero phase, the auto-zero
loop is opened and the internal differential inputs
connect to V
IN
+ and V
IN
-. The differential input signal is
then integrated for a fixed time period, which is 2000
counts (4000 clock periods). The externally set clock
frequency is divided by two before clocking the internal
counters.
The integration time period is:
EQUATION 3-3:
The differential input voltage must be within the
device's Common mode range when the converter and
measured system share the same power supply
common (ground). If the converter and measured
system do not share the same power supply common,
as in battery powered applications, V
IN
- should be tied
to analog common.
Polarity is determined at the end of signal integration
phase. The sign bit is a "true polarity" indication, in that
signals less than 1LSB are correctly determined. This
allows precision null detection that is limited only by
device noise and auto-zero residual offsets.
3.2.4 REFERENCE INTEGRATE
(DE-INTEGRATE) PHASE
The reference capacitor, which was charged during the
auto-zero phase, is connected to the input of the
integrating amplifier. The internal sign logic ensures the
polarity of the reference voltage is always connected in
the phase opposite to that of the input voltage. This
causes the integrator to ramp back to zero at a constant
rate, determined by the reference potential.
The amount of time required (t
DEINT
) for the integrating
amplifier to reach zero is directly proportional to the
amplitude of the voltage that was put on the integrating
capacitor (V
INT
) during the integration phase.
EQUATION 3-4:
The digital reading displayed by the TC820 is:
The oscillator frequency is divided by 2 prior to clock-
ing the internal decade counters. The four-phase
measurement cycle takes a total of 8000 (4000) counts
or 16,000 clock pulses. The 8000 count phase is
independent of input signal magnitude or polarity.
Each phase of the measurement cycle has the follow-
ing length:
TABLE 3-2: MEASUREMENT CYCLE
PHASE LENGTH
3.2.5 INPUT OVER RANGE
When the analog input is greater than full scale, the
LCD will display "OL" and the "OVER RANGE" LCD
annunciator will be on.
3.2.6 PEAK READING HOLD
The TC820 provides the capability of holding the high-
est (or peak) reading. Connecting the PK HOLD input
to V
DD
enables the peak hold feature. At the end of
each conversion, the contents of the TC820 counter
are compared to the contents of the display register. If
the new reading is higher than the reading being
displayed, the higher reading is transferred to the
display register. A "higher" reading is defined as the
reading with the higher absolute value.
The peak reading is held in the display register, so the
reading will not "droop" or slowly decay with time. The
held reading will be retained until a higher reading
occurs, the PK HOLD input is disconnected from V
DD
,
or power is removed.
The peak signal to be measured must be present
during the TC820 signal integrate period. The TC820
does not perform transient peak detection of the analog
input signal. However, in many cases, such as measur-
ing temperature or electric motor starting current, the
TC820 "acquisition time" will not be a limitation. If true
peak detection is required, a simple circuit will suffice.
See the applications section for details.
t
DEINT
=
R
INT
C
INT
V
INT
V
REF
V
IN
+V
IN
-
Digital Count = 2000
V
REF
Conversion Phase Counts
1) Auto-Zero 1500
2) Signal Integrate (Notes 1, 2) 2000
3) Reference Integrate 1 to 4001
4) Integrator Output Zero 499 to 4499
Note 1: This time period is fixed. The integration
period for theTC820 is:
INT (TC820) = 4000/F
OSC
= 2000 counts.
Where F
OSC
is the clock oscillator
frequency.
2: Times shown are the RANGE/FREQ at
logic low (normal operation). When
RANGE/FREQ is logic high, signal
integrate times are 200 counts. See
Section 3.2.7 “10:1 Range Change”.
