3-1/2 DIGIT ANALOG-TO-DIGITAL
CONVERTERS WITH HOLD
TC7116
TC7116A
TC7117
TC7117A
Reference Voltage
To generate full-scale output (2000 counts), the analog
input requirement is V
IN
= 2 V
REF
. Thus, for the 200 mV and
2V scale, V
REF
should equal 100 mV 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 700 mV. Instead of dividing
the input down to 200 mV, the designer should use the input
voltage directly and select V
REF
= 350 mV. Suitable values
for integrating resistor and capacitor would be 120 kΩ and
0.22
µF.
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. Temperature and weighing
systems with a variable tare are examples. This offset
reading can be conveniently generated by connecting the
voltage transducer between V
+IN
and analog common, and
the variable (or fixed) offset voltage between analog com-
–
mon and V
IN
.
1
COMPONENT VALUE SELECTION
Auto-Zero Capacitor
The size of the auto-zero capacitor has some influ-
ence on system noise. For 200 mV full scale, where noise
is very important, a 0.47
µF
capacitor is recommended. On
the 2V scale, a 0.047
µF
capacitor increases the speed of
recovery from overload and is adequate for noise on this
scale.
2
3
4
5
6
7
Reference Capacitor
A 0.1
µF
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 200-mV scale
is used, a larger value is required to prevent roll-over error.
Generally, 1
µF
will hold the roll-over error to 0.5 count in
this instance.
Integrating Capacitor
The integrating capacitor should be selected to give the
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
INT
are 0.22
µ1F
and 0.10
µF,
respectively. If different
oscillator frequencies are used, these values should be
changed in inverse proportion to maintain the output swing.
The integrating capacitor must have low dielectric ab-
sorption to prevent roll-over errors. Polypropylene capaci-
tors are recommended for this application.
TC7117/TC7117A POWER SUPPLIES
The TC7117/TC7117A are designed to operate from
±5V
supplies. However, if a negative supply is not available,
it can be generated with a TC7660 DC-to-DC converter and
two capacitors. Figure 10 shows this application.
In selected applications, a negative supply is not re-
quired. The conditions for using a single +5V supply are:
(1) The input signal can be referenced to the center of
the common-mode range of the converter.
(2) The signal is less than
±1.5V.
(3) An external reference is used.
+5V
35
+ +
36
V V
REF
LED
DRIVE
COM
TC7117
TC7117A
8
2
+
10 µF
4
TC7660
3 +
10 µF
5 (–5V)
V–
26
GND
32
+
V
IN
–
Integrating Resistor
Both the buffer amplifier and the integrator have a class
A output stage with 100
µA
of quiescent current. They can
supply 20
µA
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 kΩ is near
optimum and, similarly, 47 kΩ for 200 mV full scale.
TC04
+
V
IN
31
–
V
IN
30
21
Oscillator Components
For all frequency ranges, a 100-kΩ resistor is recom-
mended; the capacitor is selected from the equation:
f = 45 .
RC
For a 48 kHz clock (3 readings per second), C = 100 pF.
TELCOM SEMICONDUCTOR, INC.
Figure 10. Negative Power Supply Generation With TC7660
3-213
8
TELCOM [ TELCOM SEMICONDUCTOR, INC ]
