TC820 TelCom Semiconductor, TC820 Datasheet - Page 20

• Manufacturer Part Number: TC820
• Description: 3-3/4 DIGIT A/D CONVERTER WITH FREQUENCY COUNTER AND LOGIC PROBE
• Manufacturer: TelCom Semiconductor

acitors added across the input pull-down resistors to stretch
the input pulse and permit viewing short-duration input
pulses.
External Peak Detection
frequency reading indefinitely when the PK HOLD input is
connected to V
present during the A/D converter's signal integrate period.
For slowly changing signals, such as temperature, the peak
reading will be properly converted and held.
external peak detector should be added. An inexpensive
circuit can be made from an op amp and a few discrete
components, as shown in Figure 23. The droop rate of the
external peak detector should be adjusted so that the held
voltage will not decay below the desired accuracy level
during the converter's 400msec conversion time.
Liquid Crystal Display (LCD)
three backplanes. The LCD can include decimal points,
polarity sign, and annunciators for overrange, peak hold,
high and low logic levels, and low battery. Table IV shows the
assignment of the display segments to the backplanes and
segment drive lines. The backplane drive frequency is
obtained by dividing the oscillator frequency by 240.
appear on outputs BP1, BP2, and BP3. They remain the
same regardless of the segments being driven.
TC820
3-168
The TC820 will hold the highest A/D conversion or
If rapidly changing analog signals must be held, an
The TC820 drives a triplex (multiplexed 3:1) LCD with
Backplane waveforms are shown in Figure 24. These
Other display output lines have waveforms that vary
NOTE: Select R1, R2, R3 for desired logic thresholds.
PROBE
LOGIC
INPUT
1M
1M
Figure 22. Window Comparator Logic Probe
DD
. However, the analog peak input must be
R2
R1
R3
V
V
H
L
–
+
–
+
+9V
1N4148
1N4148
LOGIC
DP1/HI
DP0/LO
DGND
V
TC820
DD
40-Pin DIP
*Connect both pins 2 and 16 of LCD to TC820 BP3 output.
Table IV. LCD Backplane and Segment Assignments
depending on the displays values. Figure 25 shows a set of
waveforms for the a, g, d outputs of one digit for several
combinations of "on" segments.
Pin No.
V
10
11
12
13
14
15
IN
1
2
3
4
5
6
7
8
9
10k
TL061
+
–
Flat Pkg
Pin No.
44-Pin
3-3/4 A/D CONVERTER WITH
Figure 23. External Peak Detector
40
41
42
43
44
10
1
2
3
4
5
6
7
8
9
TELCOM SEMICONDUCTOR, INC.
FREQUENCY COUNTER
OFFSET
NULL
1N4148
Display
Pin No.
LCD
2,16*
AND LOGIC PROBE
0.01 F
10
11
12
13
14
15
3
4
5
6
7
8
9
1
+9V
OVER
PEAK
HIGH
LOW
BP1
BP1
A4
B4
A3
B3
A2
B2
A1
B1
—
—
V
PK HOLD
V
BP2
DD
BP2
IN
+
"—"
G4
G3
G2
G1
C4
C3
C2
C1
F3
F2
F1
—
—
V
SS
TC820
BATT
0V
BP3
DP3
DP2
DP1
BP3
E4
D4
E3
D3
D2
D1
E2
E1
—
—