© 2007 Microchip Technology Inc. DS21476C-page 13
TC820
When the logic probe function is selected while FREQ/
VOLTS is low (A/D mode), the ADC will remain in the
Auto-Zero mode. The LCD will read "OL" and all
decimal points will be off (Figure ).
FIGURE 3-6: LCD Reading.
If the logic probe is active while FREQ/VOLTS is high
(Counter mode), the frequency counter will continue to
operate. The display will read "OL" but the decimal
points will be visible. If the logic probe input is also
connected to the RANGE/FREQ input, bringing the
LOGIC input low will immediately display the frequency
at the logic probe input.
3.5 Analog Pin Functional Description
3.5.1 DIFFERENTIAL SIGNAL INPUTS
(V
IN
+), (V
IN
-)
The TC820 is designed with true differential inputs, and
accepts input signals within the Input Stage Common
mode voltage (V
CM
) range. The typical range is
V
DD
–1V to V
SS
+ 1.5V. Common mode voltages are
removed from the system when the TC820 operates
from a battery or floating power source (isolated from
measured system) and V
SS
is connected to analog
common (see Figure 3-5).
In systems where Common mode voltages exist, the
86 dB Common mode rejection ratio minimizes error.
Common mode voltages do, however, affect the
integrator output level. A worst case condition exists if
a large, positive V
CM
exists in conjunction with a full
scale, negative differential signal. The negative signal
drives the integrator output positive along with V
CM
(Figure 3-6). For such applications, the integrator
output swing can be reduced below the recommended
2V full scale swing. The integrator output will swing
within 0.3V of V
DD
, or V
DD
without increased linearity
error.
3.5.2 REFERENCE (V
DD
, V
SS
)
The TC820 reference, like the analog signal input, has
true differential inputs. In addition, the reference
voltage can be generated anywhere within the power
supply voltage of the converter. The differential
reference inputs permit ratiometric measurements and
simplify interfacing with sensors, such as load cells and
temperature sensors.
To prevent rollover type errors from being induced by
large Common mode voltages, C
REF
should be large
compared 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 35 ppm/°C.
3.5.3 ANALOG COMMON
The analog common pin is set at a voltage potential
approximately 3.3V below V
DD
. This potential is
between 3.15V and 3.45V below V
DD
. Analog common
is tied internally to an N-channel FET capable of sink-
ing 3 mA. This FET will hold the common line at 3.3V
below V
DD
should an external load attempt to pull the
common line toward V
DD
. Analog common source
current is limited to 12 µA, and is, therefore, easily
pulled to a more negative voltage (i.e., below V
DD
–
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 86 dB 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" system where input signals are not
referenced (float), with respect to the TC820 power
source. The analog common potential of V
DD
– 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
(V
DD
–V
SS
> 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 35 ppm/°C temperature coefficient. See the
applications section for details.
High
Low
*
**
* "High" Annuciator will be on when DP1/HI = Logic High
** "Low" Annunciator and Buzzer will be on when DP0/LO = Logic High
