TC7116/A/TC7117/A
DS21457D-page 12 2002-2012 Microchip Technology Inc.
4.0 COMPONENT VALUE
SELECTION
4.1 Auto-Zero Capacitor
The size of the auto-zero capacitor has some influence
on system noise. For 200mV full scale, where noise is
very important, a 0.47F capacitor is recommended.
On the 2V scale, a 0.047F capacitor increases the
speed of recovery from overload and is adequate for
noise on this scale.
4.2 Reference Capacitor
A 0.1F capacitor is acceptable in most applications.
However, where a large Common mode voltage exists
(i.e., the V
IN
- pin is not at analog common), and a
200mV scale is used, a larger value is required to pre-
vent rollover error. Generally, 1F will hold the rollover
error to 0.5 count in this instance.
4.3 Integrating Capacitor
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
INT
are 0.221F and 0.10F, 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 Integrating Resistor
Both the buffer amplifier and the integrator have a class
A output stage with 100A of quiescent current. They
can supply 20A 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, 470k is near optimum and, similarly,
47k for 200mV full scale.
4.5 Oscillator Components
For all frequency ranges, a 100k resistor is recom-
mended; the capacitor is selected from the equation:
EQUATION 4-1:
For a 48kHz clock (3 readings per second), C = 100pF.
4.6 Reference Voltage
To generate full scale output (2000 counts), the analog
input requirement is V
IN
= 2V
REF
. Thus, for the 200mV
and 2V scale, V
REF
should equal 100mV and 1V,
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
REF
= 350mV. Suitable values for
integrating resistor and capacitor would be 120kW and
0.22F. 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
IN
0. Tempera-
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
IN
+ and analog common, and the variable (or fixed)
offset voltage between analog common and V
IN
-.
