TC7116 Microchip, TC7116 Datasheet - Page 12

• Manufacturer Part Number: TC7116
• Description: 3-1/2 Digit Analog-to-Digital Converters with Hold
• Manufacturer: Microchip

TC7116/A/TC7117/A
4.0
4.1
The size of the auto-zero capacitor has some influence
on system noise. For 200mV full scale, where noise is
very important, a 0.47µF capacitor is recommended.
On the 2V scale, a 0.047µF capacitor increases the
speed of recovery from overload and is adequate for
noise on this scale.
4.2
A 0.1µF capacitor is acceptable in most applications.
However, where a large Common mode voltage exists
(i.e., the V
200mV scale is used, a larger value is required to pre-
vent rollover error. Generally, 1µF will hold the rollover
error to 0.5 count in this instance.
4.3
The integrating capacitor should be selected to give the
maximum voltage swing that ensures tolerance buildup
will not saturate the integrator swing (approximately
0.3V from either supply). In the TC7116/TC7116A or
the TC7117/TC7117A, when the analog common is
used as a reference, a nominal ±2V full scale integrator
swing is acceptable. For the TC7117/TC7117A, with
±5V supplies and analog common tied to supply
ground, a ±3.5V to ±4V swing is nominal. For 3 read-
ings per second (48kHz clock), nominal values for C
are 0.22µ1F and 0.10µF, respectively. If different oscil-
lator frequencies are used, these values should be
changed in inverse proportion to maintain the output
swing. The integrating capacitor must have low dielec-
tric absorption to prevent rollover errors. Polypropylene
capacitors are recommended for this application.
4.4
Both the buffer amplifier and the integrator have a class
A output stage with 100µA of quiescent current. They
can supply 20µA of drive current with negligible non-
linearity. The integrating resistor should be large
enough to remain in this very linear region over the
input voltage range, but small enough that undue leak-
age requirements are not placed on the PC board. For
2V full scale, 470kΩ is near optimum and, similarly,
47kΩ for 200mV full scale.
4.5
For all frequency ranges, a 100kΩ resistor is recom-
mended; the capacitor is selected from the equation:
EQUATION 4-1:
For a 48kHz clock (3 readings per second), C = 100pF.
DS21457B-page 12
COMPONENT VALUE
SELECTION
Auto-Zero Capacitor
Reference Capacitor
Integrating Capacitor
Integrating Resistor
Oscillator Components
IN
- pin is not at analog common), and a
f
=
0.45
---------- -
RC
INT
4.6
To generate full scale output (2000 counts), the analog
input requirement is V
and 2V scale, V
respectively. In many applications, where the ADC is
connected to a transducer, a scale factor exists
between the input voltage and the digital reading. For
instance, in a measuring system, the designer might like
to have a full scale reading when the voltage from the
transducer is 700mV. Instead of dividing the input down
to 200mV, the designer should use the input voltage
directly and select V
integrating resistor and capacitor would be 120kW and
0.22µF. This makes the system slightly quieter and also
avoids a divider network on the input. The TC7117/
TC7117A, with ±5V supplies, can accept input signals
up to ±4V. Another advantage of this system is when a
digital reading of zero is desired for V
ture and weighing systems with a variable tare are
examples. This offset reading can be conveniently gen-
erated by connecting the voltage transducer between
V
offset voltage between analog common and V
5.0
The TC7117/TC7117A are designed to operate from
±5V supplies. However, if a negative supply is not avail-
able, it can be generated with a TC7660 DC-to-DC con-
verter and two capacitors. Figure 5-1 shows this
application.
In selected applications, a negative supply is not
required. The conditions for using a single +5V supply
are:
1.
2.
3.
FIGURE 5-1:
IN
+ and analog common, and the variable (or fixed)
The input signal can be referenced to the center
of the Common mode range of the converter.
The signal is less than ±1.5V.
An external reference is used.
10µF
Reference Voltage
TC7117/TC7117A POWER
SUPPLIES
+
2
4
TC7660
LED
Drive
REF
8
3
+5V
10µF
+
REF
IN
SUPPLY GENERATION
WITH TC7660
NEGATIVE POWER
5
should equal 100mV and 1V,
©
TC7117A
= 2V
(-5V)
TC7117
= 350mV. Suitable values for
2002 Microchip Technology Inc.
REF
V+
V-
35
26
V
. Thus, for the 200mV
REF
COM
GND
V
V
IN
IN
+
+
-
36
32
31
30
21
IN
≠ 0. Tempera-
IN
-.
V
IN
+
–