TC820 TelCom Semiconductor, TC820 Datasheet - Page 19

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

applied to the reference input. If the unknown equals the
standard, the input voltage will equal the reference voltage
and the display will read 2000. The displayed reading can be
determined from the following expression:
Buffering the FREQ Input
input is low, the TC820 will count pulses at the RANGE/
FREQ input. The time base will be f
with a 40kHz clock. The signal to be measured should swing
from V
input levels without hysteresis. For best results, especially
with low-frequency sine-wave inputs, an external buffer with
hysteresis should be added. A typical circuit is shown in
Figure 20.
3-3/4 A/D CONVERTER WITH
FREQUENCY COUNTER
AND LOGIC PROBE
Figure 19. Low Parts Count Ratiometric Resistance Measurement
R UNKNOWN
R
SET V
(a) Internal Reference
TC820
Displayed reading =
The display will overrange for values of R
When the FREQ/VOLTS input is high and the LOGIC
R STANDARD
STANDARD
TELCOM SEMICONDUCTOR, INC.
V
COMMON
DD
ANALOG
SS
REF
9V
to DGND. The RANGE/FREQ input has CMOS
V
V
= 1/2 V
Figure 18. Reference Voltage Connections
V
REF
+
REF
–
+
DD
.
V
FULL SCALE
2k
REF
22k
V
V
V
V
ANALOG
COMMON
+
REF
+
IN
REF
IN
–
–
R
R
STANDARD
UNKNOWN
V
TC820
TC820
DD
(b) External Reference
COMMON
OSC
V
DD
V
V
+
REF
REF
–
/40,000, or 1 second
2000
2k
UNKNOWN
LCD
V
+
TC04A
1.2V
REF
2
Logic Probe Inputs
inputs when the LOGIC input is high. Driving either DP0/LO
or DP1/HI to a logic high will turn on the appropriate LCD
annunciator. When DP0/LO is high, the buzzer will be on.
buffers should be used. A simple circuit is shown in Figure
21. This circuit will turn the appropriate annunciator on for
high and low level inputs.
window comparator can be used. Figure 22 shows a typical
circuit. This circuit will turn on the high or low annunciators
when the logic thresholds are exceeded, but the resistors
connected from DP0/LO and DP1/HI to DGND will turn both
annunciators off when the logic probe is unconnected.
pulses of short duration will usually be difficult or impossible
to see. To display short pulses properly, the input pulse
should be "stretched." The circuit of Figure 22 shows cap-
FREQUENCY
PROBE
LOGIC
INPUT
The DP0/LO and DP1/HI inputs provide the logic probe
To provide a "single input" logic probe function, external
If carefully controlled logic thresholds are required, a
The TC820 logic inputs are not latched internally, so
INPUT
GND
Figure 20. Frequency Counter External Buffer
Figure 21. Simple External Logic Probe Buffer
DGND
+
*
1 F
*
+9V
74HC14
74HC14
*
+9V
FREQ/VOLTS
RANGE/FREQ
DGND
V
V
LOGIC
DP1/HI
DP0/LO
DGND
DD
DD
TC820
TC820
TC820
3-167
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