TC7117 TelCom, TC7117 Datasheet - Page 11

• Manufacturer Part Number: TC7117
• Description: 3-1/2 DIGIT ANALOG-TO-DIGITAL CONVERTERS WITH HOLD
• Manufacturer: TelCom

3-1/2 DIGIT ANALOG-TO-DIGITAL
CONVERTERS WITH HOLD
COMPONENT VALUE SELECTION
Auto-Zero Capacitor
ence on system noise. For 200 mV full scale, where noise
is very important, a 0.47 F capacitor is recommended. On
the 2V scale, a 0.047 F capacitor increases the speed of
recovery from overload and is adequate for noise on this
scale.
Reference Capacitor
However, where a large common-mode voltage exists (i.e.,
the V
is used, a larger value is required to prevent roll-over error.
Generally, 1 F will hold the roll-over error to 0.5 count in
this instance.
Integrating Capacitor
maximum voltage swing that ensures tolerance build-up 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 com-
mon tied to supply ground, a 3.5V to 4V swing is nominal.
For 3 readings per second (48 kHz clock), nominal values
for C
oscillator frequencies are used, these values should be
changed in inverse proportion to maintain the output swing.
sorption to prevent roll-over errors. Polypropylene capaci-
tors are recommended for this application.
Integrating Resistor
A output stage with 100 A of quiescent current. They can
supply 20 A of drive current with negligible nonlinearity.
The integrating resistor should be large enough to remain
in this very linear region over the input voltage range, but
small enough that undue leakage requirements are not
placed on the PC board. For 2V full scale, 470 k
optimum and, similarly, 47 k
Oscillator Components
mended; the capacitor is selected from the equation:
For a 48 kHz clock (3 readings per second), C = 100 pF.
The size of the auto-zero capacitor has some influ-
A 0.1 F capacitor is acceptable in most applications.
The integrating capacitor should be selected to give the
The integrating capacitor must have low dielectric ab-
Both the buffer amplifier and the integrator have a class
For all frequency ranges, a 100-k
INT
IN
TELCOM SEMICONDUCTOR, INC.
–
pin is not at analog common), and a 200-mV scale
are 0.22 1F and 0.10 F, respectively. If different
f =
RC
45
.
for 200 mV full scale.
resistor is recom-
is near
Reference Voltage
input requirement is V
2V scale, V
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 700 mV. Instead of dividing
the input down to 200 mV, the designer should use the input
voltage directly and select V
for integrating resistor and capacitor would be 120 k and
0.22 F. 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
systems with a variable tare are examples. This offset
reading can be conveniently generated by connecting the
voltage transducer between V
the variable (or fixed) offset voltage between analog com-
mon and V
TC7117/TC7117A POWER SUPPLIES
it can be generated with a TC7660 DC-to-DC converter and
two capacitors. Figure 10 shows this application.
quired. The conditions for using a single +5V supply are:
5V supplies. However, if a negative supply is not available,
10 µF
To generate full-scale output (2000 counts), the analog
The TC7117/TC7117A are designed to operate from
In selected applications, a negative supply is not re-
(1) The input signal can be referenced to the center of
(2) The signal is less than 1.5V.
(3) An external reference is used.
Figure 10. Negative Power Supply Generation With TC7660
+
the common-mode range of the converter.
2
4
IN
–
REF
.
TC7660
should equal 100 mV and 1V, respectively.
LED
DRIVE
3
8
+5V
10 µF
+
IN
5
IN
= 2 V
(–5V)
TC7117A
TC7117
REF
0. Temperature and weighing
REF
V +
V
+
35
IN
26
–
= 350 mV. Suitable values
. Thus, for the 200 mV and
V
COM
GND
and analog common, and
REF
+
V
V
+
IN
IN
–
36
32
31
30
21
TC7116A
TC7117A
TC7116
TC7117
TC04
V
+
–
IN
3-213
1
2
3
4
5
6
7
8