TC7126 TelCom, TC7126 Datasheet - Page 8

• Manufacturer Part Number: TC7126
• Description: 3-1/2 DIGIT ANALOG-TO-DIGITAL CONVERTERS
• Manufacturer: TelCom

signal, the segment is OFF. An out-of-phase segment drive
signal causes the segment to be ON, or visible. This AC drive
configuration results in negligible DC voltage across each
LCD segment, ensuring long LCD life. The polarity segment
driver is ON for negative analog inputs. If V
reversed, this indicator would reverse.
segments are turned ON. The display reads –1888. During
this mode, LCD segments have a constant DC voltage
impressed. DO NOT LEAVE THE DISPLAY IN THIS MODE
FOR MORE THAN SEVERAL MINUTES; LCDS MAY BE
DESTROYED IF OPERATED WITH DC LEVELS FOR
EXTENDED PERIODS.
shown in Figure 4.
System Timing
internal decade counters. The three-phase measurement
cycle takes a total of 4000 counts (16,000 clock pulses).
The 4000-count cycle is independent of input signal magni-
tude.
length:
3-224
TC7126
TC7126A
When a segment driver is in-phase with the backplane
On the TC7126A, when the TEST pin is pulled to V
The display font and segment drive assignment are
The oscillator frequency is
Each phase of the measurement cycle has the following
(1) Auto-zero phase: 1000 to 3000 counts
(2) Signal integrate:
(3) Reference integrate: 0 to 2000 counts
For signals less than full scale, the auto-zero phase
is assigned the unused reference integrate time
period.
This time period is fixed. The integration period is:
where f
Figure 4. Display Font and Segment Assignment
t
SI
1000's
= 4000
OSC
is the externally-set clock frequency.
100's
DISPLAY FONT
f
OSC
1
(4000 to 12,000 clock pulses)
1000 counts
(4000 clock pulses)
,
(0 to 8000 clock pulses)
10's
4 prior to clocking the
1's
IN
+
and V
IN
–
+
, all
are
where: f
placed in series with C
compensating for comparator delay. C
dielectric absorption to minimize roll-over error. A polypro-
pylene capacitor is recommended.
and ICL7126 that offers a greatly improved internal refer-
ence temperature coefficient. No external component value
changes are required to upgrade existing designs.
COMPONENT VALUE SELECTION
Auto-Zero Capacitor (C
0.33 F capacitor is recommended for 200 mV full-scale
applications where 1 LSB is 100 V. A 0.033 F capacitor is
adequate for 2V full-scale applications. A Mylar-type dielec-
tric capacitor is adequate.
Reference Voltage Capacitor (C
output voltage back to zero during the reference integrate
phase, is stored on C
when V
mode voltage exists (V
application requires a 200 mV full scale, increase C
1 F. Roll-over error will be held to less than 0.5 count. A
Mylar-type dielectric capacitor is adequate.
Integrating Capacitor (C
voltage swing without causing output saturation. Due to
the TC7126A's superior analog common temperature co-
efficient specification, analog common will normally sup-
ply the differential voltage reference. For this case, a 2V
full-scale integrator output swing is satisfactory. For 3
readings per second (f
suggested. For 1 reading per second, 0.15 F is recom-
mended. If a different oscillator frequency is used, C
must be changed in inverse proportion to maintain the
nominal 2V integrator swing.
ANALOG-TO-DIGITAL CONVERTERS
At 3 readings per second, a 750
C
The TC7126A is a drop-in replacement for the TC7126
The C
The reference voltage, used to ramp the integrator
C
An exact expression for C
INT
INT
REF
V
R
V
OSC
=
should be selected to maximize integrator output
FS
INT
INT
AZ
–
(4000)
is tied to analog common. If a large common-
size has some influence on system noise. A
= Clock frequency at pin 38
= Full-scale input voltage
= Integrating resistor
= Desired full-scale integrator output swing.
(
f
OSC
REF
TELCOM SEMICONDUCTOR, INC.
1
V
OSC
INT
REF
INT
. A 0.1 F capacitor is acceptable
( ) )
. This increases accuracy by
= 48 kHz), a 0.047 F value is
–
R
V
AZ
INT
INT
INT
FS
analog common) and the
)
is:
)
,
INT
resistor should be
REF
3-1/2 DIGIT
must have low
)
REF
INT
to